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ABSTRACTThe identification of FACT as a histone chaperone enabling transcription through chromatin in vitro has strongly shaped how its roles are envisioned. However, FACT has been implicated in essentially all aspects of chromatin biology, from transcription to DNA replication, DNA repair, and chromosome segregation. In this review, we focus on recent literature describing the role and mechanisms of FACT during transcription. We highlight the prime importance of FACT in preserving chromatin integrity during transcription and challenge its role as an elongation factor. We also review evidence for FACT’s role as a cell-type/gene-specific regulator of gene expression and briefly summarize current efforts at using FACT inhibition as an anti-cancer strategy.

ABSTRACTProper modulation of promoter chromatin architecture is crucial for gene regulation in order to precisely and efficiently orchestrate various cellular activities. Previous studies have identified the stimulatory effect of the histone-modifying complex NuA4 on the incorporation of the histone variant H2A.Z (Htz1) at the PHO5promoter (A. Auger, L. Galarneau, M. Altaf, A. Nourani, Y. Doyon, R. T. Utley, D. Cronier, S. Allard, and J. Côté, Mol Cell Biol 28:2257–2270, 2008, http://dx.doi.org/10.1128/MCB.01755-07). In vitrostudies with a reconstituted system also indicated an intriguing cross talk between NuA4 and the H2A.Z-loading complex, SWR-C (M. Altaf, A. Auger, J. Monnet-Saksouk, J. Brodeur, S. Piquet, M. Cramet, N. Bouchard, N. Lacoste, R. T. Utley, L. Gaudreau, J. Côté, J Biol Chem 285:15966–15977, 2010, http://dx.doi.org/10.1074/jbc.M110.117069). In this work, we investigated the role of the NuA4 scaffold subunit Eaf1 in global gene expression and genome-wide incorporation of Htz1. We found that loss of Eaf1 affects Htz1 levels mostly at the promoters that are normally highly enriched in the histone variant. Analysis of eaf1mutant cells by expression array unveiled a relationship between NuA4 and the gene network implicated in the purine biosynthesis pathway, as EAF1deletion cripples induction of several ADEgenes. NuA4 directly interacts with Bas1 activation domain, a key transcription factor of adenine genes. Chromatin immunoprecipitation (ChIP) experiments demonstrate that nucleosomes on the inactive ADE17promoter are acetylated already by NuA4 and enriched in Htz1. Upon derepression, these poised nucleosomes respond rapidly to activate ADEgene expression in a mechanism likely reminiscent of the PHO5promoter, leading to nucleosome disassembly. These detailed molecular events depict a specific case of cross talk between NuA4-dependent acetylation and incorporation of histone variant Htz1, presetting the chromatin structure over ADEpromoters for subsequent chromatin remodeling and activated transcription.

•In a plasma, the selection of isotopes by specific chemical reactions does not obey any more the differences of zero-point energy between reactants and products.

Heterometallic d10–d10nano systems of dicyanocuprate(I) with dicyanoaurate(I) or dicyanoargentate(I) doped in different alkali halide hosts (NaCl, NaBr, and KBr) have been studied. In addition to the characteristic emission peaks for each d10anion cluster, these doped crystals show a new luminescent peak due to the heterometallic interaction between Cu and Au or Cu and Ag. This new long lifetime emission peak occurs at much lower energy than that of single metal doped cases. The relative intensity of these emission peaks depends on the stoichiometric ratios of the corresponding metals doped in alkali halide hosts. Unlike the emission peaks originating from single Cu(I), Ag(I), and Au(I) doped systems, this emission peak from heterometallic interaction is independent of the alkali halide hosts. A spin-forbidden metal–metal-to-ligand charge-transfer transition is proposed, and the Cu–Au emission peak occurs at lower energy than that of Cu–Ag, which is due to the relativistic effect of Au. Density funcational theory and atomistic calculations have been carried out to correlate the theoretical and experimental results.

