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Kinetoplastid protozoa possess properties that are highly divergent from the mammalian, yeast and bacterial cells more commonly used in synthetic biology and represent a tantalisingly untapped source of bioengineering potential.

The African trypanosome Trypanosoma brucei is a unicellular eukaryote, which relies on a protective variant surface glycoprotein (VSG) coat for survival in the mammalian host. A single trypanosome has >2000 VSG genes and pseudogenes of which only one is expressed from one of ∼15 telomeric bloodstream form expression sites (BESs). Infectious metacyclic trypanosomes present within the tsetse fly vector also express VSG from a separate set of telomeric metacyclic ESs (MESs). All MESs are silenced in bloodstream form T. brucei. As very little is known about how this is mediated, we performed a whole genome RNAi library screen to identify MES repressors. This allowed us to identify a novel SAP domain containing DNA binding protein which we called TbSAP. TbSAP is enriched at the nuclear periphery and binds both MESs and BESs. Knockdown of TbSAP in bloodstream form trypanosomes did not result in cells becoming more ‘metacyclic-like'. Instead, there was extensive global upregulation of transcripts including MES VSGs, VSGs within the silent VSG arrays as well as genes immediately downstream of BES promoters. TbSAP therefore appears to be a novel chromatin protein playing an important role in silencing the extensive VSG repertoire of bloodstream form T. brucei.

Kinetoplastid protozoa possess properties that are highly divergent from the mammalian, yeast and bacterial cells more commonly used in synthetic biology and represent a tantalisingly untapped source of bioengineering potential. Trypanosoma brucei brucei (T. b. brucei), an established model organism for studying the Kinetoplastida, is non-pathogenic to humans and provides an interesting test case for establishing synthetic biology in this phylogenetic class. To demonstrate further the tractability of Kinetoplastida to synthetic biology, we sought to construct and demonstrate a Goodwin oscillator, the simplest oscillatory gene network, in T. b. brucei for the first time. We report one completed iteration of the archetypal synthetic biology Design-Build-Test-Learn (DBTL) cycle; firstly, using Ab initio mathematical modelling of the behaviour a theoretical, oscillatory, trypanosomal synthetic gene network (SGN) to inform the design of a plasmid encoding that network. Once assembled, the plasmid was then used to generate a stable transfectant T. b. brucei cell line. To test the performance of the oscillatory SGN, a novel experimental setup was established to capture images of the fluorescent signal from motion-restricted live cells. Data captured were consistent with oscillatory behaviour of the SGN, with cellular fluorescence observed to oscillate with a period of 50 min, with varying amplitude and linear growth trend. This first DBTL cycle establishes a foundation for future cycles in which the SGN design and experimental monitoring setup can be further refined.

The arsenal of drugs used to treat leishmaniasis, caused by Leishmania spp., is limited and beset by toxicity and emergent resistance. Furthermore, our understanding of drug mode of action and potential routes to resistance is limited. Forward genetic approaches have revolutionized our understanding of drug mode of action in the related kinetoplastid parasite Trypanosoma brucei., The arsenal of drugs used to treat leishmaniasis, caused by Leishmania spp., is limited and beset by toxicity and emergent resistance. Furthermore, our understanding of drug mode of action and potential routes to resistance is limited. Forward genetic approaches have revolutionized our understanding of drug mode of action in the related kinetoplastid parasite Trypanosoma brucei. Therefore, we screened our genome-scale T. brucei RNA interference (RNAi) library against the current antileishmanial drugs sodium stibogluconate (antimonial), paromomycin, miltefosine, and amphotericin B. Identification of T. brucei orthologues of the known Leishmania antimonial and miltefosine plasma membrane transporters effectively validated our approach, while a cohort of 42 novel drug efficacy determinants provides new insights and serves as a resource. Follow-up analyses revealed the antimonial selectivity of the aquaglyceroporin TbAQP3. A lysosomal major facilitator superfamily transporter contributes to paromomycin-aminoglycoside efficacy. The vesicle-associated membrane protein TbVAMP7B and a flippase contribute to amphotericin B and miltefosine action and are potential cross-resistance determinants. Finally, multiple phospholipid-transporting flippases, including the T. brucei orthologue of the Leishmania miltefosine transporter, a putative β-subunit/CDC50 cofactor, and additional membrane-associated hits, affect amphotericin B efficacy, providing new insights into mechanisms of drug uptake and action. The findings from this orthology-based chemogenomic profiling approach substantially advance our understanding of antileishmanial drug action and potential resistance mechanisms and should facilitate the development of improved therapies as well as surveillance for drug-resistant parasites.

The protocol is provided by the Alsford Lab at the London School of Hygiene & Tropical Medicine (https://blogs.lshtm.ac.uk/alsfordlab/protocols/bloodstream-form-culture/)

Protocol obtained from the Alsford Lab at the London School of Hygiene & Tropical Medicine (https://blogs.lshtm.ac.uk/alsfordlab/protocols/bloodstream-form-culture/)

In contrast to Trypanosoma brucei gambiense and T. b. rhodesiense (the causative agents of human African trypanosomiasis), T. b. brucei is lysed by apolipoprotein-L1 (apoL1)-containing human serum trypanolytic factors (TLF), rendering it non-infectious to humans. While the mechanisms of TLF1 uptake, apoL1 membrane integration, and T. b. gambiense and T. b. rhodesiense apoL1-resistance have been extensively characterised, our understanding of the range of factors that drive apoL1 action in T. b. brucei is limited. Selecting our bloodstream-form T. b. brucei RNAi library with recombinant apoL1 identified an array of factors that supports the trypanocidal action of apoL1, including six putative ubiquitin modifiers and several proteins putatively involved in membrane trafficking; we also identified the known apoL1 sensitivity determinants, TbKIFC1 and the V-ATPase. Most prominent amongst the novel apoL1 sensitivity determinants was a putative ubiquitin ligase. Intriguingly, while loss of this ubiquitin ligase reduces parasite sensitivity to apoL1, its loss enhances parasite sensitivity to TLF1-dominated normal human serum, indicating that free and TLF1-bound apoL1 have contrasting modes-of-action. Indeed, loss of the known human serum sensitivity determinants, p67 (lysosomal associated membrane protein) and the cathepsin-L regulator, ‘inhibitor of cysteine peptidase’, had no effect on sensitivity to free apoL1. Our findings highlight a complex network of proteins that influences apoL1 action, with implications for our understanding of the anti-trypanosomal action of human serum., Author summary Expression of the trypanolytic serum component, apolipoprotein-L1 (apoL1), by humans and the concomitant evolution of countermeasures by two African trypanosome sub-species defines their ability to cause disease in humans. A genome-scale RNAi screen identified more than 60 proteins that sensitise non-human infectious trypanosomes to apoL1. Comparing these outputs to previous screens that used different selection approaches provides insights into the mode-of-action of human serum-mediated killing of trypanosomes. The effectiveness of apoL1, and the trypanolytic factors (TLF) that carry it, is dependent on the parasite’s endocytic system for uptake, intracellular transit and delivery to target membranes. Comparing our current data to the outputs from previous screens revealed that apoL1 is able to exploit different routes to access its target membranes, depending on whether it is free or, as in human serum, complexed with TLF1 or TLF2.

