RNA polymerase III and RNA polymerase II promoter complexes are heterochromatin barriers in Saccharomyces cerevisiae

Genome stability control by checkpoint regulation of tRNA gene transcription

The RNA polymerase III pre-initiation complex (PIC) assembled on yeast tRNA genes naturally causes replication fork pausing that contributes to genome instability. Mechanistic coupling of the fork pausing activity of tRNA genes to replication has long been considered likely, but only recently demonstrated. In contrast to the expectation that this coupling might occur by a passive mechanism such as direct disruption of transcription factor-DNA complexes by a component of the replisome, it turns out that disassembly of the RNA polymerase III PIC is actively controlled by the replication stress checkpoint signal transduction pathway. This advance supports a new model in which checkpoint-dependent disassembly of the transcription machinery at tRNA genes is a vital component of an overall system of genome stability control that also targets replication and DNA repair proteins.

Pol II and its associated epigenetic marks are present at Pol III-transcribed noncoding RNA genes

Epigenetic control is an important aspect of gene regulation. Despite detailed understanding of protein-coding gene expression, the transcription of non-coding RNA genes by RNA polymerase (pol) III is less well characterized. Here we profile the epigenetic features of pol III target genes throughout the human genome. This reveals that the chromatin landscape of pol III-transcribed genes resembles that of pol II templates in many ways, although there are also clear differences. Our analysis also discovered an entirely unexpected phenomenon, namely that pol II is present at the majority of genomic loci that are bound by pol III.

An auxiliary silencer and a boundary element maintain high levels of silencing proteins at HMR in Saccharomyces cerevisiae

Heterochromatin is notable for its capacity to propagate along a chromosome. The prevailing model for this spreading process postulates that silencing proteins are first recruited to silencer sequences and then spread from these sites independently of the silencers. However, we found that in Saccharomyces cerevisiae silencers also influence the extent of silenced chromatin domains. We compared the abilities of two different silencers, HMR-E and a telomeric repeat, to promote silencing and found that the HMR-E silencer contributed to an increased steady-state association of Sir proteins over a region of several kilobase pairs compared to the telomeric repeat, even though both silencers recruited similar levels of Sir proteins. We also discovered that, although the HMR-E silencer alone was sufficient to block transcription of the HMR locus, a secondary silencer, HMR-I, boosted the level of Sir proteins at HMR, apparently beyond the level necessary to repress transcription. Finally, we discovered that a tRNA(Thr) gene near HMR-I helped maintain silenced chromatin and transcriptional repression under conditions of reduced deacetylase activity. This study highlights the importance of auxiliary elements, such as HMR-I and the tRNA(Thr) gene, in enhancing the association of Sir silencing proteins with appropriate genomic locations, thereby buffering the capacity of silenced chromatin to assemble under suboptimal conditions.

Rpd3-dependent boundary formation at telomeres by removal of Sir2 substrate

Boundaries between euchromatic and heterochromatic regions until now have been associated with chromatin-opening activities. Here, we identified an unexpected role for histone deacetylation in this process. Significantly, the histone deacetylase (HDAC) Rpd3 was necessary for boundary formation in Saccharomyces cerevisiae. rpd3Delta led to silent information regulator (SIR) spreading and repression of subtelomeric genes. In the absence of a known boundary factor, the histone acetyltransferase complex SAS-I, rpd3Delta caused inappropriate SIR spreading that was lethal to yeast cells. Notably, Rpd3 was capable of creating a boundary when targeted to heterochromatin. Our data suggest a mechanism for boundary formation whereby histone deacetylation by Rpd3 removes the substrate for the HDAC Sir2, so that Sir2 no longer can produce O-acetyl-ADP ribose (OAADPR) by consumption of NAD(+) in the deacetylation reaction. In essence, OAADPR therefore is unavailable for binding to Sir3, preventing SIR propagation.

Unique functions of repetitive transcriptomes

Repetitive sequences occupy a huge fraction of essentially every eukaryotic genome. Repetitive sequences cover more than 50% of mammalian genomic DNAs, whereas gene exons and protein-coding sequences occupy only ~3% and 1%, respectively. Numerous genomic repeats include genes themselves. They generally encode "selfish" proteins necessary for the proliferation of transposable elements (TEs) in the host genome. The major part of evolutionary "older" TEs accumulated mutations over time and fails to encode functional proteins. However, repeats have important functions also on the RNA level. Repetitive transcripts may serve as multifunctional RNAs by participating in the antisense regulation of gene activity and by competing with the host-encoded transcripts for cellular factors. In addition, genomic repeats include regulatory sequences like promoters, enhancers, splice sites, polyadenylation signals, and insulators, which actively reshape cellular transcriptomes. TE expression is tightly controlled by the host cells, and some mechanisms of this regulation were recently decoded. Finally, capacity of TEs to proliferate in the host genome led to the development of multiple biotechnological applications.

The SNF2-family member Fun30 promotes gene silencing in heterochromatic loci

Chromatin regulates many key processes in the nucleus by controlling access to the underlying DNA. SNF2-like factors are ATP-driven enzymes that play key roles in the dynamics of chromatin by remodelling nucleosomes and other nucleoprotein complexes. Even simple eukaryotes such as yeast contain members of several subfamilies of SNF2-like factors. The FUN30/ETL1 subfamily of SNF2 remodellers is conserved from yeasts to humans, but is poorly characterized. We show that the deletion of FUN30 leads to sensitivity to the topoisomerase I poison camptothecin and to severe cell cycle progression defects when the Orc5 subunit is mutated. We demonstrate a role of FUN30 in promoting silencing in the heterochromatin-like mating type locus HMR, telomeres and the rDNA repeats. Chromatin immunoprecipitation experiments demonstrate that Fun30 binds at the boundary element of the silent HMR and within the silent HMR. Mapping of nucleosomes in vivo using micrococcal nuclease demonstrates that deletion of FUN30 leads to changes of the chromatin structure at the boundary element. A point mutation in the ATP-binding site abrogates the silencing function of Fun30 as well as its toxicity upon overexpression, indicating that the ATPase activity is essential for these roles of Fun30. We identify by amino acid sequence analysis a putative CUE motif as a feature of FUN30/ETL1 factors and show that this motif assists Fun30 activity. Our work suggests that Fun30 is directly involved in silencing by regulating the chromatin structure within or around silent loci.