Anionic dicyanocuprate(I) ions in nano and bulky crystals have been investigated. The doped nanosystems show unusual photophysical properties such as multiple emission bands, thermochromism, and optical memory. In contrast, NaCu(CN)2and KCu(CN)2have different polymeric structures and exhibit only a single emission peak. This was confirmed by infrared, Raman, and reflectance spectroscopy results. The emission peaks of these doped systems show tunability over a wide wavelength interval, indicating the existence of different {[Cu(CN)2]}nnanoclusters. The thermochromism and optical memory phenomena are due to the population changes of these different-sized nanoclusters. Ab initio DFT calculations for the impurity copper(I) ion in different alkali halides show that the {[Cu(CN)2]}nnanoclusters are examples of excimers and exciplexes. Atomistic modeling describes how the distribution of different nanoclusters varies with the alkali halide hosts.

Luminescence spectra for dithiocarbamate complexes of gold(I) and silver(I) display very different luminescence properties. At room temperature, a narrow band with a maximum at approximately 18 000 cm–1and a broad band with a maximum at approximately 13 000 cm–1are observed for the gold(I) and silver(I) compounds, respectively. The luminescence from the gold(I) compounds is strongly affected by external pressure, with maxima shifting to lower energy by approximately −120 cm–1/kbar. In contrast, a silver compound shows a small shift to higher energy by approximately +20 cm–1/kbar. These shifts are analyzed in terms of structure and bridging ligand geometry, revealing the important influence of unsupported metal–metal interactions on pressure-induced shifts of luminescence bands.

Luminescent nanoclusters of linear dicyanoargentate(I) ions doped in different alkali halide single crystals (NaF, NaCl, NaBr, and KCl) have been investigated. Site-selective excitation of the different nanoclusters results in the appearance of emission bands which are assigned to different luminescent {[Ag(CN)2]−}nnanoclusters in the host lattices. The nanoclusters of [Ag(CN)2]−in the alkali halides display energy tunability over a wide wavelength interval. The ab initio DFT calculations show that for nanoclusters, the first excited state has a deeper potential and shorter internuclear separation than the nanocluster ground state, indicative of excimer/exciplex behavior. Molecular modeling indicates how the distribution of nanoclusters varies with the host lattice. This emission energy tunability makes these systems attractive candidates for potential applications, such as tunable solid state lasers, photocatalysts, and photosensitizers for water splitting.

Despite the fact that it has been intensively studied during the last decade, the function of the histone variant H2A.Z remains enigmatic. In the last few years, we and others have determined the localization of H2A.Z in various organisms. These studies have revealed that H2A.Z occupies different well defined regions in the genome. Interestingly, H2A.Z occupies the promoters and regulatory regions of active genes, as well as constitutive and facultative heterochromatin. The localization of H2A.Z on genomic regions with so diverse functions suggests that the roles of H2A.Z are multiple, only increasing the mystery around this molecule. The way H2A.Z finds its way into these regions is not fully understood but mechanisms that direct the specific incorporation of H2A.Z in promoters and enhancers have been well described. In this “point of view,” we will review our recent work suggesting that non-targeted incorporation, coupled to targeted depletion of H2A.Z, is a novel mechanism for localizing H2A.Z to some regions. We propose that the various functions of H2A.Z may be a consequence of the mechanism used for its incorporation, as well as its interactions with other histone variants.

A supramolecular engineering approach has been developed for a novel family of Nsalicylidene aniline derivatives to control their thermo and photochromic behaviours. Hsaltrz, Habs, Hsalphen, Hsaltz and Ksaltz are versatile molecules built from Nheterocycles, which drive the molecular arrangement to form a controlled crystal packing with predesigned optical properties. A complete structural, optical and computational study of powders of these new molecular nanoswitches is presented. An Nsalicylidene aniline derivative possessing no thermo or photoinduced chromic properties thanks to a specific molecular geometry was sought, and Habs and Hsalphen were designed to enhance π–π stacking interactions. A theoretical study of the crystal packing, combined with timedependent diffuse reflectance studies of Habs, have confirmed the appearance of photochromism at room temperature completed with an unprecedented “springtype effect” observed during the photochemical relaxation of the metastable transketo form. The absence of thermochromism in an Nsalicylidene aniline derivative, induced by the absence of cisketo form, is a unique behaviour, firstly identified and explained for these types of compounds. Finally, Hsaltz and Ksaltz are the result of a molecular derivation of Hsaltrz which allow enhancing the amplitude of thermochromism of these functional materials.