All cells are subject to structural damage that must be addressed for continued growth. A wide range of damage affects the genome, meaning multiple pathways have evolved to repair or bypass the resulting DNA lesions. Though many repair pathways are conserved, their presence or function can reflect the life style of individual organisms. To identify genome maintenance pathways in a divergent eukaryote and important parasite, Trypanosoma brucei, we performed RNAi screens to identify genes important for survival following exposure to the alkylating agent methyl methanesulphonate. Amongst a cohort of broadly conserved and, therefore, early evolved repair pathways, we reveal multiple activities not so far examined functionally in T. brucei, including DNA polymerases, DNA helicases and chromatin factors. In addition, the screens reveal Trypanosoma- or kinetoplastid-specific repair-associated activities. We also provide focused analyses of repair-associated protein kinases and show that loss of at least nine, and potentially as many as 30 protein kinases, including a nuclear aurora kinase, sensitises T. brucei to alkylation damage. Our results demonstrate the potential for synthetic lethal genome-wide screening of gene function in T. brucei and provide an evolutionary perspective on the repair pathways that underpin effective responses to damage, with particular relevance for related kinetoplastid pathogens. By revealing that a large number of diverse T. brucei protein kinases act in the response to damage, we expand the range of eukaryotic signalling factors implicated in genome maintenance activities., Author summary Damage to the genome is a universal threat to life. Though the repair pathways used to tackle damage can be widely conserved, lineage-specific specialisations are found, reflecting the differing life styles of extant organisms. Using RNAi coupled with next generation sequencing we have screened for genes that are important for growth of Trypanosoma brucei, a diverged eukaryotic microbe and important parasite, in the presence of alkylation damage caused by methyl methanesulphonate. We reveal both repair pathway conservation relative to characterised eukaryotes and specialisation, including uncharacterised roles for translesion DNA polymerases, DNA helicases and chromatin factors. Furthermore, we demonstrate that loss of around 15% of T. brucei protein kinases sensitises the parasites to alkylation, indicating phosphorylation signalling plays widespread and under-investigated roles in the damage response pathways of eukaryotes.

Trypanosoma brucei, protozoan parasites that cause human African trypanosomiasis (HAT), depend on ornithine uptake and metabolism by ornithine decarboxylase (ODC) for survival. Indeed, ODC is the target of the WHO “essential medicine” eflornithine, which is antagonistic to another anti-HAT drug, suramin. Thus, ornithine uptake has important consequences in T. brucei, but the transporters have not been identified. We describe these amino acid transporters (AATs). In a heterologous expression system, TbAAT10-1 is selective for ornithine, whereas TbAAT2-4 transports both ornithine and histidine. These AATs are also necessary to maintain intracellular ornithine and polyamine levels in T. brucei, thereby decreasing sensitivity to eflornithine and increasing sensitivity to suramin. Consistent with competition for histidine, high extracellular concentrations of this amino acid phenocopied a TbAAT2-4 genetic defect. Our findings established TbAAT10-1 and TbAAT2-4 as the parasite ornithine transporters, one of which can be modulated by histidine, but both of which affect sensitivity to important anti-HAT drugs.—Macedo, J. P., Currier, R. B., Wirdnam, C., Horn, D., Alsford, S., Rentsch, D. Ornithine uptake and the modulation of drug sensitivity in Trypanosoma brucei.

One of the most promising new targets for trypanocidal drugs to emerge in recent years is the cyclic AMP (cAMP) phosphodiesterase (PDE) activity encoded by TbrPDEB1 and TbrPDEB2 . These genes were genetically confirmed as essential, and a high-affinity inhibitor, CpdA, displays potent antitrypanosomal activity. To identify effectors of the elevated cAMP levels resulting from CpdA action and, consequently, potential sites for adaptations giving resistance to PDE inhibitors, resistance to the drug was induced. Selection of mutagenized trypanosomes resulted in resistance to CpdA as well as cross-resistance to membrane-permeable cAMP analogues but not to currently used trypanocidal drugs. Resistance was not due to changes in cAMP levels or in PDEB genes. A second approach, a genome-wide RNA interference (RNAi) library screen, returned four genes giving resistance to CpdA upon knockdown. Validation by independent RNAi strategies confirmed resistance to CpdA and suggested a role for the identified cA MP R esponse P roteins (CARPs) in cAMP action. CARP1 is unique to kinetoplastid parasites and has predicted cyclic nucleotide binding-like domains, and RNAi repression resulted in >100-fold resistance. CARP2 and CARP4 are hypothetical conserved proteins associated with the eukaryotic flagellar proteome or with flagellar function, with an orthologue of CARP4 implicated in human disease. CARP3 is a hypothetical protein, unique to Trypanosoma . CARP1 to CARP4 likely represent components of a novel cAMP signaling pathway in the parasite. As cAMP metabolism is validated as a drug target in Trypanosoma brucei , cAMP effectors highly divergent from the mammalian host, such as CARP1 , lend themselves to further pharmacological development.

Bloodstream-form cells of Trypanosoma brucei exhibit massively increased endocytic activity relative to the insect midgut stage, enabling rapid recycling of variant surface glycoprotein and antibody clearance from the surface. In addition, recent advances have identified a role for receptor-mediated endocytosis in the uptake of the antitrypanosomal drug, suramin, via invariant surface glycoprotein 75, and in the uptake of trypanosome lytic factor 1 via haptoglobin-haemoglobin receptor. Here, we argue that receptor-mediated endocytosis represents both a validated drug target and a promising route for the delivery of novel therapeutics into trypanosomes.

SUMMARYThe trypanosomes cause two neglected tropical diseases, Chagas disease in the Americas and African trypanosomiasis in sub-Saharan Africa. Over recent years a raft of molecular tools have been developed enabling the genetic dissection of many aspects of trypanosome biology, including the mechanisms underlying resistance to some of the current clinical and veterinary drugs. This has led to the identification and characterization of key resistance determinants, including transporters for the anti-Trypanosoma bruceidrugs, melarsoprol, pentamidine and eflornithine, and the activator of nifurtimox-benznidazole, the anti-Trypanosoma cruzidrugs. More recently, advances in sequencing technology, combined with the development of RNA interference libraries in the clinically relevant bloodstream form ofT. bruceihave led to an exponential increase in the number of proteins known to interact either directly or indirectly with the anti-trypanosomal drugs. In this review, we discuss these findings and the technological developments that are set to further revolutionise our understanding of drug-trypanosome interactions. The new knowledge gained should inform the development of novel interventions against the devastating diseases caused by these parasites.