Expanded roles of the origin recognition complex in the architecture and function of silenced chromatin in Saccharomyces cerevisiae

The silenced chromatin at the cryptic mating-type loci (HML and HMR) of Saccharomyces cerevisiae requires a cell cycle event between early S phase and G(2)/M phase to achieve repression. Although DNA replication per se is not essential for silencing, mutations in many of the proteins involved in DNA replication affect silencing. Each of the four silencers, which flank the silenced loci, includes an origin recognition complex (ORC) binding site (ACS). ORC directly interacted with Sir1 and recruits Sir1 to the silencers. This study describes additional roles for ORC in the architecture of silenced chromatin. Using chromatin immunoprecipitation (ChIP) analysis, we found that ORC physically interacts throughout the internal regions of HMR as well as with silencers. This interaction depended on the presence of Sir proteins and, in part, on the HMR-I silencer. ORC remained associated with the internal regions of HMR even when these regions were recombinationally separated from the silencers. Moreover, ORC could be recruited to the silencers lacking an ACS through its Sir1 interaction.

A positive but complex association between meiotic double-strand break hotspots and open chromatin in Saccharomyces cerevisiae

During meiosis, chromatin undergoes extensive changes to facilitate recombination, homolog pairing, and chromosome segregation. To investigate the relationship between chromatin organization and meiotic processes, we used formaldehyde-assisted isolation of regulatory elements (FAIRE) to map open chromatin during the transition from mitosis to meiosis in the budding yeast Saccharomyces cerevisiae. We found that meiosis-induced opening of chromatin is associated with meiotic DSB hotpots. The positive association between open chromatin and DSB hotspots is most prominent 3 h into meiosis, when the early meiotic genes DMC1 and HOP1 exhibit maximum transcription and the early recombination genes SPO11 and RAD51 are strongly up-regulated. While the degree of chromatin openness is positively associated with the occurrence of recombination hotspots, many hotspots occur outside of open chromatin. Of particular interest, many DSB hotspots that fell outside of meiotic open chromatin nonetheless occurred in chromatin that had recently been open during mitotic growth. Finally, we find evidence for meiosis-specific opening of chromatin at the regions adjacent to boundaries of subtelomeric sequences, which exhibit specific crossover control patterns hypothesized to be regulated by chromatin.

The SAGA subunit Ada2 functions in transcriptional silencing

The cellular role of the Ada2 coactivator is currently understood in the context of the SAGA histone acetyltransferase (HAT) complex, where Ada2 increases the HAT activity of Gcn5 and interacts with transcriptional activators. Here we report a new function for Ada2 in promoting transcriptional silencing at telomeres and ribosomal DNA. This silencing function is the first characterized role for Ada2 distinct from its involvement with Gcn5. Ada2 binds telomeric chromatin and the silencing protein Sir2 in vivo. Loss of ADA2 causes the spreading of Sir2 and Sir3 into subtelomeric regions and decreased histone H4 K16 acetylation. This previously uncharacterized boundary activity of Ada2 is functionally similar to, but mechanistically distinct from, that of the MYST family HAT Sas2. Mounting evidence in the literature indicates that boundary activities create chromosomal domains important for regulating gene expression in response to environmental changes. Consistent with this, we show that upon nutritional changes, Ada2 occupancy increases at a subtelomeric region proximal to a SAGA-inducible gene and causes derepression of a silenced telomeric reporter gene. Thus, Ada2, likely in the context of SAGA, is positioned at chromosomal termini to participate in both transcriptional repression and activation in response to nutrient signaling.

Spatial organization of genes as a component of regulated expression

The DNA of living cells is highly compacted. Inherent in this spatial constraint is the need for cells to organize individual genetic loci so as to facilitate orderly retrieval of information. Complex genetic regulatory mechanisms are crucial to all organisms, and it is becoming increasingly evident that spatial organization of genes is one very important mode of regulation for many groups of genes. In eukaryotic nuclei, it appears not only that DNA is organized in three-dimensional space but also that this organization is dynamic and interactive with the transcriptional state of the genes. Spatial organization occurs throughout evolution and with genes transcribed by all classes of RNA polymerases in all eukaryotic nuclei, from yeast to human. There is an increasing body of work examining the ways in which this organization and consequent regulation are accomplished. In this review, we discuss the diverse strategies that cells use to preferentially localize various classes of genes.

In vivo expression of MHC class I genes depends on the presence of a downstream barrier element

Regulation of MHC class I gene expression is critical to achieve proper immune surveillance. In this work, we identify elements downstream of the MHC class I promoter that are necessary for appropriate in vivo regulation: a novel barrier element that protects the MHC class I gene from silencing and elements within the first two introns that contribute to tissue specific transcription. The barrier element is located in intergenic sequences 3' to the polyA addition site. It is necessary for stable expression in vivo, but has no effect in transient transfection assays. Accordingly, in both transgenic mice and stably transfected cell lines, truncation of the barrier resulted in transcriptional gene silencing, increased nucleosomal density and decreased histone H3K9/K14 acetylation and H3K4 di-methylation across the gene. Significantly, distinct sequences within the barrier element govern anti-silencing and chromatin modifications. Thus, this novel barrier element functions to maintain transcriptionally permissive chromatin organization and prevent transcriptional silencing of the MHC class I gene, ensuring it is poised to respond to immune signaling.

DNA polymerase epsilon, acetylases and remodellers cooperate to form a specialized chromatin structure at a tRNA insulator

Insulators bind transcription factors and use chromatin remodellers and modifiers to mediate insulation. In this report, we identified proteins required for the efficient formation and maintenance of a specialized chromatin structure at the yeast tRNA insulator. The histone acetylases, SAS-I and NuA4, functioned in insulation, independently of tRNA and did not participate in the formation of the hypersensitive site at the tRNA. In contrast, DNA polymerase epsilon, functioned with the chromatin remodeller, Rsc, and the histone acetylase, Rtt109, to generate a histone-depleted region at the tRNA insulator. Rsc and Rtt109 were required for efficient binding of TFIIIB to the tRNA insulator, and the bound transcription factor and Rtt109 in turn were required for the binding of Rsc to tRNA. Robust insulation during growth and cell division involves the formation of a hypersensitive site at the insulator during chromatin maturation together with competition between acetylases and deacetylases.

Transcription independent insulation at TFIIIC-dependent insulators

Chromatin insulators separate active from repressed chromatin domains. In yeast the RNA pol III transcription machinery bound to tRNA genes function with histone acetylases and chromatin remodelers to restrict the spread of heterochromatin. Our results collectively demonstrate that binding of TFIIIC is necessary for insulation but binding of TFIIIB along with TFIIIC likely improves the probability of complex formation at an insulator. Insulation by this transcription factor occurs in the absence of RNA polymerase III or polymerase II but requires specific histone acetylases and chromatin remodelers. This analysis identifies a minimal set of factors required for insulation.