All light and no heat?Structure–property relationships for photochromic and thermochromic N‐salicylideneanils have been revised (see picture). One derivative has a packed crystal structure, is photochromic and exhibits an infinitely slow thermal back relaxation of technological interest. Complexation of the anils to transition metals has also been studied.

Abstract—Kainsaz CO3 insoluble organic matter (IOM) was studied using Curie point pyrolysis, electronic paramagnetic resonance (EPR), and high‐resolution transmission electron microscopy (HRTEM) to determine the effect of thermal metamorphism on molecular chondritic fingerprints. Pyrolysis released a very low amount of products that consist of one‐ and two‐ring aromatic units with methyl, dimethyl, and ethyl substituents. Moreover, Kainsaz IOM contains two orders of magnitude fewer radicals than Orgueil, Murchison, and Tagish Lake IOM. In addition, no diradicaloids were found in Kainsaz, although they are thought to constitute a specific signature for weakly organized extraterrestrial organic compounds in contrast to terrestrial ones. HRTEM reveals a very heterogeneous structure, with microporous disordered carbon, mesoporous graphitic carbons and graphite. Graphitization likely occurs and explains the differences between Kainsaz and CI or CM IOM. Heating stress experienced by Kainsaz IOM, on the parent body and/or prior its accretion, is likely responsible for the differences in molecular and structural organizations compared with those of CI and CM IOM.

Upc2p, a transcription factor of the zinc cluster family, is an important regulator of sterol biosynthesis and azole drug resistance in Candida albicans. To better understand Upc2p function in C. albicans, we used genomewide location profiling to identify the transcriptional targets of Upc2p in vivo. A triple hemagglutinin epitope, introduced at the C terminus of Upc2p, conferred a gain-of-function effect on the fusion protein. Location profiling identified 202 bound promoters (P < 0.05). Overrepresented functional groups of genes whose promoters were bound by Upc2p included 12 genes involved in ergosterol biosynthesis (NCP1, ERG11, ERG2, and others), 18 genes encoding ribosomal subunits (RPS30, RPL32, RPL12, and others), 3 genes encoding drug transporters (CDR1, MDR1, and YOR1), 4 genes encoding transcription factors (INO2, ACE2, SUT1, and UPC2), and 6 genes involved in sulfur amino acid metabolism (MET6, SAM2, SAH1, and others). Bioinformatic analyses suggested that Upc2p binds to the DNA motif 5'-VNCGBDTR that includes the previously characterized Upc2p binding site 5'-TCGTATA. Northern blot analysis showed that increased binding correlates with increased expression for the analyzed Upc2p targets (ERG11, MDR1, CDR1, YOR1, SUT1, SMF12, and CBP1). The analysis of ERG11, MDR1, and CDR1 transcripts in wild-type and upc2/upc2 strains grown under Upc2p-activating conditions (lovastatin treatment and hypoxia) showed that Upc2p regulates its targets in a complex manner, acting as an activator or as a repressor depending upon the target and the activating condition. Taken together, our results indicate that Upc2p is a key regulator of ergosterol metabolism. They also suggest that Upc2p may contribute to azole resistance by regulating the expression of drug efflux pump-encoding genes in addition to ergosterol biosynthesis genes.

A NanoSIMS ion microprobe was used to map the submicron-scale distributions of carbon, nitrogen, sulfur, silicon, and oxygen in organic microfossils and laminae in a thin section of the ˜0.85 billion year old Bitter Springs Formation of Australia. The data provide clues about the original chemistry of the microfossils, the silicification process, and the biosignatures of specific microorganisms and microbial communities. Chemical maps of fossil unicells and filaments revealed distinct wall- and sheath-like structures enriched in C, N, and S, consistent with their accepted biological origin. Surprisingly, organic laminae, previously considered to be amorphous, also exhibited filamentous and apparently compressed spheroidal structures defined by strong enrichments in C, N, and S. By analogy to NanoSIMS data from the well-preserved microfossils, these structures were interpreted as being of biological origin, most likely representing densely packed remnants of microbial mats. Given that the preponderance of organic matter in Precambrian sediments is similarly "amorphous," our findings indicate that a re-evaluation of ancient specimens via in situ structural, chemical, and isotopic study is warranted. Our analyses have led us to propose new criteria for assessing the biogenicity of problematic kerogenous materials, and, thus, these criteria can be applied to assessments of poorly preserved or fragmentary organic residues in early Archean sediments and any that might occur in meteorites or other extraterrestrial samples.