The nuclear lamina is a filamentous structure subtending the nuclear envelope and required for chromatin organization, transcriptional regulation and maintaining nuclear structure. The trypanosomatid coiled-coil NUP-1 protein is a lamina component functionally analogous to lamins, the major lamina proteins of metazoa. There is little evidence for shared ancestry, suggesting the presence of a distinct lamina system in trypanosomes. To find additional trypanosomatid lamina components we identified NUP-1 interacting proteins by affinity capture and mass-spectrometry. Multiple components of the nuclear pore complex (NPC) and a second coiled-coil protein, which we termed NUP-2, were found. NUP-2 has a punctate distribution at the nuclear periphery throughout the cell cycle and is in close proximity to NUP-1, the NPCs and telomeric chromosomal regions. RNAi-mediated silencing of NUP-2 leads to severe proliferation defects, gross alterations to nuclear structure, chromosomal organization and nuclear envelope architecture. Further, transcription is altered at telomere-proximal variant surface glycoprotein (VSG) expression sites (ESs), suggesting a role in controlling ES expression, although NUP-2 silencing does not increase VSG switching. Transcriptome analysis suggests specific alterations to Pol I-dependent transcription. NUP-1 is mislocalized in NUP-2 knockdown cells and vice versa, implying that NUP-1 and NUP-2 form a co-dependent network and identifying NUP-2 as a second trypanosomatid nuclear lamina component.

The genome of African trypanosomes contains a large number of minichromosomes. Their only proposed role is in the expansion of the parasites' repertoire of telomeric variant surface glycoprotein (VSG) genes as minichromosomes carry silent VSG gene copies in telomeric locations. Despite their importance as VSG gene donors, little is known about the actual composition of the minichromosomal karyotype and the stability of its inheritance. In this study we show, by using high-resolution pulsed-field electrophoresis, that a non-clonal trypanosome population contains an extremely diverse pattern of minichromosomes, which can be resolved into less complex clone-specific karyotypes by non-selective cloning. We show that the minichromosome patterns of such clones are stable over at least 360 generations. Furthermore, using DNA markers for specific minichromosomes, we demonstrate the mitotic stability of these minichromosomes within the population over a period of more than 5 years. Length variation is observed for an individual minichromosome and is most likely caused by a continuous telomeric growth of approximately 6 bp per telomere per cell division. This steady telomeric growth, counteracted by stochastic large losses of telomeric sequences is the most likely cause of minichromosome karyotype heterogeneity within a population.

Most African trypanosomes, including the veterinary species Trypanosoma brucei brucei and T. congolense are susceptible to lysis by human serum. A recent study by Alsford et al. [PLoS Pathogens (2014) 10, e1004130] has identified a T. b. brucei lysosomal cathepsin with an inhibitory effect on human serum’s trypanolytic action.

Allelic exclusion underpins antigenic variation and immune evasion in African trypanosomes. These bloodstream parasites use RNA polymerase-I (pol-I) to transcribe just one telomeric variant surface glycoprotein (VSG) gene at a time, producing superabundant and switchable VSG coats. We identified trypanosome VSG exclusion-1 (VEX1) using a genetic screen for defects in telomere-exclusive expression. VEX1 was sequestered by the active VSG and silencing of other VSGs failed when VEX1 was either ectopically expressed or depleted, indicating positive and negative regulation, respectively. Positive regulation affected VSGs and nontelomeric pol-I-transcribed genes, whereas negative regulation primarily affected VSGs. Negative regulation by VEX1 also affected telomeric pol-I-transcribed reporter constructs, but only when they contained blocks of sequence sharing homology with a pol-I-transcribed locus. We conclude that restricted positive regulation due to VEX1 sequestration, combined with VEX1-dependent, possibly homology-dependent silencing, drives a "winner-takes-all" mechanism of allelic exclusion.

Eukaryotic cells limit ribosomal DNA (rDNA) transcription by RNA polymerase I (RNAP-I) to maintain genome integrity. African trypanosomes present an excellent model for studies on RNAP-I regulation because they possess a bifunctional RNAP-I and because RNAP-II transcription appears unregulated. Since Elp3, the catalytic component of Elongator, controls RNAP-II transcription in yeast and human cells, we predicted a role for a trypanosome Elp3-related protein, ELP3a or ELP3b, in RNAP-I regulation. elp3b null and conditional strains specifically exhibited resistance to a transcription elongation inhibitor, suggesting that ELP3b negatively impacts elongation. Nascent RNA analysis and expression of integrated reporter cassettes supported this interpretation and revealed negative control of rDNA transcription. ELP3b specifically localized to the nucleolus, and ELP3b loss rendered cells hypersensitive to DNA damage and to translation inhibition, suggesting that anti-Elongator function was important to maintain genome integrity rather than to modulate ribosome production. Finally, ELP3b displayed discrimination between RNAP-I compartments in the same cell. Our results establish ELP3b as a major negative regulator of rDNA transcription and extend the roles of the Elp3-related proteins to RNAP-I transcription units. ELP3b is also the first trypanosome protein shown to distinguish between rDNA and variant surface glycoprotein transcription within different RNAP-I compartments.

A major obstacle to reproducible expression of recombinant transcripts lies in the epigenetic effects of the flanking chromatin following integration. We previously presented a strategy to overcome this problem in bloodstream form Trypanosoma brucei, using a reporter to identify a ribosomal-spacer locus that supports optimal expression and then marking that locus for subsequent targeting. Advantages include elimination of variable-expression position-effects and the easy confirmation of correct integration. We now report a set of validated constructs that exploit this system for expression of dsRNA or recombinant protein. The current construct-set allows expression of intramolecular dsRNA for RNA interference knockdown or expression of proteins that can incorporate c-Myc epitope(s) or a fluorescent-tag for subcellular localisation, interaction and/or other functional analysis. The constructs are integrated at a single, marked locus and deliver reliable and reproducible expression.

Chromatin modification is important for virtually all aspects of DNA metabolism but little is known about the consequences of such modification in trypanosomatids, early branching protozoa of significant medical and veterinary importance. MYST-family histone acetyltransferases in other species function in transcription regulation, DNA replication, recombination and repair. Trypanosoma brucei HAT3 was recently shown to acetylate histone H4K4 and we now report characterization of all three T. brucei MYST acetyltransferases (HAT1-3). First, GFP-tagged HAT1-3 all localize to the trypanosome nucleus. While HAT3 is dispensable, both HAT1 and HAT2 are essential for growth. Strains with HAT1 knock-down display mitosis without nuclear DNA replication and also specific de-repression of a telomeric reporter gene, a rare example of transcription control in an organism with widespread and constitutive polycistronic transcription. Finally, we show that HAT2 is responsible for H4K10 acetylation. By analogy to the situation in Saccharomyces cerevisiae, we discuss low-level redundancy of acetyltransferase function in T. brucei and suggest that two MYST-family acetyltransferases are essential due to the absence of a Gcn5 homologue. The results are also consistent with the idea that HAT1 contributes to establishing boundaries between transcriptionally active and repressed telomeric domains in T. brucei.