Diverse transcription influences can be insulated by the Drosophila SF1 chromatin boundary

Chromatin boundaries regulate gene expression by modulating enhancer–promoter interactions and insulating transcriptional influences from organized chromatin. However, mechanistic distinctions between these two aspects of boundary function are not well understood. Here we show that SF1, a chromatin boundary located in the Drosophila Antennapedia complex (ANT-C), can insulate the transgenic miniwhite reporter from both enhancing and silencing effects of surrounding genome, a phenomenon known as chromosomal position effect or CPE. We found that the CPE-blocking activity associates with different SF1 sub-regions from a previously characterized insulator that blocks enhancers in transgenic embryos, and is independent of GAF-binding sites essential for the embryonic insulator activity. We further provide evidence that the CPE-blocking activity cannot be attributed to an enhancer-blocking activity in the developing eye. Our results suggest that SF1 contains multiple non-overlapping activities that block diverse transcriptional influences from embryonic or adult enhancers, and from positive and negative chromatin structure. Such diverse insulating capabilities are consistent with the proposed roles of SF1 to functionally separate fushi tarazu (ftz), a non-Hox gene, from the enhancers and the organized chromatin of the neighboring Hox genes.

Limiting the extent of the RDN1 heterochromatin domain by a silencing barrier and Sir2 protein levels in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, transcriptional silencing occurs at the cryptic mating-type loci (HML and HMR), telomeres, and ribosomal DNA (rDNA; RDN1). Silencing in the rDNA is unusual in that polymerase II (Pol II) promoters within RDN1 are repressed by Sir2 but not Sir3 or Sir4. rDNA silencing unidirectionally spreads leftward, but the mechanism of limiting its spreading is unclear. We searched for silencing barriers flanking the left end of RDN1 by using an established assay for detecting barriers to HMR silencing. Unexpectedly, the unique sequence immediately adjacent to RDN1, which overlaps a prominent cohesin binding site (CARL2), did not have appreciable barrier activity. Instead, a fragment located 2.4 kb to the left, containing a tRNA(Gln) gene and the Ty1 long terminal repeat, had robust barrier activity. The barrier activity was dependent on Pol III transcription of tRNA(Gln), the cohesin protein Smc1, and the SAS1 and Gcn5 histone acetyltransferases. The location of the barrier correlates with the detectable limit of rDNA silencing when SIR2 is overexpressed, where it blocks the spreading of rDNA heterochromatin. We propose a model in which normal Sir2 activity results in termination of silencing near the physical rDNA boundary, while tRNA(Gln) blocks silencing from spreading too far when nucleolar Sir2 pools become elevated.

Assembling heterochromatin in the appropriate places: A boost is needed

Heterochromatin, or condensed chromatin, has the potential to encroach into what ordinarily would be euchromatin and repress resident genes. We explore how heterochromatin is restricted to the appropriate regions of the genome, using Saccharomyces cerevisiae as a case study and emphasizing two under-appreciated aspects of silenced chromatin. First, the capacity of silenced chromatin to propagate along a chromosome is limited by the intrinsic instability of the structure. We argue that this limited potential to spread is an important factor restricting silenced chromatin to the vicinity of recruitment sites (silencers). Second, this limited capacity to spread creates the need for additional mechanisms to stabilize silenced chromatin at the required locations. Such mechanisms include the use of multiple silencers and higher-order arrangements of the chromatin fiber. Therefore, to understand how silenced chromatin is restricted to the appropriate genomic locations, researchers must take into account the mechanisms by which silenced chromatin is stabilized in appropriate locations. J. Cell. Physiol. 219: 525-528, 2009. (c) 2009 Wiley-Liss, Inc.

Histone deacetylase Rpd3 antagonizes Sir2-dependent silent chromatin propagation

In the eukaryotic genome, transcriptionally silent chromatin tends to propagate along a chromosome and encroach upon adjacent active chromatin. The silencing machinery can be stopped by chromatin boundary elements. We performed a screen in Saccharomyces cerevisiae for proteins that may contribute to the establishment of a chromatin boundary. We found that disruption of histone deacetylase Rpd3p results in defective boundary activity, leading to a Sir-dependent local propagation of transcriptional repression. In rpd3 Delta cells, the amount of Sir2p that was normally found in the nucleolus decreased and the amount of Sir2p found at telomeres and at HM and its adjacent loci increased, leading to an extension of silent chromatin in those areas. In addition, Rpd3p interacted directly with chromatin at boundary regions to deacetylate histone H4 at lysine 5 and at lysine 12. Either the mutation of histone H4 at lysine 5 or a decrease in the histone acetyltransferase (HAT) activity of Esa1p abrogated the silencing phenotype associated with rpd3 mutation, suggesting a novel role for the H4 amino terminus in Rpd3p-mediated heterochromatin boundary regulation. Together, these data provide insight into the molecular mechanisms for the anti-silencing functions of Rpd3p during the formation of heterochromatin boundaries.

The impact of local genome sequence on defining heterochromatin domains

Characterizing how genomic sequence interacts with trans-acting regulatory factors to implement a program of gene expression in eukaryotic organisms is critical to understanding genome function. One means by which patterns of gene expression are achieved is through the differential packaging of DNA into distinct types of chromatin. While chromatin state exerts a major influence on gene expression, the extent to which cis-acting DNA sequences contribute to the specification of chromatin state remains incompletely understood. To address this, we have used a fission yeast sequence element (L5), known to be sufficient to nucleate heterochromatin, to establish de novo heterochromatin domains in the Schizosaccharomyces pombe genome. The resulting heterochromatin domains were queried for the presence of H3K9 di-methylation and Swi6p, both hallmarks of heterochromatin, and for levels of gene expression. We describe a major effect of genomic sequences in determining the size and extent of such de novo heterochromatin domains. Heterochromatin spreading is antagonized by the presence of genes, in a manner that can occur independent of strength of transcription. Increasing the dosage of Swi6p results in increased heterochromatin proximal to the L5 element, but does not result in an expansion of the heterochromatin domain, suggesting that in this context genomic effects are dominant over trans effects. Finally, we show that the ratio of Swi6p to H3K9 di-methylation is sequence-dependent and correlates with the extent of gene repression. Taken together, these data demonstrate that the sequence content of a genomic region plays a significant role in shaping its response to encroaching heterochromatin and suggest a role of DNA sequence in specifying chromatin state.