Abstract—Interactions between nebular gas and molten silicates or oxides could have played a major role in the formation and differentiation of the first solids formed in the solar system. In order to simulate such interactions, we set up a new experimental device in which isothermal condensation experiments have been conducted. Partially molten chondrule‐like samples have been exposed to high SiO(g) partial pressures, for intervals between 80 and 300 s and at temperatures ranging from 1600 to 1750 K. Results show that silica entering from the gas phase could be responsible for several textural and mineralogical features observed in natural chondrules. For instance, these experiments reproduce not only the mineralogical zonation of porphyritic olivine‐pyroxene chondrules with the peripheral location of pyroxenes, but also olivine resorption textures and the common poikilitical enclosure of olivines in pyroxenes. In the light of these similarities, we advocate that gas‐melt interactions through condensation are viable mechanisms for chondrule formation and hence may place severe constraints on the history of these primitive objects. In the nebula, high SiO(g) partial pressures could have been established by the volatilization of regions with high dust/gas ratio. A possible scenario for this stochastic thermal activity is the intense activity of the protosun in its young stellar object phase.

Genome-wide location analysis indicates that the yeast nucleosome-remodeling complex RSC has approximately 700 physiological targets and that the Rsc1 and Rsc2 isoforms of the complex behave indistinguishably. RSC is associated with numerous tRNA promoters, suggesting that the complex is recruited by the RNA polymerase III transcription machinery. At RNA polymerase II promoters, RSC specifically targets several gene classes, including histones, small nucleolar RNAs, the nitrogen discrimination pathway, nonfermentative carbohydrate metabolism, and mitochondrial function. At the histone HTA1/HTB1 promoter, RSC recruitment requires the Hir1 and Hir2 corepressors, and it is associated with transcriptional inactivity. In contrast, RSC binds to promoters involved in carbohydrate metabolism in response to transcriptional activation, but prior to association of the Pol II machinery. Therefore, the RSC complex is generally recruited to Pol III promoters and it is specifically recruited to Pol II promoters by transcriptional activators and repressors.

Transcription and splicing are coordinated processes in mammalian cells. We have used affinity chromatography with immobilized transcription elongation factor SII to purify a protein complex that contains core RNA polymerase II (RNA Pol II), the general transcription initiation factors, and several splicing factors, including the U1, U2, and U4 small nuclear RNPs, the U2AF65, and serine/arginine-rich proteins. The splicing factors and the transcription machinery co-purify through a gel filtration column and co-immunoprecipitate in experiments using an anti-U2AF65antibody, indicating that they are part of a unique complex. Although the RNA Pol II-containing complex does not possess splicing activity, it can complement small nuclear RNP-inactivated extracts and can promote the formation of a pre-spliceosome complex. Because interactions between components of the splicing and transcription machineries occur in the context of a complex containing a hypophosphorylated RNA Pol II capable of initiating transcription, our results suggest that the coupling between transcription and splicing begins before transcription initiation.

Ion microprobe quantitative imaging was performed for H, D, 12,13C, 16,18O, 27Al, and 28,29,30Si in five stratospheric particles with a 1.5 × 1.5 μm spatial resolution to determine the carriers of high D/H ratios and to give new clues about the parent bodies of interplanetary dust particles (IDPs). Among these particles, four appear to be of extraterrestrial origin. Using imaging, the large variations of D/H ratios can be correlated at the micrometer scale with chemical composition so that endmembers can be identified. From the systematics of variation of C/H and D/H ratios, water present as hydroxyls in phyllosilicates (at C/H = 0) and three different types of organic matter (OM1, 2, and 3) were identified. Water exhibits D/H ratios lying in the chondritic domain (D/H ∼150 × 10−6), whereas OMs are enriched in deuterium. OM1 is similar to the macromolecular organic matter of carbonaceous chondrites (D/H = 250 × 10−6and C/H = 1.5). OM2 (D/H = 1500 × 10−6and C/H = 1.0) is close to cometary HCN. OM3 (D/H = 2000 × 10−6and C/H = 3.0) is a highly condensed carbonaceous phase with no counterpart in known extraterrestrial objects. The C, O, and Si isotopic compositions are solar within ± 10%.

