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During embryogenesis, the genome shifts from transcriptionally quiescent to extensively active in a process known as Zygotic Genome Activation (ZGA). In Drosophila, the pioneer factor Zelda is known to be essential for the progression of development; still, it regulates the activation of only a small subset of genes at ZGA. However, thousands of genes do not require Zelda, suggesting that other mechanisms exist. By conducting GRO-seq, HiC and ChIP-seq in Drosophila embryos, we demonstrate that up to 65% of zygotically activated genes are enriched for the histone variant H2A.Z. H2A.Z enrichment precedes ZGA and RNA Polymerase II loading onto chromatin. In vivo knockdown of maternally contributed Domino, a histone chaperone and ATPase, reduces H2A.Z deposition at transcription start sites, causes global downregulation of housekeeping genes at ZGA, and compromises the establishment of the 3D chromatin structure. We infer that H2A.Z is essential for the de novo establishment of transcriptional programs during ZGA via chromatin reorganization., Nature Communications, 12 (1), ISSN:2041-1723

Whether non-integrated viral DNAs distribute randomly or target specific positions within the higher-order architecture of mammalian genomes remains largely unknown. Here we use Hi-C and viral DNA capture (CHi-C) in primary human hepatocytes infected by either hepatitis B virus (HBV) or adenovirus type 5 (Ad5) virus to show that they adopt different strategies in their respective positioning at active chromatin. HBV contacts preferentially CpG islands (CGIs) enriched in Cfp1 a factor required for its transcription. These CGIs are often associated with highly expressed genes (HEG) and genes deregulated during infection. Ad5 DNA interacts preferentially with transcription start sites (TSSs) and enhancers of HEG, as well as genes upregulated during infection. These results show that DNA viruses use different strategies to infiltrate genomic 3D networks and target specific regions. This targeting may facilitate the recruitment of transcription factors necessary for their own replication and contribute to the deregulation of cellular gene expression., Whether DNA viruses contact specific regions of host genomes or make random contacts is unclear. Here, the authors use Hi-C and show that HBV cccDNA and Ad5 DNA contact preferentially active chromatin at CpG islands for the former and at transcription start sites and enhancers for the latter.

International audience; Retrotransposons, the ancestors of retroviruses, have the potential for gene disruption and genomic takeover if not kept in check. Paradoxically, although host cells repress these elements by multiple mechanisms, they are transcribed and are even activated under stress conditions. Here, we describe a new mechanism of retrotransposon regulation through transcription start site (TSS) selection by altered nucleosome occupancy. We show that Fun30 chromatin remodelers cooperate to maintain a high level of nucleosome occupancy at retrotransposon-flanking long terminal repeat (LTR) elements. This enforces the use of a downstream TSS and the production of a truncated RNA incapable of reverse transcription and retrotransposition. However, in stressed cells, nucleosome occupancy at LTR elements is reduced, and the TSS shifts to allow for productive transcription. We propose that controlled retrotransposon transcription from a nonproductive TSS allows for rapid stress-induced activation, while preventing uncontrolled transposon activity in the genome.

Pervasive transcription generates many unstable non-coding transcripts in budding yeast. The transcription of such noncoding RNAs, in particular antisense RNAs (asRNAs), has been shown in a few examples to repress the expression of the associated mRNAs. Yet, such mechanism is not known to commonly contribute to the regulation of a given class of genes. Using a mutant context that stabilised pervasive transcripts, we observed that the least expressed mRNAs during the exponential phase were associated with high levels of asRNAs. These asRNAs also overlapped their corresponding gene promoters with a much higher frequency than average. Interrupting antisense transcription of a subset of genes corresponding to quiescence-enriched mRNAs restored their expression. The underlying mechanism acts in cis and involves several chromatin modifiers. Our results convey that transcription interference represses up to 30% of the 590 least expressed genes, which includes 163 genes with quiescence-enriched mRNAs. We also found that pervasive transcripts constitute a higher fraction of the transcriptome in quiescence relative to the exponential phase, consistent with gene expression itself playing an important role to suppress pervasive transcription. Accordingly, the HIS1 asRNA, normally only present in quiescence, is expressed in exponential phase upon HIS1 mRNA transcription interruption.