Epigenetic packaging of DNA sequence into chromatin is a major force in shaping the function of complex genomes. Different types of chromatin have distinct effects on gene expression, and thus chromatin state imparts distinct features on the associated genomic DNA. Our study focuses on the transition between two opposing chromatin states: euchromatin, which generally correlates with gene expression, and heterochromatin, which is typically refractive to gene expression. While heterochromatin is capable of spreading into euchromatic domains, the parameters that influence such spreading are unknown. We established heterochromatin at ectopic sites in the genome and evaluated whether specific DNA sequences affected the extent of heterochromatin spreading and the transition between heterochromatin and euchromatin. We found that the nature of the genomic DNA neighboring the heterochromatic sequence dramatically affected the extent of heterochromatin spreading. In particular, the presence of genes antagonized the spread of heterochromatin, whereas neutral sequence elements were incorporated into the domain. This study demonstrates that genome sequence and chromatin identity are inextricably linked; features of both interact to determine the structural and functional fate of underlying DNA sequences.

Heterochromatin and the cohesion of sister chromatids

Heterochromatin, once thought to be the useless junk of chromosomes, is now known to play significant roles in biology. Underlying much of this newfound fame are links between the repressive chromatin structure and cohesin, the protein complex that mediates sister chromatid cohesion. Heterochromatin-mediated recruitment and retention of cohesin to domains flanking centromeres promotes proper attachment of chromosomes to the mitotic and meiotic spindles. Heterochromatin assembled periodically between convergently transcribed genes also recruits cohesin, which promotes a novel form of transcription termination. Heterochromatin-like structures in budding yeast also recruit cohesin. Here the complex appears to regulate transcriptional silencing and recombination between repeated DNA sequences. The link between heterochromatin and cohesin is particularly relevant to human health. In Roberts-SC phocomelia syndrome, heterochromatic cohesion is selectively lost due to mutation of the acetyltransferase responsible for cohesin activation. In this review I discuss recent work that relates to these relationships between heterochromatin and cohesin.

Analysis of chromatin boundary activity in Drosophila cells

Background

Chromatin boundaries, also known as insulators, regulate gene activity by organizing active and repressive chromatin domains and modulate enhancer-promoter interactions. However, the mechanisms of boundary action are poorly understood, in part due to our limited knowledge about insulator proteins, and a shortage of standard assays by which diverse boundaries could be compared.

Results

We report here the development of an enhancer-blocking assay for studying insulator activity in Drosophila cultured cells. We show that the activities of diverse Drosophila insulators including suHw, SF1, SF1b, Fab7 and Fab8 are supported in these cells. We further show that double stranded RNA (dsRNA)-mediated knockdown of SuHw and dCTCF factors disrupts the enhancer-blocking function of suHw and Fab8, respectively, thereby establishing the effectiveness of using RNA interference in our cell-based assay for probing insulator function.

Conclusion

The novel boundary assay provides a quantitative and efficient method for analyzing insulator mechanism and can be further exploited in genome-wide RNAi screens for insulator components. It provides a useful tool that complements the transgenic and genetic approaches for studying this important class of regulatory elements.

A silencer promotes the assembly of silenced chromatin independently of recruitment

In Saccharomyces cerevisiae, silenced chromatin occurs at telomeres and the silent mating-type loci HMR and HML. At these sites, the Sir proteins are recruited to a silencer and then associate with adjacent chromatin. We used chromatin immunoprecipitation to compare the rates of Sir protein assembly at different genomic locations and discovered that establishment of silenced chromatin was much more rapid at HMR than at the telomere VI-R. Silenced chromatin also assembled more quickly on one side of HMR-E than on the other. Despite differences in spreading, the Sir proteins were recruited to HMR-E and telomeric silencers at equivalent rates. Additionally, insertion of HMR-E adjacent to the telomere VI-R increased the rate of Sir2p association with the telomere. These data suggest that HMR-E functions to both recruit Sir proteins and promote their assembly across several kilobases. Observations that association of Sir2p occurs simultaneously throughout HMR and that silencing at HMR is insensitive to coexpression of catalytically inactive Sir2p suggest that HMR-E acts by enabling assembly to occur in a nonlinear fashion. The ability of silencers to promote assembly of silenced chromatin over several kilobases is likely an important mechanism for maintaining what would otherwise be unstable chromatin at the correct genomic locations.

TFIIIC binding sites function as both heterochromatin barriers and chromatin insulators in Saccharomyces cerevisiae

Chromosomal sites of RNA polymerase III (Pol III) transcription have been demonstrated to have "extratranscriptional" functions, as the assembled Pol III complex can act as chromatin boundaries or pause sites for replication forks, can alter nucleosome positioning or affect transcription of neighboring genes, and can play a role in sister chromatid cohesion. Several studies have demonstrated that assembled Pol III complexes block the propagation of heterochromatin-mediated gene repression. Here we show that in Saccharomyces cerevisiae tRNA genes (tDNAs) and even partially assembled Pol III complexes containing only the transcription factor TFIIIC can exhibit chromatin boundary functions both as heterochromatin barriers and as insulators to gene activation. Both the TRT2 tDNA and the ETC4 site which binds only the TFIIIC complex prevented an upstream activation sequence from activating the GAL promoters in our assay system, effectively acting as chromatin insulators. Additionally, when placed downstream from the heterochromatic HMR locus, ETC4 blocked the ectopic spread of Sir protein-mediated silencing, thus functioning as a barrier to repression. Finally, we show that TRT2 and the ETC6 site upstream of TFC6 in their natural contexts display potential insulator-like functions, and ETC6 may represent a novel case of a Pol III factor directly regulating a Pol II promoter. The results are discussed in the context of how the TFIIIC transcription factor complex may function to demarcate chromosomal domains in yeast and possibly in other eukaryotes.

Different functional modes of p300 in activation of RNA polymerase III transcription from chromatin templates

Transcriptional coactivators that regulate the activity of human RNA polymerase III (Pol III) in the context of chromatin have not been reported. Here, we describe a completely defined in vitro system for transcription of a human tRNA gene assembled into a chromatin template. Transcriptional activation and histone acetylation in this system depend on recruitment of p300 by general initiation factor TFIIIC, thus providing a new paradigm for recruitment of histone-modifying coactivators. Beyond its role as a chromatin-modifying factor, p300 displays an acetyltransferase-independent function at the level of preinitiation complex assembly. Thus, direct interaction of p300 with TFIIIC stabilizes binding of TFIIIC to core promoter elements and results in enhanced transcriptional activity on histone-free templates. Additional studies show that p300 is recruited to the promoters of actively transcribed tRNA and U6 snRNA genes in vivo. These studies identify TFIIIC as a recruitment factor for p300 and thus may have important implications for the emerging concept that tRNA genes or TFIIIC binding sites act as chromatin barriers to prohibit spreading of silenced heterochromatin domains.