International audience; The ability to simultaneously assess every gene in a genome for a role in a particular process has obvious appeal. This protocol describes how to perform genome-scale RNAi library screens in bloodstream-form African trypanosomes, a family of parasites that causes lethal human and animal diseases and also serves as a model for studies on basic aspects of eukaryotic biology and evolution. We discuss strain assembly, screen design and implementation, the RNAi target sequencing approach and hit validation, and we provide a step-by-step protocol. A screen can yield from one to thousands of 'hits' associated with the phenotype of interest. The screening protocol itself takes 2 weeks or less to be completed, and high-throughput sequencing may also be completed within weeks. Pre- and post-screen strain assembly, validation and follow-up can take several months, depending on the type of screen and the number of hits analyzed.

International audience; In Trypanosoma brucei, RNA interference (RNAi) and recombinant protein expression are established as powerful approaches for functional genomics, particularly when combined with inducible expression. The favoured methods involve exploiting homologous recombination to target expression cassettes to a chromosome sub-set to establish stable cell lines. Unfortunately, bloodstream-form cells, those that cause disease in mammals, exhibit low efficiency stable transfection. Current expression systems can also exhibit other undesirable features, including variable position effects and leaky, inducible expression. We have developed systems in bloodstream-form cells that alleviate these problems. Using constructs for RNAi and expression of (GFP) tagged proteins, we target a (hyg) tagged ribosomal RNA (RRNA) locus which circumvents position effects and allows increased targeting efficiency. We also report a compatible double-inducible system for tight regulation of highly toxic products. This system exploits a new inducible RRNA promoter to drive T7 RNA polymerase (T7RNAP) transcription which then drives expression from inducible T7 promoters. The developments described should facilitate functional analysis and increased throughput.

Closely related African trypanosomes cause lethal diseases but display distinct host ranges. Specifically, Trypanosoma brucei brucei causes nagana in livestock but fails to infect humans, while Trypanosoma brucei gambiense and Trypanosoma brucei rhodesiense cause sleeping sickness in humans. T. b. brucei fails to infect humans because it is sensitive to innate immune complexes found in normal human serum known as trypanolytic factor (TLF) 1 and 2; the lytic component is apolipoprotein-L1 in both TLFs. TLF resistance mechanisms of T. b. gambiense and T. b. rhodesiense are now known to arise through either gain or loss-of-function, but our understanding of factors that render T. b. brucei susceptible to lysis by human serum remains incomplete. We conducted a genome-scale RNA interference (RNAi) library screen for reduced sensitivity to human serum. Among only four high-confidence ‘hits’ were all three genes previously shown to sensitize T. b. brucei to human serum, the haptoglobin-haemoglobin receptor (HpHbR), inhibitor of cysteine peptidase (ICP) and the lysosomal protein, p67, thereby demonstrating the pivotal roles these factors play. The fourth gene identified encodes a predicted protein with eleven trans-membrane domains. Using chemical and genetic approaches, we show that ICP sensitizes T. b. brucei to human serum by modulating the essential cathepsin, CATL, a lysosomal cysteine peptidase. A second cathepsin, CATB, likely to be dispensable for growth in in vitro culture, has little or no impact on human-serum sensitivity. Our findings reveal major and novel determinants of human-serum sensitivity in T. b. brucei. They also shed light on the lysosomal protein-protein interactions that render T. b. brucei exquisitely sensitive to lytic factors in human serum, and indicate that CATL, an important potential drug target, has the capacity to resist these factors., Author Summary The interplay among host innate immunity and resistance mechanisms in African trypanosomes has a major impact on the host range of these tsetse-fly transmitted parasites, defining their ability to cause disease in humans. A genome-scale RNAi screen identified a highly restricted set of four genes that sensitise trypanosomes to human serum: those encoding the haptoglobin-haemoglobin receptor, a predicted trans-membrane channel, a lysosomal membrane-protein and the cysteine peptidase inhibitor. An analysis of the cysteine peptidases revealed cathepsin-L as the protease regulated by the inhibitor – and with the capacity to render the parasite resistant to lysis by human serum. These findings emphasise the importance of parasite factors for the delivery and stability of host toxins. They also shed light on the control of proteolysis by parasites and potential unanticipated consequences of therapies that target the parasite proteases.

Most African trypanosomes, including the veterinary species Trypanosoma brucei brucei and T. congolense are susceptible to lysis by human serum. A recent study by Alsford et al. [PLoS Pathogens (2014) 10, e1004130] has identified a T. b. brucei lysosomal cathepsin with an inhibitory effect on human serum’s trypanolytic action.

In addition to 11 pairs of housekeeping chromosomes, the genome of Trypanosoma brucei contains approximately 100 minichromosomes that are probably involved in the ability of the parasite to evade the host's immune response. This minichromosomal population is segregated on the mitotic spindle. How this is achieved provides insight into potential segregation mechanisms for small DNA molecules in eukaryotic microorganisms.