Summary We investigated the relationship among ERK signaling, histone modifications, and transcription factor activity, focusing on the ERK-regulated ternary complex factor family of SRF partner proteins. In MEFs, activation of ERK by TPA stimulation induced a common pattern of H3K9acS10ph, H4K16ac, H3K27ac, H3K9acK14ac, and H3K4me3 at hundreds of transcription start site (TSS) regions and remote regulatory sites. The magnitude of the increase in histone modification correlated well with changes in transcription. H3K9acS10ph preceded the other modifications. Most induced changes were TCF dependent, but TCF-independent TSSs exhibited the same hierarchy, indicating that it reflects gene activation per se. Studies with TCF Elk-1 mutants showed that TCF-dependent ERK-induced histone modifications required Elk-1 to be phosphorylated and competent to activate transcription. Analysis of direct TCF-SRF target genes and chromatin modifiers confirmed this and showed that H3S10ph required only Elk-1 phosphorylation. Induction of histone modifications following ERK stimulation is thus directed by transcription factor activation and transcription., Graphical Abstract, Highlights • TPA-driven ERK activation induces a common pattern of histone modifications at TSSs • Most ERK-induced histone modifications are dependent on TCF family of SRF cofactors • At these TSSs, induced histone modifications require ERK-dependent TCF phosphorylation • TCF-dependent histone modifications require TCF to recruit the transcription machinery, In MEFs, TPA-induced ERK activation induces a common pattern of histone modifications at more than 2,000 TSSs. Modifications at most TSSs are dependent on the TCF family of ERK-regulated SRF cofactors. At these TSSs, induced modifications are dependent both on phosphorylation of TCF and on its ability to recruit the transcriptional machinery.

International audience; The bacterium Legionella pneumophila is found ubiquitously in aquatic environments and can cause a severe pneumonia in humans called Legionnaires' disease. How this bacterium switches from intracellular to extracellular life and adapts to different hosts and environmental conditions is only partly understood. Here we used RNA deep sequencing from exponentially (replicative) and post exponentially (virulent) grown L. pneumophila to analyze the transcriptional landscape of its entire genome. We established the complete operon map and defined 2561 primary transcriptional start sites (TSS). Interestingly, 187 of the 1805 TSS of protein-coding genes contained tandem promoters of which 93 show alternative usage dependent on the growth phase. Similarly, over 60% of 713 here identified ncRNAs are phase dependently regulated. Analysis of their conservation among the seven L. pneumophila genomes sequenced revealed many strain specific differences suggesting that L. pneumophila contains a highly dynamic pool of ncRNAs. Analysis of six ncRNAs exhibiting the same expression pattern as virulence genes showed that two, Lppnc0584 and Lppnc0405 are indeed involved in intracellular growth of L. pneumophila in A. castellanii. Furthermore, L. pneumophila encodes a small RNA named RsmX that functions together with RsmY and RsmZ in the LetA-CsrA regulatory pathway, crucial for the switch to the virulent phenotype. Together our data provide new insight into the transcriptional organization of the L. pneumophila genome, identified many new ncRNAs and will provide a framework for the understanding of virulence and adaptation properties of L. pneumophila.

International audience; BubR1 is a key protein mediating spindle checkpoint activation. Loss of this checkpoint control results in chromosomal instability and aneuploidy. The transcriptional regulation of the cell cycle regulated human BUB1B gene, which encodes BubR1, was investigated in this report. A minimal BUB1B gene promoter containing 464 bp upstream from the translation initiation codon was sufficient for cell cycle regulated promoter activity. A pivotal role for transcription factor hStaf/ZNF143 in the expression of the BUB1B gene was demonstrated through gel retardation assays, transient expression of mutant BUB1B promoter-reporter gene constructs and chromatin immunoprecipitation assay. Two phylogenetically conserved hStaf/ZNF143-binding sites (SBS) were identified which are indispensable for BUB1B promoter activity. In addition, we found that the domain covering the transcription start sites contains conserved boxes homologous to initiator (Inr), cell cycle dependent (CDE) and cell cycle genes homology regions (CHR) elements. Mutations within the CDE and CHR elements led to diminished cell cycle regulation of BUB1B transcription. These results demonstrate that BUB1B gene transcription is positively regulated by hStaf/ZNF143, a ubiquitously expressed factor, and that the CDE-CHR tandem element was essential for G2/M-specific transcription of the BUB1B gene.