Insights into the impact of histone acetylation and methylation on Sir protein recruitment, spreading, and silencing in Saccharomyces cerevisiae

Silent chromatin formation in Saccharomyces cerevisiae begins with the recruitment of silent information regulator (Sir) proteins to silencers at the silent mating-type loci and to telomere ends. Next, Sir2/3/4 proteins propagate across these loci as histones are deacetylated by the NAD(+)-dependent histone deacetylase Sir2p, ultimately resulting in the cessation of transcription and in the loss of SET1- and DOT1-dependent methylation of histone H3 within silent chromatin. We analyzed the effects of modifiable lysine residues on histones H3 and H4 on experimentally defined steps in silencing: recruitment of Sir proteins to silencers, Sir protein spreading, and transcriptional repression. Loss of acetylation, but not methylation, facilitated both Sir recruitment and spreading, and Sir spreading across hypoacetylated chromatin could disrupt SET1- and DOT1-dependent histone methylation without silencing underlying genes. Our data indicate that loss of methylation of K4 and K79 on histone H3 reflects intermediate events during the formation of silent chromatin, and that retention of a positive charge at a single residue on histone H4 (K16) was both necessary and sufficient to permit Sir spreading beyond sites of their recruitment.

Boundaries. Boundaries...Boundaries???

One way to modulate transcription is by partitioning the chromatin fiber within the nucleus into the active or inactive domains through the establishment of higher-order chromatin structure. Such subdivision of chromatin implies the existence of insulators and boundaries that delimit differentially regulated chromosomal loci. Recently published data on transcriptional interference from the repeated component of the genome fits the classic definition of insulator/boundary activity. This review discusses the phenomena of transcriptional interference and raises the question about functionality of genomic "junk" along with the need to stimulate a dialogue on how we would define the insulators and boundaries in the light of contemporary data. Rule 19 (a) (Boundaries)"Before the toss, the umpires shall agree the boundary of the field of play with both captains. The boundary shall, if possible, be marked along its whole length" Rules of Cricket.

MYSTs mark chromatin for chromosomal functions

The MYST family of lysine acetyltransferases has been intensely studied because of its broad conservation and biological significance. In humans, there are multiple correlations between the enzymes and development and disease. In model organisms, genetic and biochemical studies have been particularly productive because of mechanistic insights they provide in defining substrate specificity, the complexes through which the enzymes function, and the sites of their activity within the genome. Established and emerging data from yeast reveal roles for the three MYST enzymes in diverse chromosomal functions. In particular, recent studies help explain how MYST complexes coordinate with other modifiers, the histone variant H2A.Z, and remodeling complexes to demarcate silent and active chromosomal domains, facilitate transcription, and enable repair of DNA damage.

Chromatin structure and expression of a gene transcribed by RNA polymerase III are independent of H2A.Z deposition

The genes transcribed by RNA polymerase III (Pol III) generally have intragenic promoter elements. One of them, the yeast U6 snRNA (SNR6) gene is activated in vitro by a positioned nucleosome between its intragenic box A and extragenic, downstream box B separated by approximately 200 bp. We demonstrate here that the in vivo chromatin structure of the gene region is characterized by the presence of an array of positioned nucleosomes, with only one of them in the 5' end of the gene having a regulatory role. A positioned nucleosome present between boxes A and B in vivo does not move when the gene is repressed due to nutritional deprivation. In contrast, the upstream nucleosome which covers the TATA box under repressed conditions is shifted approximately 50 bp further upstream by the ATP-dependent chromatin remodeler RSC upon activation. It is marked with the histone variant H2A.Z and H4K16 acetylation in active state. In the absence of H2A.Z, the chromatin structure of the gene does not change, suggesting that H2A.Z is not required for establishing the active chromatin structure. These results show that the chromatin structure directly participates in regulation of a Pol III-transcribed gene under different states of its activity in vivo.

A genetic modifier screen identifies multiple genes that interact with Drosophila Rap/Fzr and suggests novel cellular roles

In the developing Drosophila eye, Rap/Fzr plays a critical role in neural patterning by regulating the timely exit of precursor cells. Rap/Fzr (Retina aberrant in pattern/Fizzy related) is an activator of the E3 Ubiquitin ligase, the APC (Anaphase Promoting Complex-cyclosome) that facilitates the stage specific proteolytic destruction of mitotic regulators, such as cyclins and cyclin-dependent kinases. To identify novel functional roles of Rap/Fzr, we conducted an F(1) genetic modifier screen to identify genes which interact with the partial-loss-function mutations in rap/fzr. We screened 2741 single P-element, lethal insertion lines and piggyBac lines on the second and third chromosome for dominant enhancers and suppressors of the rough eye phenotype of rap/fzr. From this screen, we have identified 40 genes that exhibit dosage-sensitive interactions with rap/fzr; of these, 31 have previously characterized cellular functions. Seven of the modifiers identified in this study are regulators of cell cycle progression with previously known interactions with rap/fzr. Among the remaining modifiers, 27 encode proteins involved in other cellular functions not directly related to cell-cycle progression. The newly identified variants fall into at least three groups based on their previously known cellular functions: transcriptional regulation, regulated proteolysis, and signal transduction. These results suggest that, in addition to cell cycle regulation, rap/fzr regulates ubiquitin-ligase-mediated protein degradation in the developing nervous system as well as in other tissues.

Long-range communication between the silencers of HMR

Gene regulation involves long-range communication between silencers, enhancers, and promoters. In Saccharomyces cerevisiae, silencers flank transcriptionally repressed genes to mediate regional silencing. Silencers recruit the Sir proteins, which then spread along chromatin to encompass the entire silenced domain. In this report we have employed a boundary trap assay, an enhancer activity assay, chromatin immunoprecipitations, and chromosome conformation capture analyses to demonstrate that the two HMR silencer elements are in close proximity and functionally communicate with one another in vivo. We further show that silencing is necessary for these long-range interactions, and we present models for Sir-mediated silencing based upon these results.