ABSTRACTEvolutionarily conserved variant histone H2A.Z has been recently shown to regulate gene transcription in Saccharomyces cerevisiae. Here we show that loss of H2A.Z in this organism negatively affects the induction of GALgenes. Importantly, fusion of the H2A.Z C-terminal region to S phase H2A without its corresponding C-terminal region can mediate the variant histone's specialized function in GAL1-10gene induction, and it restores the slow-growth phenotype of cells with a deletion of HTZ1. Furthermore, we show that the C-terminal region of H2A.Z can interact with some components of the transcriptional apparatus. In cells lacking H2A.Z, recruitment of RNA polymerase II and TATA-binding protein to the GAL1-10promoters is significantly diminished under inducing conditions. Unexpectedly, we also find that H2A.Z is required to globally maintain chromatin integrity under GALgene-inducing conditions. We hypothesize that H2A.Z can positively regulate gene transcription, at least in part, by modulating interactions with RNA polymerase II-associated factors at certain genes under specific cell growth conditions.

Abstract. A distinct group of gold deposits in the Abitibi greenstone belt is spatially associated with quartz-monzonite to syenite stocks and dikes. The deposits occur mainly along major fault zones, in association with preserved slivers of alluvial-fluvial, Timiskaming-type, sedimentary rocks. The deposits consist of disseminated sulfide replacement zones with variably developed stockworks of quartz-carbonate±K-feldspar veinlets, within zones of carbonate, albite, K-feldspar, and sericite alteration. The syenitic intrusions are broadly contemporaneous with deposition of Timiskaming sedimentary rocks and, together with disseminated gold mineralization, they have been overprinted by subsequent regional folding and related penetrative cleavage. Disseminated gold orebodies occur within composite syenitic stocks or along their margins, along satellite dikes and sills, and along faults and lithologic contacts away from intrusions. Orebodies in these different positions are interpreted to represent proximal to distal components of large magmatic-hydrothermal systems centered on, and possibly genetically related to, composite syenitic stocks.

Evolutionarily conserved variant histone H2A.Z has been recently shown to regulate gene transcription in Saccharomyces cerevisiae. Here we show that loss of H2A.Z in this organism negatively affects the induction of GALgenes. Importantly, fusion of the H2A.Z C-terminal region to S phase H2A without its corresponding C-terminal region can mediate the variant histone's specialized function in GAL1-10gene induction, and it restores the slow-growth phenotype of cells with a deletion of HTZ1. Furthermore, we show that the C-terminal region of H2A.Z can interact with some components of the transcriptional apparatus. In cells lacking H2A.Z, recruitment of RNA polymerase II and TATA-binding protein to the GAL1-10promoters is significantly diminished under inducing conditions. Unexpectedly, we also find that H2A.Z is required to globally maintain chromatin integrity under GALgene-inducing conditions. We hypothesize that H2A.Z can positively regulate gene transcription, at least in part, by modulating interactions with RNA polymerase II-associated factors at certain genes under specific cell growth conditions.

Solid state CP/MAS 13C nuclear magnetic resonance (NMR) study of the insoluble organic fraction isolated by HF/HCl treatment under an inert atmosphere from the Orgueil and Murchison carbonaceous meteorites is reported. Based on these spectra, eight different types of carbons (aro- and ali-linked CH3, CH2, aliphatic C-linked to heteroelements, protonated and non-protonated aromatic C, carboxyls and carbonyls) were identified, their relative amounts were determined and theoretical NMR-derived H/C atomic ratios were calculated. Comparison of these H/C ratios with those obtained from elemental analysis revealed that a large part of the carbons of the insoluble fraction of both meteorites are not detected by CP/MAS 13C NMR. Taking into account these undetected carbons, the relative abundance of aromatic carbons (as % of the total carbons) was calculated as between 69 and 78% in Orgueil and 61 and 67% in Murchison and thus shown to be much higher than previously thought. In addition, the NMR data allowed to calculate the ratio of non-protonated and protonated aromatic carbons. Based on previous studies on chemical and thermal degradation, the present NMR data were interpreted as reflecting the occurrence of highly substituted rather small aromatic moieties in these insoluble organic materials. Along with FTIR data, NMR results also pointed to a relatively high branching level in the aliphatic chains, especially for Murchison. According to the above results, the insoluble organic matter in these meteorites did not suffer a marked thermal maturation during the late hydrothermalism of the parent body. Its synthesis involved a statistical combination of all the possible bonds between -CH3, -CH2and -CH radicals, producing both the aliphatic and the aromatic moieties. This model explains not only the chemical features of the chondritic insoluble organic material but also the typical FTIR signature of the interstellar medium. Therefore, organic material found in the interstellar medium and that incorporated in the most primitive objects of the solar system seem to share a common organosynthesis.