Enterococcus faecalis is the third cause of nosocomial infections. To obtain the first snapshot of transcriptional organizations in this bacterium, we used a modified RNA-seq approach enabling to discriminate primary from processed 5′ RNA ends. We also validated our approach by confirming known features in Escherichia coli. We mapped 559 transcription start sites (TSSs) and 352 processing sites (PSSs) in E. faecalis. A blind motif search retrieved canonical features of SigA- and SigN-dependent promoters preceding transcription start sites mapped. We discovered 85 novel putative regulatory RNAs, small- and antisense RNAs, and 72 transcriptional antisense organizations. Presented data constitute a significant insight into bacterial RNA landscapes and a step toward the inference of regulatory processes at transcriptional and post-transcriptional levels in a comprehensive manner.

International audience; PromAn is a modular web-based tool dedicated to promoter analysis that integrates distinct complementary databases, methods and programs. PromAn provides automatic analysis of a genomic region with minimal prior knowledge of the genomic sequence. Prediction programs and experimental databases are combined to locate the transcription start site (TSS) and the promoter region within a large genomic input sequence. Transcription factor binding sites (TFBSs) can be predicted using several public databases and user-defined motifs. Also, a phylogenetic footprinting strategy, combining multiple alignment of large genomic sequences and assignment of various scores reflecting the evolutionary selection pressure, allows for evaluation and ranking of TFBS predictions. PromAn results can be displayed in an interactive graphical user interface, PromAnGUI. It integrates all of this information to highlight active promoter regions, to identify among the huge number of TFBS predictions those which are the most likely to be potentially functional and to facilitate user refined analysis. Such an integrative approach is essential in the face of a growing number of tools dedicated to promoter analysis in order to propose hypotheses to direct further experimental validations. PromAn is publicly available at http://bips.u-strasbg.fr/PromAn.

International audience; The differentiation potential of stem cells is determined by the ability of these cells to establish and maintain developmentally regulated gene expression programs that are specific to different lineages. Although transcriptionally potentiated epigenetic states of genes have been described for haematopoietic progenitors, the developmental stage at which the formation of lineage-specific gene expression domains is initiated remains unclear. In this study, we show that an intergenic cis-acting element in the mouse lambda5-VpreB1 locus is marked by histone H3 acetylation and histone H3 lysine 4 methylation at a discrete site in embryonic stem (ES) cells. The epigenetic modifications spread from this site toward the VpreB1 and lambda5 genes at later stages of B-cell development, and a large, active chromatin domain is established in pre-B cells when the genes are fully expressed. In early B-cell progenitors, the binding of haematopoietic factor PU.1 coincides with the expansion of the marked region, and the region becomes a center for the recruitment of general transcription factors and RNA polymerase II. In pre-B cells, E2A also binds to the locus, and general transcription factors are distributed across the active domain, including the gene promoters and the intergenic region. These results suggest that localized epigenetic marking is important for establishing the transcriptional competence of the lambda5 and VpreB1 genes as early as the pluripotent ES cell stage.

International audience; We have previously cloned and characterized three estrogen receptors (ER) in the zebrafish (zfERalpha, zfERbeta1 and zfERbeta2). We have also shown that they are functional in vitro and exhibit a distinct expression pattern, although partially overlapping, in the brain of zebrafish. In this paper, we have shown that the hepatic expression of these zfER genes responds differently to estradiol (E2). In fact, a 48-h direct exposure of zebrafish to E2 resulted in a strong stimulation of zfERalpha gene expression while zfERbeta1 gene expression was markedly reduced and zfERbeta2 remained virtually unchanged. To establish the potential implication of each zfER in the E2 upregulation of the zfERalpha gene, the promoter region of this gene was isolated and characterized. Transfection experiments with promoter-luciferase reporter constructs together with different zfER expression vectors were carried out in different cell contexts. The data showed that in vivo E2 upregulation of the zfERalpha gene requires ERalpha itself and a conserved transcription unit sequence including at least an imperfect estrogen-responsive element (ERE) and an AP-1/ERE half site at the proximal transcription initiation site. Interestingly, although in the presence of E2 zfERalpha was the most potent at inducing the expression of its own gene, the effect of E2 mediated by zfERbeta2 represented 50% of the zfERalpha activity. In contrast, zfERbeta1 was unable to upregulate the zfERalpha gene whereas this receptor form was able to tightly bind E2 and activate a reporter plasmid containing a consensus ERE. Altogether, these results indicated that the two ERbeta forms recently characterized in teleost fish could have partially distinct and not redundant functions.