Sir2 deacetylates histone H3 lysine 56 to regulate telomeric heterochromatin structure in yeast

At telomeric heterochromatin in yeast, the Sir protein complex spreads from Rap1 sites to silence adjacent genes. This cascade is believed to occur when Sir2, an NAD(+)-dependent enzyme, deacetylates histone H3 and H4 N termini, in particular histone H4 K16, enabling more Sir protein binding. Lysine 56 of histone H3 is located at the entry-exit points of the DNA superhelix surrounding the nucleosome, where it may control DNA compaction. We have found that K56 substitutions disrupt silencing severely without decreasing Sir protein binding at the telomere. Our in vitro and in vivo data indicate that Sir2 deacetylates K56 directly in telomeric heterochromatin to compact chromatin and prevent access to RNA polymerase and ectopic bacterial dam methylase. Since the spread of Sir proteins is necessary but not sufficient for silencing, we propose that silencing occurs when Sir2 deacetylates H3 K56 to close the nucleosomal entry-exit gates, enabling compaction of heterochromatin.

An RNA polymerase III-dependent heterochromatin barrier at fission yeast centromere 1

Heterochromatin formation involves the nucleation and spreading of structural and epigenetic features along the chromatin fiber. Chromatin barriers and associated proteins counteract the spreading of heterochromatin, thereby restricting it to specific regions of the genome. We have performed gene expression studies and chromatin immunoprecipitation on strains in which native centromere sequences have been mutated to study the mechanism by which a tRNA^Alanine gene barrier (cen1 tDNA^Ala) blocks the spread of pericentromeric heterochromatin at the centromere of chromosome 1 (cen1) in the fission yeast, Schizosaccharomyces pombe. Within the centromere, barrier activity is a general property of tDNAs and, unlike previously characterized barriers, requires the association of both transcription factor IIIC and RNA Polymerase III. Although the cen1 tDNA^Ala gene is actively transcribed, barrier activity is independent of transcriptional orientation. These findings provide experimental evidence for the involvement of a fully assembled RNA polymerase III transcription complex in defining independent structural and functional domains at a eukaryotic centromere.

Functions of site-specific histone acetylation and deacetylation

Histone acetylation regulates many cellular processes, and specific acetylation marks, either singly or in combination, produce distinct outcomes. For example, the acetylation pattern on newly synthesized histones is important for their assembly into nucleosomes by histone chaperones. Additionally, the degree of chromatin compaction and folding may be regulated by acetylation of histone H4 at lysine 16. Histone acetylation also regulates the formation of heterochromatin; deacetylation of H4 lysine 16 is important for spreading of heterochromatin components, whereas acetylation of this site serves as a barrier to this spreading. Finally, histone acetylation is critical for gene transcription, but recent results suggest that deacetylation of certain sites also plays an important role. There are many histone acetyltransferases (HATs) and deacetylases, with differing preferences for the various histone proteins and for specific sites on individual histones. Determining how these enzymes create distinct acetylation patterns and regulate the functional outcome is an important challenge.

A tDNA establishes cohesion of a neighboring silent chromatin domain

DNA replication generates sister chromatid pairs that are bound to one another until anaphase onset. The process, termed sister chromatid cohesion, requires the multisubunit cohesin complex that resides at centromeres and sites where genes converge. At the HMR mating-type locus of budding yeast, cohesin associates with a heterochromatin-like structure known as silent chromatin. In this report, we show that silent chromatin is necessary but not sufficient for cohesion of the replicating locus. A tRNA gene (tDNA) that delimits the silent chromatin domain is also required, as are subunits of the TFIIIB and RSC complexes that bind the gene. Non-tDNA boundary elements do not substitute for tDNAs in cohesion, suggesting that barrier activity is not responsible for the phenomenon. The results reveal an unexpected role for tDNAs and RNA polymerase III-associated proteins in establishment of sister chromatid cohesion.

The role of insulator elements in large-scale chromatin structure in interphase

Insulator elements can be classified as enhancer-blocking or barrier insulators depending on whether they interfere with enhancer-promoter interactions or act as barriers against the spreading of heterochromatin. The former class may exert its function at least in part by attaching the chromatin fiber to a nuclear substrate such as the nuclear matrix, resulting in the formation of chromatin loops. The latter class functions by recruiting histone-modifying enzymes, although some barrier insulators have also been shown to create chromatin loops. These loops may correspond to functional nuclear domains containing clusters of co-expressed genes. Thus, insulators may determine specific patterns of nuclear organization that are important in establishing specific programs of gene expression during cell differentiation and development.

Tup1-Ssn6 and Swi-Snf remodelling activities influence long-range chromatin organization upstream of the yeast SUC2 gene

The traditional model for chromatin remodelling during transcription has focused upon the remodelling of nucleosomes at gene promoters. However, in this study, we have determined that Tup1-Ssn6 and Swi-Snf chromatin remodelling activities extend far upstream of the SUC2 gene promoter into the intergenic region of the Saccharomyces cerevisiae chromosome. We mapped the nucleosomal array over a 7.5 kb region that encompassed the SUC2 gene promoter and upstream region but was devoid of other transcriptionally active genes. Nucleosome positioning over this region was determined under conditions of glucose repression and derepression, and in snf2, ssn6 and snf2 ssn6 mutant strains. A map detailing remodelling events extending as much as 5 kb upstream of the SUC2 gene promoter underlines the roles of the Tup1-Ssn6 and Swi-Snf complexes in respectively organizing and disrupting nucleosome arrays. The gene specificity of these events suggests a role in gene regulation. We propose that long-range chromatin remodelling activities of Swi-Snf and Tup1-Ssn6 may ultimately influence whether the chromosomal state of the SUC2 gene is proficient for transcription. These data raise the possibility that remodelling of extensive chromatin domains may be a general property of the Swi-Snf and Tup1-Ssn6 complexes.

The nuclear envelope and transcriptional control

Cells have evolved sophisticated multi-protein complexes that can regulate gene activity at various steps of the transcription process. Recent advances highlight the role of nuclear positioning in the control of gene expression and have put nuclear envelope components at centre stage. On the inner face of the nuclear envelope, active genes localize to nuclear-pore structures whereas silent chromatin localizes to non-pore sites. Nuclear-pore components seem to not only recruit the RNA-processing and RNA-export machinery, but contribute a level of regulation that might enhance gene expression in a heritable manner.

A novel role for histone chaperones CAF-1 and Rtt106p in heterochromatin silencing

The histone chaperones CAF-1 and Rtt106p are required for heterochromatin silencing in the yeast Saccharomyces cerevisiae. Although it has been suggested that CAF-1 is involved in the maintenance of heterochromatin silencing, their exact functions during this process are not well understood. Here, we show that CAF-1 and Rtt106p are involved in the early stages of heterochromatin formation. The binding of Sir proteins to telomeric heterochromatin is significantly reduced and, additionally, Sir proteins are mislocalized in cells lacking CAF-1 and Rtt106p. At the HMR locus, CAF-1 and Rtt106p are required for the initial recruitment of Sir2p and Sir3p, but not Sir4p, to the HMR-E silencer, where silencing initiates, as well as the efficient spreading of all of these Sir proteins to the distal a1 gene. Moreover, silencing at the HMR locus is dramatically reduced in cells lacking CAF-1, Rtt106p, and Sir1p. Thus, these studies reveal a novel role for CAF-1 and Rtt106p in epigenetic silencing and indicate that the spreading of heterochromatin, a poorly understood process, requires histone chaperones.