The measured D/H ratios in interstellar environments and in the solar system are reviewed. The two extreme D/H ratios in solar system water - (720±120)×10−6in clay minerals and (88±11)×10−6in chondrules, both from LL3 chondritic meteorites - are interpreted as the result of a progressive isotopic exchange in the solar nebula between deuterium-rich interstellar water and protosolar H2. According to a turbulent model describing the evolution of the nebula (Drouart et al., 1999), water in the solar system cannot be a product of thermal (neutral) reactions occurring in the solar nebula. Taking 720×10−6as a face value for the isotopic composition of the interstellar water that predates the formation of the solar nebula, numerical simulations show that the water D/H ratio decreases via an isotopic exchange with H2. During the course of this process, a D/H gradient was established in the nebula. This gradient was smoothed with time and the isotopic homogenization of the solar nebula was completed in 106years, reaching a D/H ratio of 88×10−6. In this model, cometary water should have also suffered a partial isotopic re-equilibration with H2. The isotopic heterogeneity observed in chondrites result from the turbulent mixing of grains, condensed at different epochs and locations in the solar nebula. Recent isotopic determinations of water ice in cold interstellar clouds are in agreement with these chondritic data and their interpretation (Texeira et al., 1999).

Abstract—The distribution of Li, Be, and B was studied by ion microprobe mass spectrometry in 36 chondrules from the Semarkona, Bishunpur, Allende, Clovis #1, and Hedjaz meteorites. Within a single chondrule, Li‐Be‐B concentrations can vary up to one order of magnitude. For example, in a chondrule from Hedjaz, concentrations range from 0.3 to 2.4 ppm for Li, from <0.001 to 0.17 ppm for Be, and from 0.4 to 5.5 ppm for B. Among chondrules from Semarkona and Bishunpur, clear crystal‐mesostasis partitioning was observed in nine chondrules for Li, in nine chondrules for Be, and in three chondrules for B. The heterogeneities in the distribution of Li, Be, and B in chondrules from Semarkona and Bishunpur appear to be primary features that were inherited from the chondrules' precursors and not totally obscured during the chondrules' formation. A redistribution of B was nevertheless observed at the whole‐rock scale for Allende (B‐Al2O3correlation) and Hedjaz (B–SiO2correlation).

Abstract—The D/H ratios and water contents were measured by ion microprobe analysis in 52 individual Antarctic micrometeorites (AMMs) and 10 Antarctic cosmic spherules (ACSs) containing nuggets of iron hydroxide (COPS phase). In AMMs, δD values vary from −366 to +249%‰ and water contents lie between 0.4‐3.7 wt%. The COPS nuggets in cosmic spherules have high water contents (2 to 8 wt%) and exhibit δD values from −144 to +167%‰, which is indicative of an extraterrestrial origin of their constituent water. The silicate portion of ACSs also contain extraterrestrial H equivalent to ∼0.l to 1.2 wt% water. Deuterium‐exchange experiments were performed with isotopically spiked water. These experiments demonstrate that water in mineral phases of AMMs and ACSs is indigenous and does not result from contamination during residence in Antarctic ice. The frequency distribution of D/H ratios in AMMs allows us to further narrow the relationship between AMMs and carbonaceous chondrites to CM and CI chondrites but contrasts with that of stratospheric interplanetary dust particles (IDPs) of similar sizes (from −10 to 50 μm). The relatively narrow range of D/H ratios measured in AMMs as well as in ACSs (which are more resistant and thus less susceptible to collection biases) suggests that D‐rich IDP‐like particles are very rare in our AMMs collections. This indicates that these D‐rich grains might constitute a minor fraction of the micrometeorite flux in the interplanetary medium and that possible collection biases in Antarctica would not be responsible for their strong depletion in the AMMs collections.