International audience; Members of a family of ATP-dependent proteases related to Lon from Escherichia coli are present in most prokaryotes and eukaryotes. These proteases are generally reported to be heat induced, and various regulatory systems have been described. The authors cloned and disrupted the lon gene and studied the regulation of its expression in Streptomyces lividans. lon is negatively regulated by the HspR/HAIR repressor/operator system, suggesting that Lon is produced concomitantly with the other members of this regulon, DnaK and ClpB. The lon mutant grew more slowly than the wild-type and spore germination was impaired at high temperature. Nevertheless its cell cycle was not greatly affected and it sporulated normally.

International audience; GDNF is a Sertoli-cell-derived factor that controls the balance between self-renewal and differentiation of the spermatogonial stem cells. Although research in recent years has concentrated on the impact of GDNF on target germ cells rather little attention has been paid to the molecular control of GDNF expression in Sertoli cells. Here, we aimed to characterize the promoter region of the mouse gdnf gene active in Sertoli cells. We identified the transcriptional start sites and analyzed the promoter activity of the 5'-flanking regions. By in-silico analysis of evolutionarily conserved DNA sequences we identified several putative transcription factor-binding regions. Deletion analysis showed the involvement of the three CRE sites for basal and cAMP-induced expression of gdnf in murine Sertoli cells. These results provide the basis for future studies to analyze how hormonal or paracrine signals modulate the transcriptional activity of gdnf in Sertoli cells.

International audience; Deep-sequencing technologies applied to RNA have tremendous potential to identify novel transcripts with single-nucleotide resolution. By combining whole-transcript cDNA sequencing (RNA-seq) and genome-wide identification of transcription start sites (dRNA-seq), it is possible to characterize long 5'-untranslated regions potentially endowed with regulatory capacities and to detect premature termination of transcription. This can be used to identify new potential riboswitches. In this chapter, we provide a detailed protocol of the dRNA-seq method based on differential pretreatment of RNAs with tobacco acid pyrophosphatase to differentiate between 5'-ends of primary and processed RNAs. We also give a briefer protocol of the preparation of RNA-seq libraries and of how to go through data bioinformatics analysis and data visualization using genome browsers. This approach is powerful to identify novel riboswitches and to demonstrate the functionality of riboswitches predicted in silico.

International audience; Clostridium difficile intestinal disease is mediated largely by the actions of toxins A (TcdA) and B (TcdB), whose production occurs after the initial steps of colonization involving different surface or flagellar proteins. In B. subtilis, the sigma factor SigD controls flagellar synthesis, motility, and vegetative autolysins. A homolog of SigD encoding gene is present in the C.difficile 630 genome. We constructed a sigD mutant in C. difficile 630 ∆erm to analyze the regulon of SigD using a global transcriptomic approach. A total of 103 genes were differentially expressed between the wild-type and the sigD mutant, including genes involved in motility, metabolism and regulation. In addition, the sigD mutant displayed decreased expression of genes involved in flagellar biosynthesis, and also of genes encoding TcdA and TcdB as well as TcdR, the positive regulator of the toxins. Genomic analysis and RACE-PCR experiments allowed us to characterize promoter sequences of direct target genes of SigD including tcdR and to identify the SigD consensus. We then established that SigD positively regulates toxin expression via direct control of tcdR transcription. Interestingly, the overexpression of FlgM, a putative anti-SigD factor, inhibited the positive regulation of motility and toxin synthesis by SigD. Thus, SigD appears to be the first positive regulator of the toxin synthesis in C. difficile.