Chromatin insulators

Active and silenced chromatin domains are often in close juxtaposition to one another, and enhancer and silencer elements operate over large distances to regulate the genes in these domains. The lack of promiscuity in the function of these elements suggests that active mechanisms exist to restrict their activity. Insulators are DNA elements that restrict the effects of long-range regulatory elements. Studies on different insulators from different organisms have identified common themes in their mode of action. Numerous insulators map to promoters of genes or have binding sites for transcription factors and like active chromatin hubs and silenced loci, insulators also cluster in the nucleus. These results bring into focus potential conserved mechanisms by which these elements might function in the nucleus.

Histone H3 lysine 36 methylation antagonizes silencing in Saccharomyces cerevisiae independently of the Rpd3S histone deacetylase complex

In yeast, methylation of histone H3 on lysine 36 (H3-K36) is catalyzed by the NSD1 leukemia oncoprotein homolog Set2. The histone deacetylase complex Rpd3S is recruited to chromatin via binding of the chromodomain protein Eaf3 to methylated H3-K36 to prevent erroneous transcription initiation. Here we identify a distinct function for H3-K36 methylation. We used random mutagenesis of histones H3 and H4 followed by a reporter-based screen to identify residues necessary to prevent the ectopic spread of silencing from the silent mating-type locus HMRa into flanking euchromatin. Mutations in H3-K36 or deletion of SET2 caused ectopic silencing of a heterochromatin-adjacent reporter. Transcriptional profiling revealed that telomere-proximal genes are enriched for those that display decreased expression in a set2Delta strain. Deletion of SIR4 rescued the expression defect of 26 of 37 telomere-proximal genes with reduced expression in set2Delta cells, implying that H3-K36 methylation prevents the spread of telomeric silencing. Indeed, Sir3 spreads from heterochromatin into neighboring euchromatin in set2Delta cells. Furthermore, genetic experiments demonstrated that cells lacking the Rpd3S-specific subunits Eaf3 or Rco1 did not display the anti-silencing phenotype of mutations in SET2 or H3-K36. Thus, antagonism of silencing is independent of the only known effector of this conserved histone modification.

Requirement of Nhp6 proteins for transcription of a subset of tRNA genes and heterochromatin barrier function in Saccharomyces cerevisiae

A key event in tRNA gene (tDNA) transcription by RNA polymerase (Pol) III is the TFIIIC-dependent assembly of TFIIIB upstream of the transcription start site. Different tDNA upstream sequences bind TFIIIB with different affinities, thereby modulating tDNA transcription. We found that in the absence of Nhp6 proteins, the influence of the 5'-flanking region on tRNA gene transcription is dramatically enhanced in Saccharomyces cerevisiae. Expression of a tDNA bearing a suboptimal TFIIIB binding site, but not of a tDNA preceded by a strong TFIIIB binding region, was strongly dependent on Nhp6 in vivo. Upstream sequence-dependent stimulation of tRNA gene transcription by Nhp6 could be reproduced in vitro, and Nhp6 proteins were found associated with tRNA genes in yeast cells. We also show that both transcription and silencing barrier activity of a tDNA(Thr) at the HMR locus are compromised in the absence of Nhp6. Our data suggest that Nhp6 proteins are important components of Pol III chromatin templates that contribute both to the robustness of tRNA gene expression and to positional effects of Pol III transcription complexes.

Bypassing the catalytic activity of SIR2 for SIR protein spreading in Saccharomyces cerevisiae

Sir protein spreading along chromosomes and silencing in Saccharomyces cerevisiae requires the NAD+-dependent histone deacetylase activity of Sir2p. We tested whether this requirement could be bypassed at the HM loci and telomeres in cells containing a stably expressed, but catalytically inactive mutant of Sir2p, sir2-345p, plus histone mutants that mimic the hypoacetylated state normally created by Sir2p. Sir protein spreading was rescued in sir2-345 mutants expressing histones in which key lysine residues in their N-termini had been mutated to arginine. Mating in these mutants was also partially restored upon overexpression of Sir3p. Together, these results indicate that histone hypoacetylation is sufficient for Sir protein spreading in the absence of production of 2'-O-acetyl-ADP ribose by sir2p and Sir2p's enzymatic function for silencing can be bypassed in a subset of cells in a given population. These results also provide genetic evidence for the existence of additional critical substrates of Sir2p for silencing in vivo.

Spatial organization of transcription by RNA polymerase III

RNA polymerase III (pol III) transcribes many essential, small, noncoding RNAs, including the 5S rRNAs and tRNAs. While most pol III-transcribed genes are found scattered throughout the linear chromosome maps or in multiple linear clusters, there is increasing evidence that many of these genes prefer to be spatially clustered, often at or near the nucleolus. This association could create an environment that fosters the coregulation of transcription by pol III with transcription of the large ribosomal RNA repeats by RNA polymerase I (pol I) within the nucleolus. Given the high number of pol III-transcribed genes in all eukaryotic genomes, the spatial organization of these genes is likely to affect a large portion of the other genes in a genome. In this Survey and Summary we analyze the reports regarding the spatial organization of pol III genes and address the potential influence of this organization on transcriptional regulation.

Insulators: exploiting transcriptional and epigenetic mechanisms

Insulators are DNA sequence elements that prevent inappropriate interactions between adjacent chromatin domains. One type of insulator establishes domains that separate enhancers and promoters to block their interaction, whereas a second type creates a barrier against the spread of heterochromatin. Recent studies have provided important advances in our understanding of the modes of action of both types of insulator. These new insights also suggest that the mechanisms of action of both enhancer blockers and barriers might not be unique to these types of element, but instead are adaptations of other gene-regulatory mechanisms.