Antigenic variation in African trypanosomes requires monoallelic transcription and switching of variant surface glycoprotein (VSG) genes. The transcribed VSG, always flanked by ‘70 bp’-repeats and telomeric-repeats, is either replaced through DNA double-strand break (DSB) repair or transcriptionally inactivated. However, little is known about the subtelomeric DSBs that naturally trigger antigenic variation in Trypanosoma brucei, the subsequent DNA damage responses, or how these responses determine the mechanism of VSG switching. We found that DSBs naturally accumulate close to both transcribed and non-transcribed telomeres. We then induced high-efficiency meganuclease-mediated DSBs and monitored DSB-responses and DSB-survivors. By inducing breaks at distinct sites within both transcribed and silent VSG transcription units and assessing local DNA resection, histone modification, G2/M-checkpoint activation, and both RAD51-dependent and independent repair, we reveal how breaks at different sites trigger distinct responses and, in ‘active-site’ survivors, different switching mechanisms. At the active site, we find that promoter-adjacent breaks typically failed to trigger switching, 70 bp-repeat-adjacent breaks almost always triggered switching through 70 bp-repeat recombination (∼60% RAD51-dependent), and telomere-repeat-adjacent breaks triggered switching through loss of the VSG expression site (25% of survivors). Expression site loss was associated with G2/M-checkpoint bypass, while 70 bp-repeat-recombination was associated with DNA-resection, γH2A-focus assembly and a G2/M-checkpoint. Thus, the probability and mechanism of antigenic switching are highly dependent upon the location of the break. We conclude that 70 bp-repeat-adjacent and telomere-repeat-adjacent breaks trigger distinct checkpoint responses and VSG switching pathways. Our results show how subtelomere fragility can generate the triggers for the major antigenic variation mechanisms in the African trypanosome., Author Summary Previous studies on antigenic variation in African trypanosomes relied upon positive or negative selection, yielding only cells that underwent variation. This made it difficult to define individual switched clones as independent, potentially introduced bias in the relative contribution of each switching mechanism and precluded analysis of cells undergoing switching. We show that DNA double-strand breaks (DSBs) naturally accumulate close to Trypanosoma brucei telomeres. Using the I-SceI meganuclease, we then established a system to trigger breaks in all cells in a population. The specificity, temporal constraint and efficiency of cleavage facilitated the application of a quantitative approach to dissecting subtelomeric break responses and their consequences. Accordingly, we show that the DSB-site determines probability and mechanism of antigenic switching, that DSBs can trigger switching via recombination or transcription inactivation and that a checkpoint-bypass mechanism can explain switching via VSG expression site deletion. Our results provide major new insights into the mechanisms underlying antigenic variation and provide a new model to explain how the repeats flanking VSG genes serve distinct roles in fragility and recombination. The findings are also relevant to telomeric gene rearrangements that control immune evasion in other protozoal, fungal and bacterial pathogens such as Plasmodium, Pneumocystis and Borrelia species, respectively.

We have analysed the regulation of histone H2A, H2B, H4 and β-tubulin RNA levels during the cell cycle of asynchronous cultures of Trypanosoma brucei by fluorescence in situ hybridisation. Whereas tubulin mRNA is detectable at high levels during the entire cell cycle, histone mRNA presence peaks during S phase and is not detectable during all other stages of the cell cycle within the sensitivity limits of this technique. We show that fluorescence in situ hybridisation can be used to characterise the distribution patterns of cell cycle regulated transcripts in asynchronous cell culture systems and discuss the possibilities and limitations of quantification of hybridisation patterns by means of computer-assisted image analysis.

SummaryAcetylcholinesterase (ACHE) is present in the muscle and on the tegument of schistosomes. Molecular forms of schistosome AChE were examined because particular AChEs are found in tissues of distinct function elsewhere. The dimeric globular form (G2) is the only form evident in adult Schistosoma haematobium: 32 % of the muscle AChE is hydrophilic and 61 % is membrane associated. A substantial amount of this enzyme is phosphatidylinositol (PI) anchored since it could be released by PI-specific phospholipase C from both muscle and tegumental membranes.

Blood dwelling stages of schistosomes have acetylcholinesterase (AChE) on their teguments. As an initial step towards understanding the function of tegumental AChE, we have used specific ligand-binding assays to identify nicotinic acetylcholine receptors (nAChR) on the schistosome surface. AChR could not be detected on migratory stages using fluoroscein isothiocyanate-alpha-bungarotoxin binding but the amount of specific labelling increased on sexual pairing and as the parasites matured into egg-producing adults. Both AChE and nAChR were concentrated on the dorsal surface of the adult male. These results indicate a role for AChE and AChR associated with the transporting function of this membrane.

Antigenic variation in African trypanosomes involves monoallelic expression and reversible silencing of variant surface glycoprotein (VSG) genes found adjacent to telomeres in polycistronic expression sites (ESs). We assessed the impact on ES silencing of five candidate essential chromatin-associated factors that emerged from a genome-wide RNA interference viability screen. Using this approach, we demonstrate roles in VSG ES silencing for two histone chaperones. Defects in S-phase progression in cells depleted for histone H3, or either chaperone, highlight in particular the link between chromatin assembly and DNA replication control. S-phase checkpoint arrest was incomplete, however, allowing G2/M-specific VSG ES derepression following knockdown of histone H3. In striking contrast, knockdown of anti-silencing factor 1A (ASF1A) allowed for derepression at all cell cycle stages, whereas knockdown of chromatin assembly factor 1b (CAF-1b) revealed derepression predominantly in S-phase and G2/M. Our results support a central role for chromatin in maintaining VSG ES silencing. ASF1A and CAF-1b appear to play constitutive and DNA replication-dependent roles, respectively, in the recycling and assembly of chromatin. Defects in these functions typically lead to arrest in S-phase but defective cells can also progress through the cell cycle leading to nucleosome depletion and derepression of telomeric VSG ESs.

The control of gene expression, and more significantly gene cohorts, requires tight transcriptional coordination and is an essential feature of probably all cells. In higher eukaryotes, the mechanisms used involve controlled modifications to both local and global DNA environments, principally through changes in chromatin structure as well ascis-element-driven mechanisms. Although the mechanisms regulating chromatin in terms of transcriptional permissiveness and the relation to developmental programmes and responses to the environment are becoming better understood for animal and fungal cells, it is only just beginning to become clear how these processes operate in other taxa, including the trypanosomatids. Recent advances are now illuminating how African trypanosomes regulate higher-order chromatin structure, and, further, how these mechanisms impact on the expression of major surface antigens that are of fundamental importance to life-cycle progression. It is now apparent that several mechanisms are rather more similar between animal and fungal cells and trypanosomes than it originally appeared, but some aspects do involve gene products unique to trypanosomes. Therefore, both evolutionarily common and novel mechanisms cohabit in trypanosomes, offering both important biological insights and possible therapeutic opportunity.

Temporal and spatial organization of the nucleus is critical for the control of transcription, mRNA processing and the assembly of ribosomes. This includes the occupancy of specific territories by mammalian chromosomes, the presence of subnuclear compartments such as the nucleolus and Cajal bodies and the division of chromatin between active and inactive states. These latter are commonly associated with the location of DNA within euchromatin and heterochromatin respectively; critically these distinctions arise through modifications to chromatin-associated proteins, including histones, as well as the preferential localization of heterochromatin at the nuclear periphery. Most research on nuclear organization has focused on metazoa and fungi; however, recent technical advances have made more divergent eukaryotes accessible to study, with some surprising results. For example, the organization of heterochromatin is mediated in metazoan nuclei in large part by lamins, the prototypical intermediate filament proteins. Despite the presence of heterochromatin, detected both biochemically and by EM in most eukaryotic organisms, until this year lamins were thought to be restricted to metazoan taxa, and the proteins comprising the lamina in other lineages were unknown. Recent work indicates the presence of lamin orthologs in amoeba, while trypanosomatids possess a large coiled-coil protein, NUP-1, that performs functions analogous to lamins. These data indicate that the presence of a nuclear lamina is substantially more widespread than previously thought, with major implications for the evolution of eukaryotic gene expression mechanisms. We discuss these and other recent findings on the organization of nuclei in diverse organisms, and the implications of these findings for the evolutionary origin of eukaryotes.