Bacterial Subversion Tactics Intracellular bacterial pathogens such as Listeria monocytogenes can change host cell transcription programs to promote infection. Eskandarian et al. ( 1238858 ) found that during infection, the Listeria effector protein InlB promoted the movement of a host protein deacetylase, SIRT2, from its normal location in the cytosol to the nucleus. In the nucleus, SIRT2 helped to repress a number of host cell genes by deacetylating one of their associated histones. In mice, reduced levels of SIRT2 impaired bacterial infection.

test; International audience; The SOX8 and SOX9 transcription factors are involved in, among others, sex differentiation, male gonad development and adult maintenance of spermatogenesis. Sox8(-/-) mice lacking Sox9 in Sertoli cells fail to form testis cords and cannot establish spermatogenesis. Although genetic and histological data show an important role for these transcription factors in regulating spermatogenesis, it is not clear which genes depend upon them at a genome-wide level. To identify transcripts that respond to the absence of Sox8 in all cells and Sox9 in Sertoli cells we measured mRNA concentrations in testicular samples from mice at 0, 6 and 18 days post-partum. In total, 621 and 629 transcripts were found at decreased or increased levels, respectively, at different time points in the mutant as compared to the control samples. These mRNAs were categorized as preferentially expressed in Sertoli cells or germ cells using data obtained with male and female gonad samples and enriched testicular cell populations. Five candidate genes were validated at the protein level. Furthermore, we identified putative direct SOX8 and SOX9 target genes by integrating predicted SOX-binding sites present in potential regulatory regions upstream of the transcription start site. Finally, we used protein network data to gain insight into the effects on regulatory interactions that occur when Sox8 and Sox9 are absent in developing Sertoli cells. The integration of testicular samples with enriched Sertoli cells, germ cells and female gonads enabled us to broadly distinguish transcripts directly affected in Sertoli cells from others that respond to secondary events in testicular cell types. Thus, combined RNA profiling signals, motif predictions and network data identified putative SOX8/SOX9 target genes in Sertoli cells and yielded insight into regulatory interactions that depend upon these transcription factors. In addition, our results will facilitate the interpretation of genome-wide in vivo SOX8 and SOX9 DNA binding data.

International audience; The adaptation of Legionella pneumophila to the different conditions it encounters in the environment and in the host is governed by a complex regulatory system. Current knowledge of these regulatory networks and the transcriptome responses of L. pneumophila is mainly based on microarray analysis and limited to transcriptional products of annotated protein-coding genes. The application of the Next-Generation Sequencing (NGS) technology allows now genome-wide strand-specific sequencing and accurate determination of all expressed regions of the genome to reveal the complete transcriptional network and the dynamic interplay of specific regulators on a genome-wide level. NGS-based techniques promote deeper understanding of the global transcriptional organization of L. pneumophila by identifying transcription start sites (TSS), alternative TSS and operon organization, noncoding RNAs, antisense RNAs, and 5'-/3'-untranslated regions. In this chapter we describe the construction of cDNA libraries for (1) RNA deep sequencing (RNA-seq) and (2) TSS mapping using the Illumina technology.

International audience; Bacteria use biofilm structures to colonize surfaces and to survive in hostile conditions, and numerous bacteria produce cellulose as a biofilm matrix polymer. Hence, expression of the bcs operon, responsible for cellulose biosynthesis, must be finely regulated in order to allow bacteria to adopt the proper surface-associated behaviours. Here we show that in the phytopathogenic bacterium, Dickeya dadantii, production of cellulose is required for pellicle-biofilm formation and resistance to chlorine treatments. Expression of the bcs operon is growth phase-regulated and is stimulated in biofilms. Furthermore, we unexpectedly found that the nucleoid-associated protein and global regulator of virulence functions, Fis, directly represses bcs operon expression by interacting with an operator that is absent from the bcs operon of animal pathogenic bacteria and the plant pathogenic bacterium Pectobacterium. Moreover, production of cellulose enhances plant surface colonization by D. dadantii. Overall, these data suggest that cellulose production and biofilm formation may be important factors for surface colonization by D. dadantii and its subsequent survival in hostile environments. This report also presents a new example of how bacteria can modulate the action of a global regulator to co-ordinate basic metabolism, virulence and modifications of lifestyle.