A role for TFIIIC transcription factor complex in genome organization

Eukaryotic genome complexity necessitates boundary and insulator elements to partition genomic content into distinct domains. We show that inverted repeat (IR) boundary elements flanking the fission yeast mating-type heterochromatin domain contain B-box sequences, which prevent heterochromatin from spreading into neighboring euchromatic regions by recruiting transcription factor TFIIIC complex without RNA polymerase III (Pol III). Genome-wide analysis reveals TFIIIC with Pol III at all tRNA genes, many of which cluster at pericentromeric heterochromatin domain boundaries. However, a single tRNA(phe) gene with modest TFIIIC enrichment is insufficient to serve as boundary and requires RNAi-associated element to restrain heterochromatin spreading. Remarkably, we found TFIIIC localization without Pol III at many sites located between divergent promoters. These sites appear to act as chromosome-organizing clamps by tethering distant loci to the nuclear periphery, at which TFIIIC is concentrated into several distinct bodies. Our analyses uncover a general genome organization mechanism involving conserved TFIIIC complex.

Nucleosome depletion activates poised RNA polymerase III at unconventional transcription sites in Saccharomyces cerevisiae

RNA polymerase (pol) III, assisted by the transcription factors TFIIIC and TFIIIB, transcribes small untranslated RNAs, such as tRNAs. In addition to known pol III-transcribed genes, the Saccharomyces cerevisiae genome contains loci (ZOD1, ETC1-8) associated to incomplete pol III transcription complexes (Moqtaderi, Z., and Struhl, K. (2004) Mol. Cell. Biol. 24, 4118-4127). We show that a short segment of the ZOD1 locus, containing box A and box B promoter elements and a termination signal between them, directs the pol III-dependent production of a small RNA both in vitro and in vivo. In yeast cells, the levels of both ZOD1- and ETC5-specific transcripts were dramatically enhanced upon nucleosome depletion. Remarkably, transcription factor and pol III occupancy at the corresponding loci did not change significantly upon derepression, thus suggesting that chromatin opening activates poised pol III to transcription. Comparative genomic analysis revealed that the ZOD1 promoter is the only surviving portion of a tDNA(Ile) ancestor, whose transcription capacity has been preserved throughout evolution independently from the encoded RNA product. Similarly, another TFIIIC/TFIIIB-associated locus, close to the YGR033c open reading frame, was found to be the strictly conserved remnant of an ancient tDNA(Arg). The maintenance, by eukaryotic genomes, of chromatin-repressed, non-coding transcription units has implications for both genome expression and organization.

Sir2 represses endogenous polymerase II transcription units in the ribosomal DNA nontranscribed spacer

Silencing at the rDNA, HM loci, and telomeres in Saccharomyces cerevisiae requires histone-modifying enzymes to create chromatin domains that are refractory to recombination and RNA polymerase II transcription machineries. To explore how the silencing factor Sir2 regulates the composition and function of chromatin at the rDNA, the association of histones and RNA polymerase II with the rDNA was measured by chromatin immunoprecipitation. We found that Sir2 regulates not only the levels of K4-methylated histone H3 at the rDNA but also the levels of total histone H3 and RNA polymerase II. Furthermore, our results demonstrate that the ability of Sir2 to limit methylated histones at the rDNA requires its deacetylase activity. In sir2Delta cells, high levels of K4-trimethylated H3 at the rDNA nontranscribed spacer are associated with the expression of transcription units in the nontranscribed spacer by RNA polymerase II and with previously undetected alterations in chromatin structure. Together, these data suggest a model where the deacetylase activity of Sir2 prevents euchromatinization of the rDNA and silences naturally occurring intergenic transcription units whose expression has been associated with disruption of cohesion complexes and repeat amplification at the rDNA.

A heterochromatin barrier partitions the fission yeast centromere into discrete chromatin domains

BACKGROUND

Centromeres are cis-acting chromosomal domains that direct kinetochore formation, enabling faithful chromosome segregation. Centromeric regions of higher eukaryotes are structurally complex, consisting of various epigenetically modified chromatin types including specialized chromatin at the kinetochore itself, pericentromeric heterochromatin, and flanking euchromatin. Although the features necessary for the establishment and maintenance of discrete chromatin domains remain poorly understood, two models have been proposed based either on the passive convergence of competing activities involved in individual domain formation or, alternatively, on the action of specific genomic sequences and associated proteins to actively block the propagation of one chromatin type into another.

RESULTS

Functional analysis of centromeric sequences located at the intersection of Schizosaccharomyces pombe central core chromatin and outer repeat heterochromatin identified a chromatin barrier that contains a transfer RNA (tRNA) gene. Deletion or modification of the barrier sequences result in the propagation of pericentromeric heterochromatin beyond its normal boundary. The tRNA gene is transcriptionally active, and barrier activity requires sequences necessary for RNA polymerase III transcription. Moreover, absence of the barrier results in abnormal meiotic chromosome segregation.

CONCLUSIONS

The identification of DNA sequences with chromatin barrier activity at the fission yeast centromere provides a model for establishment of centromeric chromatin domains in higher eukaryotes.

Histone H1 of Saccharomyces cerevisiae inhibits transcriptional silencing

Eukaryotic genomes contain euchromatic regions, which are transcriptionally active, and heterochromatic regions, which are repressed. These domains are separated by "barrier elements": DNA sequences that protect euchromatic regions from encroachment by neighboring heterochromatin. To identify proteins that play a role in the function of barrier elements we have carried out a screen in S. cerevisiae. We recovered the gene HHO1, which encodes the yeast ortholog of histone H1, as a high-copy modifier of barrier activity. Histone H1 is a linker histone that binds the outside of nucleosomes and modifies chromatin dynamics. Here we show that Hho1p reinforces the action of several types of barrier elements, and also inhibits silencing on its own.

Dominant mutants of the Saccharomyces cerevisiae ASF1 histone chaperone bypass the need for CAF-1 in transcriptional silencing by altering histone and Sir protein recruitment

Transcriptional silencing involves the formation of specialized repressive chromatin structures. Previous studies have shown that the histone H3-H4 chaperone known as chromatin assembly factor 1 (CAF-1) contributes to transcriptional silencing in yeast, although the molecular basis for this was unknown. In this work we have identified mutations in the nonconserved C terminus of antisilencing function 1 (Asf1) that result in enhanced silencing of HMR and telomere-proximal reporters, overcoming the requirement for CAF-1 in transcriptional silencing. We show that CAF-1 mutants have a drastic reduction in DNA-bound histone H3 levels, resulting in reduced recruitment of Sir2 and Sir4 to the silent loci. C-terminal mutants of another histone H3-H4 chaperone Asf1 restore the H3 levels and Sir protein recruitment to the silent loci in CAF-1 mutants, probably as a consequence of the weakened interaction between these Asf1 mutants and histone H3. As such, these studies have identified the nature of the molecular defect in the silent chromatin structure that results from inactivation of the histone chaperone CAF-1.