An analytical procedure has been developed to measure in situ the 11B/10B ratio in terrestrial basaltic rocks and meteoritic chondrules having B concentration of less than 1 μg g−1using a small radius ims3f ion microprobe. The central difficulties for these measurements are (i) the removal of the trace amount of B contamination introduced in the sample during polishing, (ii) the precise calibration of instrumental mass fractionation of B isotopes and (iii) the low count rates of 10B and 11B. Contamination experiments conducted with isotopically labelled B enriched in 10B showed that ultrasonic cleaning in bi‐distilled water (< 1 ng g−1B) and pre‐sputtering of the analysed area decrease B contamination to the level of 0.01 μg g−1. Analyses of isotope standards spanning a range of 11B/10B between 3.93 and 4.20 showed that instrumental mass fractionation was constant within? during one session of analyses. Repeated analyses of a standard glass showed a reproducibility of instrumental mass fractionation between February 1991 and October 1996 of 1.3. Taking into account all sources of error, boron isotope measurements are accurate to within 5 for meteoritic samples having B contents in the range 0.1 to 1 μg g−1. A slightly better accuracy of 1.5 can be achieved for basaltic glasses which can be sputtered with very intense primary beams.

Oxygen isotope compositions of the carbon-rich, acid-resistant fraction of four carbonaceous chondrites (Orgueil, Renazzo, Murchison and Murray) are reported, along with results of experiments wherein bulk samples of Orgueil and Renazzo were selectively outgassed.

Summary The aim of this note is to present a synthesis of recent applications or extensions of the Stein-Rosenberg theorem on nonnegative matrices: convergence ofnon linear iterative process; truncated, or chaotic, or boolean Stein-Rosenberg theorems.

The assembly of a preinitiation complex containing RNA polymerase II on promoter DNA is a complex process that involves several general transcription factors. Using 5-[N-(p-azidobenzoyl)-3-aminoallyl] photocross-linking, we previously determined the locations of the two large subunits of transcription factor (TF) IIA (A35 and A21), TATA box-binding protein (TBP), RNA polymerase II-associated protein (RAP) 30, and TFIIB along the Ad2 ML promoter. We have now localized TFIIE34 and RAP74 just upstream of the transcription start site. The two subunits of TFIIF, RAP74 and RAP30, cross-linked to nucleotides that probed adjacent spaces on the same face of the DNA helix beginning just downstream of TBP at −19 and extending to −5. Specific photocross-linking of TFIIE34 required the presence TFIIE56. In addition, TFIIE and RAP74 strongly stimulated cross-linking of RAP30 and the large subunits of RNA polymerase II to position −19. Our topological data support the idea that RAP74 and TFIIE34 may be involved in melting of the promoter DNA upstream of the initiation site.

Genomic DNA is framed by additional layers of information, referred to as the epigenome. Epigenomic marks such as DNA methylation, histone modifications, and histone variants are concentrated on specific genomic sites, where they can both instruct and reflect gene expression. How this information is maintained, notably in the face of transcription, is not completely understood. Specifically, the extent to which modified histones themselves are retained through RNA polymerase II passage is unclear. Here, we show that several histone modifications are mislocalized when the transcription-coupled histone chaperones FACT or Spt6 are disrupted in Saccharomyces cerevisiae. In the absence of functional FACT or Spt6, transcription generates nucleosome loss, which is partially compensated for by the increased activity of non-transcription-coupled histone chaperones. The random incorporation of transcription-evicted modified histones scrambles epigenomic information. Our work highlights the importance of local recycling of modified histones by FACT and Spt6 during transcription in the maintenance of the epigenomic landscape.

The article offers information on the Biobaler, a harvesting system developed by Anderson Group.

The cover picture showsa panel of functional Nsalicylidene derivatives that can be rationally designed and assembled by crystal engineering. The amplitude of their thermochromism andor photochromism can be finely tuned as shown by the central needle. This study is supported by a multidisciplinary approach syntheses, physical and structural studies, calculations. Details are discussed in the article by Y. Garcia et al. on p. 621 ff.