Several Bacillus strains secrete phytase, an enzyme catalyzing dephosphorylation of myo -inositol hexakisphosphate (phytate). We identified the phyC (phytase) gene from environmental Bacillus amyloliquefaciens FZB45 as a member of the phosphate starvation-inducible PhoPR regulon. In vivo and in vitro assays revealed that PhoP∼P is essential for phy C transcription. The transcriptional start site was identified downstream of a σ A -like promoter region located 27 bp upstream of the probable translation ATG start codon. Inspection of the phyC promoter sequence revealed an unusual structure. The− 35 and −10 regions are separated by a window of 21 bp. A pair of tandemly repeated PhoP TT(T/A/C)ACA binding boxes was located within and upstream of the −35 consensus promoter region. A single PhoP box was found within the −10 consensus promoter region. DNase I footprinting experiments performed with isolated PhoP confirmed that PhoP∼P binds at two sites overlapping with the phy C −35 and −10 consensus promoter region. While binding of dimeric PhoP∼P at −35 is essential for activation of the phyC promoter, binding of PhoP∼P at− 10 suppresses promoter activity. A sixfold enhancement of phyC gene expression was registered after T:G substitution of nucleotide −13 (mutant MUT13), which eliminates PhoP binding at the single PhoP box without impairing the −10 consensus sequence. Moreover, MUT13 also expressed phyC during phosphate-replete growth, suggesting that the repressing effect due to binding of PhoP∼P at −10 was abolished. A model is presented in which transcription initiation of phyC is positively and negatively affected by the actual concentration of the PhoP∼P response regulator.

International audience; The vertebrate hindbrain is subject to a transient segmentation process leading to the formation of seven or eight metameric territories termed rhombomeres (r). This segmentation provides the basis for the subsequent establishment of hindbrain neuronal organization and participates in the patterning of the neural crest involved in craniofacial development. The zinc-finger gene Krox20 is expressed in r3 and r5, and encodes a transcription factor that plays a key role in hindbrain segmentation, coordinating segment formation, specification of odd- and even-numbered rhombomeres, and cell segregation between adjacent segments, through the regulation of numerous downstream genes. In order to further elucidate the genetic network underlying hindbrain segmentation, we have undertaken the analysis of the cis-regulatory sequences governing Krox20 expression. We have found that the control of Krox20 transcription relies on three very long-range (200 kb) enhancer elements (A, B and C) that are conserved between chick, mouse and human genomes. Elements B and C are activated at the earliest stage of Krox20 expression in r5 and r3-r5, respectively, and do not require the Krox20 protein. These elements are likely to function as initiators of Krox20 expression. Element B contains a binding site for the transcription factor vHNF1, the mutation of which abolishes its activity, suggesting that vHNF1 is a direct initiator of Krox20 expression in r5. Element A contains Krox20-binding sites, which are required, together with the Krox20 protein, for its activity. This element therefore allows the establishment of a direct positive autoregulatory loop, which takes the relay of the initiator elements and maintains Krox20 expression. Together, our studies provide a basis for a model of the molecular mechanisms controlling Krox20 expression in the developing hindbrain and neural crest.

International audience; We report here the complete nucleotide sequence of the 23.5-kb mitochondrial genome from the yeast Kluyveromyces thermotolerans. It encodes, all on the same DNA strand, three subunits of cytochrome oxidase (COX1, COX2 and COX3), three subunits of ATP synthetase (ATP6, ATP8 and ATP9), the apocytochrome b (COB), the ribosomal protein VAR1, 24 tRNAs, the small and large ribosomal RNAs, and the RNA subunit of RNase P. Three intronic ORFs are present within the COX1 gene group I introns. The K. thermotolerans mitochondrial genome is very similar to the Candida glabrata mitochondrial genome, as judged from clusters of gene order, gene transcription units and sequence similarities. Interestingly, the predicted secondary structure of the abnormal tRNAThr1 contains 10 nucleotides in its anticodon loop. This sequence is available under EMBL Accession No. AJ634268.

International audience; Eukaryotic genomes are organised into independent domains through the establishment of boundaries which allow to have distinct pattern of gene expression both during development and in differentiated cells. The previously reported site independent expression of the mammary-specific goat alpha-lactalbumin gene in transgenic mice suggested the existence of cis-regulatory elements located upstream of this gene. The nearby presence of a second ubiquitously expressed gene (the cyclin T1 gene) allowed to define two chromatin domains putatively separated by a boundary insulator-like element. In this study, the characterisation of a third putative gene (FLJ20436) present between the alpha-lactalbumin and the cyclin T1 loci is reported. It was found to be a functional gene, ubiquitously expressed both in goat and transgenic mice. A complex pattern of alternative splicing events was observed in several analysed tissues leading to various mRNAs and putative FLJ20436 proteins, as suggested in human and mouse species. This allowed us to assign this gene to one of the two hypothesised chromatin domains, refining the location of the potential insulator element.