NUP1, the first example of a nuclear lamin analog in nonmetazoans, performs roles similar to those of lamins in maintaining the structure and organization of the nucleus in Trypanosoma brucei., A unifying feature of eukaryotic nuclear organization is genome segregation into transcriptionally active euchromatin and transcriptionally repressed heterochromatin. In metazoa, lamin proteins preserve nuclear integrity and higher order heterochromatin organization at the nuclear periphery, but no non-metazoan lamin orthologues have been identified, despite the likely presence of nucleoskeletal elements in many lineages. This suggests a metazoan-specific origin for lamins, and therefore that distinct protein elements must compose the nucleoskeleton in other lineages. The trypanosomatids are highly divergent organisms and possess well-documented but remarkably distinct mechanisms for control of gene expression, including polycistronic transcription and trans-splicing. NUP-1 is a large protein localizing to the nuclear periphery of Trypanosoma brucei and a candidate nucleoskeletal component. We sought to determine if NUP-1 mediates heterochromatin organization and gene regulation at the nuclear periphery by examining the influence of NUP-1 knockdown on morphology, chromatin positioning, and transcription. We demonstrate that NUP-1 is essential and part of a stable network at the inner face of the trypanosome nuclear envelope, since knockdown cells have abnormally shaped nuclei with compromised structural integrity. NUP-1 knockdown also disrupts organization of nuclear pore complexes and chromosomes. Most significantly, we find that NUP-1 is required to maintain the silenced state of developmentally regulated genes at the nuclear periphery; NUP-1 knockdown results in highly specific mis-regulation of telomere-proximal silenced variant surface glycoprotein (VSG) expression sites and procyclin loci, indicating a disruption to normal chromatin organization essential to life-cycle progression. Further, NUP-1 depletion leads to increased VSG switching and therefore appears to have a role in control of antigenic variation. Thus, analogous to vertebrate lamins, NUP-1 is a major component of the nucleoskeleton with key roles in organization of the nuclear periphery, heterochromatin, and epigenetic control of developmentally regulated loci., Author Summary Eukaryotes—fungi, plants, animals, and many unicellular organisms—are defined by the presence of a cell nucleus that contains the chromosomes and is enveloped by a lipid membrane lined on the inner face with a protein network called the lamina. Among other functions, the lamina serves as an anchorage site for the ends of chromosomes. In multicellular animals (metazoa), the lamina comprises a few related proteins called lamins, which are very important for many functions related to the nucleus; abnormal lamins result in multiple nuclear defects and diseases, including inappropriate gene expression and premature aging. Until now, however, lamins had been found only in metazoa; no protein of equivalent function had been identified in plants, fungi, or unicellular organisms. Here, we describe a protein from African trypanosomes—the single-cell parasites that cause sleeping sickness—that fulfils many lamin-like roles, including maintaining nuclear structure and organizing the chromosomes of this organism. We show that this protein, which we call NUP-1 for nuclear periphery protein-1, is vital for the antigenic variation mechanisms that allow the parasite to escape the host immune response. We propose that NUP-1 is a lamin analogue that performs similar functions in trypanosomes to those of authentic lamins in metazoa. These findings, we believe, have important implications for understanding the evolution of the nucleus.

Summary The concept of specific chemotherapy was developed a century ago by Paul Ehrlich and others. Dyes and arsenical compounds that displayed selectivity against trypanosomes were central to this work 1,2, and the drugs that emerged remain in use for treating Human African Trypanosomiasis (HAT) 3. Ehrlich recognised the importance of understanding the mechanisms underlying selective drug action and resistance for the development of improved HAT therapies, but these mechanisms have remained largely mysterious. Here, we use all five current HAT drugs for genome-scale RNA interference (RNAi) target sequencing (RIT-seq) screens in Trypanosoma brucei, revealing the transporters, organelles, enzymes and metabolic pathways that function to facilitate anti-trypanosomal drug action. RIT-seq profiling identifies both known drug importers 4,5 and the only known pro-drug activator 6, and links more than fifty additional genes to drug action. A specific bloodstream stage invariant surface glycoprotein (ISG75) family mediates suramin uptake while the AP-1 adaptin complex, lysosomal proteases and major lysosomal transmembrane protein, as well as spermidine and N-acetylglucosamine biosynthesis all contribute to suramin action. Further screens link ubiquinone availability to nitro-drug action, plasma membrane P-type H+-ATPases to pentamidine action, and trypanothione and multiple putative kinases to melarsoprol action. We also demonstrate a major role for aquaglyceroporins in pentamidine and melarsoprol cross-resistance. These advances in our understanding of mechanisms of anti-trypanosomal drug efficacy and resistance will aid the rational design of new therapies and help to combat drug resistance, and provide unprecedented levels of molecular insight into the mode of action of anti-trypanosomal drugs.

African trypanosomes are major pathogens of humans and livestock and represent a model for studies of unusual protozoal biology. We describe a high-throughput phenotyping approach termed RNA interference (RNAi) target sequencing, or RIT-seq that, using Illumina sequencing, maps fitness-costs associated with RNAi. We scored the abundance of >90,000 integrated RNAi targets recovered from trypanosome libraries before and after induction of RNAi. Data are presented for 7435 protein coding sequences, >99% of a non-redundant set in the Trypanosoma brucei genome. Analysis of bloodstream and insect life-cycle stages and differentiated libraries revealed genome-scale knockdown profiles of growth and development, linking thousands of previously uncharacterized and “hypothetical” genes to essential functions. Genes underlying prominent features of trypanosome biology are highlighted, including the constitutive emphasis on post-transcriptional gene expression control, the importance of flagellar motility and glycolysis in the bloodstream, and of carboxylic acid metabolism and phosphorylation during differentiation from the bloodstream to the insect stage. The current data set also provides much needed genetic validation to identify new drug targets. RIT-seq represents a versatile new tool for genome-scale functional analyses and for the exploitation of genome sequence data.

Graphical abstract Genome-scale RNA interference (RNAi) library screens in bloodstream-form Trypanosoma brucei, using nifurtimox and benznidazole, revealed type I nitroreductase, NTR, knockdown. A similar screen with eflornithine, revealed amino-acid transporter, AAT6, knockdown. . Research highlights ▶ A genome-scale RNA interference library is used to identify loss-of-function resistance mechanisms in bloodstream-form Trypanosoma brucei. ▶ Screens for resistance to nifurtimox or benznidazole identified loss of nitroreductase function. ▶ A screen for resistance to eflornithine identified loss of amino-acid transporter function., To be effective, therapeutic compounds must typically enter target cells and, in some cases, must be concentrated or modified. Thus, uptake and activation mechanisms often form the basis of selectivity against infectious agents. Loss-of-function screens can be used to identify proteins involved in drug uptake and metabolism and may also identify clinically relevant potential resistance mechanisms. We used a genome-scale RNA interference (RNAi) library to identify loss-of-function resistance mechanisms in bloodstream-form Trypanosoma brucei. Nifurtimox–Eflornithine Combination Therapy (NECT) was recently introduced for Human African Trypanosomiasis and we focus on these drugs here. Screens for resistance to nifurtimox and a related drug, benznidazole, identified loss of nitroreductase (NTR) pro-drug activator function. A screen for resistance to the amino-acid analogue, eflornithine, identified loss of amino-acid transporter (AAT6) function. Our results confirm recent findings and suggest that NTR or AAT6 loss-of-function represent major potential mechanisms of resistance to these drugs. Thus, bloodstream-form T. brucei RNAi libraries present a versatile tool for selective genetic screening and for the rapid identification of drug-activation, uptake and potential resistance mechanisms.

Silent information regulator 2 (Sir2)-related proteins or sirtuins function as NAD(+)-dependent deacetylases or ADP ribosylases that target a range of substrates, thereby influencing chromatin structure and a diverse range of other biological functions. Genes encoding three Sir2-related proteins (SIR2rp1-3) have been identified in the parasitic trypanosomatids, early branching protozoa with no previously reported transcriptional silencing machinery. Here we show that, in the mammalian-infective bloodstream-stage of the African trypanosome, Trypanosoma brucei, SIR2rp1 localizes to the nucleus while SIR2rp2 and SIR2rp3 are both mitochondrial proteins. The nuclear protein, SIR2rp1, controls DNA repair and repression of RNA polymerase I-mediated expression immediately adjacent to telomeres. Antigenic variation, however, which involves the silencing and Pol I-mediated transcriptional switching of subtelomeric variant surface glycoprotein genes, continues to operate independent of SIR2rp1.

International audience; Eukaryotic chromosomes are capped with telomeres which allow complete chromosome replication and prevent the ends from being recognized by the repair machinery. The African trypanosome, Trypanosoma brucei, is a protozoan parasite where antigenic variation requires reversible silencing of a repository of telomere-adjacent variant surface glycoprotein (VSG) genes. We have investigated the role of the telomere adjacent to a repressed VSG. In cells lacking telomerase, the rate of telomere-repeat loss appeared to be inversely proportional to telomere length. We therefore constructed strains in which a single telomere could be immediately removed by conditional I-SceI meganuclease cleavage. Following telomere deletion, cells maintain and segregate the damaged chromosome without repairing it. These cells continue to proliferate at the normal rate but progressively lose terminal DNA at the broken end. Although sirtuin-dependent repression is lost along with the telomere, VSG-silencing is preserved. The results provide direct evidence for telomere-dependent repression but suggest a telomere-independent mode of VSG-silencing. They also indicate the absence of a telomere-loss checkpoint in T. brucei.

In Trypanosoma brucei, homologous recombination (HR) is important for antigenic variation and is widely exploited for genetic manipulation. Thus, parameters that influence HR are of considerable interest. We have evaluated the effect of target site transcription on HR in bloodstream-form T. brucei. Strains were created with a conditional RNA polymerase (RNAP) I promoter driving transcription of an HR-target integrated within chromosomal DNA. Gene targeting efficiency was measured in the absence and presence of tetracycline-induced target-site transcription. The results indicate that transcription stimulates HR more than three-fold. HR predominates over non-homologous end joining (NHEJ) in T. brucei and participates in the main mode of antigenic variation [1,2]. HR is also routinely and widely exploited to manipulate genes and other sequences and to integrate (conditional) expression cassettes into the genome, particularly for experiments involving RNA interference or expression of tagged proteins. A few different targets have been used for the latter approaches but non-transcribed ribosomal RNA (RRNA) spacer loci are probably the most popular. To facilitate reproducible targeting and circumvent problems with position effects, we tagged one of 15-20 spacer loci with a unique target sequence [3]. An unexpected but welcome outcome was that the tagged locus was reproducibly targeted more efficiently than un-tagged loci. One possible explanation is transcriptional stimulation of HR at the tagged target. This arises from the fact that, along with the tag, an RNAP-I promoter was integrated to drive selectable marker gene expression; RNAP-I naturally transcribes certain protein-coding genes in T. brucei including the single active Variant Surface Glycoprotein (VSG) gene [4]. Because HR predominates in T. brucei, the efficiency of stable integrative transformation has proven to be a useful measure of HR. The approach involves electroporation with a linear DNA construct containing any one of several resistance markers. Powerful antibiotic selection is then used to eliminate non-transformed cells. Transformation efficiency is low in bloodstream form cells, 5 μg of plasmid DNA [5] and is independent of target site copy number when a single transcriptionally active and several transcriptionally silent loci are compared [7]. Although target site transcription is not required for HR in T. brucei, transcription does stimulate HR in yeast [12] and human cells [13]. To examine the relationship between transcription and HR in T. brucei, we established an assay where target transcription is the only altered parameter. The assay was based on HR between a chromosomal target and a donor cassette introduced into bloodstream form cells by electroporation. Plasmid-based molecules capable of stable, episomal propagation are yet to be reported in bloodstream form cells so recombination in vivo is a pre-requisite for stable antibiotic resistance in this life-cycle stage. We established strains with HR targets downstream of a tetracycline (Tet)-inducible RRNA promoter (Fig. 1A). The cassette was integrated at a tagged RRNA spacer locus to ensure reliable, inducible transcription [3]. Selectable marker transcription was constitutive and independent of HR-target transcription at this locus. For the HR target, we chose a T. brucei sequence encoding a dispensable mitochondrial protein [14]. Selection of a T. brucei sequence meant that the native sequences (T. brucei is diploid) would serve as critical internal control targets where RNAP-II transcription should remain constant. We chose a gene encoding a dispensable protein so there would be no selection against native locus targeting and a mitochondrial protein so there was little chance of interference with nuclear recombination processes. We also fused the ectopic HR-target to a Green Fluorescent Protein (GFP) gene such that GFP was expressed in the single mitochondrion following Tet-induction. This allowed us to confirm conditional expression at the target locus (Fig. 1B) and also facilitated scoring of HR at the native versus ectopic targets; clones in which the ectopic target participates in HR can no longer express GFP (Fig. 1). The donor cassette consists of terminal recombination targets flanking a BSD selectable marker. A promoter (PPRO, another RNAP-I promoter) is an essential component of this cassette because it ensures selectable marker expression regardless of the transcriptional status of the target. Thus, although transcription apparatus may be recruited and donor sequence transcribed prior to recombination, any impact on HR should be Tet-independent. Figure 1 Transcription stimulates homologous recombination. The assay itself consisted of a series of electroporation experiments in strains with HR-target transcription either repressed or induced (Fig. 1). We measured transformation efficiency and used loss of GFP expression to measure integration at the ectopic target. Of 120 transformed clones, 83 were screened for the ability to express GFP (Table 1). The results (Fig. 1C) indicate no change in HR at the control loci (GFP-positive clones) and more than three-fold increase in HR at the transcribed locus (GFP-negative clones). Fifteen-fold and three to twenty-fold transcription-stimulated HR have been reported in yeast [12] and human cells [13] respectively. Table 1 Gene targeting data There are a number of possible explanations for increased HR within transcribed DNA. Strand-invasion and/or DNA lesions can initiate HR [15]. Transcription may increase accessibility to the HR machinery or promote strand invasion through topological changes that create a more accessible structure or lead to the generation of regions of single-stranded DNA. Transcribed DNA may also be more prone to lesions introduced by nucleases involved in transcription or DNA replication. We report a modest increase in targeting efficiency that has potential technical application. Low efficiency transformation limits the application of forward-genetic approaches but also renders reverse-genetics approaches more cumbersome. This effect has already been exploited to facilitate increased throughput in 2T1 cells. In these cells, a downstream target is constitutively transcribed by RNAP-I at a tagged RRNA locus [3]. We routinely obtain several transformed clones from a single electroporation experiment with these cells and now have an explanation for this improved output. It is also worth considering how our findings might impact on T. brucei biology. RNAP-I and II transcribed domains may be more prone to recombination at stages of the cell and life-cycle when they are actively transcribed. This may be particularly important at the single active VSG locus transcribed by RNAP-I only in bloodstream-form cells [4]. High rates of HR have been reported at VSG loci and this is known to be important for accessing the large reservoir of sub-telomeric VSGs and VSG fragments available for antigenic variation [16]. It will be necessary to directly assess the effect of transcription on VSG recombination but our results indicate that recombination involving the active VSG could be favoured.

Recently we identified multiple suramin-sensitivity genes with a genome wide screen in Trypanosoma brucei that includes the invariant surface glycoprotein ISG75, the adaptin-1 (AP-1) complex and two deubiquitylating enzymes (DUBs) orthologous to ScUbp15/HsHAUSP1 and pVHL-interacting DUB1 (type I), designated TbUsp7 and TbVdu1, respectively. Here we have examined the roles of these genes in trafficking of ISG75, which appears key to suramin uptake. We found that, while AP-1 does not influence ISG75 abundance, knockdown of TbUsp7 or TbVdu1 leads to reduced ISG75 abundance. Silencing TbVdu1 also reduced ISG65 abundance. TbVdu1 is a component of an evolutionarily conserved ubiquitylation switch and responsible for rapid receptor modulation, suggesting similar regulation of ISGs in T. brucei. Unexpectedly, TbUsp7 knockdown also blocked endocytosis. To integrate these observations we analysed the impact of TbUsp7 and TbVdu1 knockdown on the global proteome using SILAC. For TbVdu1, ISG65 and ISG75 are the only significantly modulated proteins, but for TbUsp7 a cohort of integral membrane proteins, including the acid phosphatase MBAP1, that is required for endocytosis, and additional ISG-related proteins are down-regulated. Furthermore, we find increased expression of the ESAG6/7 transferrin receptor and ESAG5, likely resulting from decreased endocytic activity. Therefore, multiple ubiquitylation pathways, with a complex interplay with trafficking pathways, control surface proteome expression in trypanosomes., Author Summary The mechanisms by which pathogens interact with their environment are of major importance, both for fulfilling the basic needs of the parasite and understanding immune evasion. For African trypanosomes, the surface is dominated by the variant surface glycoprotein (VSG), but recent data has demonstrated an important role for ubiquitylation in mediating turnover of invariant surface glycoproteins (ISGs) and maintaining ISG copy number independent of VSG. Further, ISG expression is required for suramin-sensitivity. Here we describe mechanisms mediating ISG turnover, uncovered using a screen for genes involved in sensitivity to suramin. These involve multiple aspects of the ubiquitylation machinery, and connect ISG turnover with additional surface proteins. Our data provide a first insight into the complexity of regulation of the ISG family, identifying further aspects to the control of a drug-sensitivity pathway in trypanosomes, and offering insights into metabolism of the parasite surface proteome.

Genome sequencing is now complete for a number of trypanosomatids [1] (see www.genedb.org) but functional analysis techniques that allow an increase in throughput are urgently needed if we are to fully exploit this data. RNA interference (RNAi) is a biological response to double-stranded RNA (dsRNA) that knocks down expression from complementary genes. RNAi has revolutionised research on gene function, been exploited as a high-throughput experimental tool [2] and is currently the method of choice for loss-of-function experiments in Trypanosoma brucei. We have established a simple quantitative RNAi barcode methodology that generates a read-out of relative cell number from a mixed T. brucei culture engineered for inducible RNAi. The RNAi target sequence, usually between 400 and 600 bp, is flanked by head-to-head inducible promoters and stably integrated into the genomic DNA of each cell type. Each target then serves as the template for dsRNA synthesis and as the barcode that can be amplified and labelled through multiplex competitive PCR with a single primer. The mixed barcode probe is then hybridised to specific sequences to produce the read-out. Proof-of-principle experiments demonstrate that the read-out can provide a quantitative report of relative drug resistance, relative RNAi-induced growth defects and RNAi-induced auxotrophy. Increased throughput, a range of alternative assay systems and read-out using microarray should all be compatible with the methodology.

The histones are responsible for packaging and regulating access to eukaryotic genomes. Trypanosomatids are flagellated protists that diverged early from the eukaryotic lineage and include parasites that cause disease in humans and other mammals. Here, we review the properties of histones in parasitic trypanosomatids, from gene organization and sequence to expression, post-translational modification and function within chromatin. Phylogenetic and experimental analysis indicates that certain specifically conserved histone sequence motifs, particularly within the N-terminal 'tail' domains, possibly represent functionally important modification substrates conserved throughout the eukaryotic lineage. For example, histone H3 contains a highly conserved methylation substrate. Trypanosomatids also possess at least three variant histones. Among these is an orthologue of H2A.Z, a histone involved in protecting 'active' chromatin from silencing in yeast. Histones provide docking platforms for a variety of regulatory factors. The presence of histone modification and variant histones in trypanosomatids therefore represents evidence for a network that provides the discrimination required to regulate transcription, recombination, repair and chromosome replication and segregation.

Double-strand breaks initiate coat protein switching in African trypanosomes., The DNA repair machinery has been co-opted for antigenic variation in African trypanosomes. New work directly demonstrates that a double-strand break initiates a switch in the expressed variant surface coat.