Assays for nucleosome positioning in yeast

Subtracting the sequence bias from partially digested MNase-seq data reveals a general contribution of TFIIS to nucleosome positioning

BACKGROUND

TFIIS stimulates RNA cleavage by RNA polymerase II and promotes the resolution of backtracking events. TFIIS acts in the chromatin context, but its contribution to the chromatin landscape has not yet been investigated. Co-transcriptional chromatin alterations include subtle changes in nucleosome positioning, like those expected to be elicited by TFIIS, which are elusive to detect. The most popular method to map nucleosomes involves intensive chromatin digestion by micrococcal nuclease (MNase). Maps based on these exhaustively digested samples miss any MNase-sensitive nucleosomes caused by transcription. In contrast, partial digestion approaches preserve such nucleosomes, but introduce noise due to MNase sequence preferences. A systematic way of correcting this bias for massively parallel sequencing experiments is still missing.

RESULTS

To investigate the contribution of TFIIS to the chromatin landscape, we developed a refined nucleosome-mapping method in Saccharomyces cerevisiae. Based on partial MNase digestion and a sequence-bias correction derived from naked DNA cleavage, the refined method efficiently mapped nucleosomes in promoter regions rich in MNase-sensitive structures. The naked DNA correction was also important for mapping gene body nucleosomes, particularly in those genes whose core promoters contain a canonical TATA element. With this improved method, we analyzed the global nucleosomal changes caused by lack of TFIIS. We detected a general increase in nucleosomal fuzziness and more restricted changes in nucleosome occupancy, which concentrated in some gene categories. The TATA-containing genes were preferentially associated with decreased occupancy in gene bodies, whereas the TATA-like genes did so with increased fuzziness. The detected chromatin alterations correlated with functional defects in nascent transcription, as revealed by genomic run-on experiments.

CONCLUSIONS

The combination of partial MNase digestion and naked DNA correction of the sequence bias is a precise nucleosomal mapping method that does not exclude MNase-sensitive nucleosomes. This method is useful for detecting subtle alterations in nucleosome positioning produced by lack of TFIIS. Their analysis revealed that TFIIS generally contributed to nucleosome positioning in both gene promoters and bodies. The independent effect of lack of TFIIS on nucleosome occupancy and fuzziness supports the existence of alternative chromatin dynamics during transcription elongation.

Histone Chaperone Nap1 Is a Major Regulator of Histone H2A-H2B Dynamics at the Inducible GAL Locus

Histone chaperones, like nucleosome assembly protein 1 (Nap1), play a critical role in the maintenance of chromatin architecture. Here, we use the GAL locus in Saccharomyces cerevisiae to investigate the influence of Nap1 on chromatin structure and histone dynamics during distinct transcriptional states. When the GAL locus is not expressed, cells lacking Nap1 show an accumulation of histone H2A-H2B but not histone H3-H4 at this locus. Excess H2A-H2B interacts with the linker DNA between nucleosomes, and the interaction is independent of the inherent DNA-binding affinity of H2A-H2B for these particular sequences as measured in vitro When the GAL locus is transcribed, excess H2A-H2B is reversed, and levels of all chromatin-bound histones are depleted in cells lacking Nap1. We developed an in vivo system to measure histone exchange at the GAL locus and observed considerable variability in the rate of exchange across the locus in wild-type cells. We recapitulate this variability with in vitro nucleosome reconstitutions, which suggests a contribution of DNA sequence to histone dynamics. We also find that Nap1 is required for transcription-dependent H2A-H2B exchange. Altogether, these results indicate that Nap1 is essential for maintaining proper chromatin composition and modulating the exchange of H2A-H2B in vivo.

Mediator, TATA-binding protein, and RNA polymerase II contribute to low histone occupancy at active gene promoters in yeast

Transcription by RNA polymerase II (Pol II) in eukaryotes requires the Mediator complex, and often involves chromatin remodeling and histone eviction at active promoters. Here we address the role of Mediator in recruitment of the Swi/Snf chromatin remodeling complex and its role, along with components of the preinitiation complex (PIC), in histone eviction at inducible and constitutively active promoters in the budding yeast Saccharomyces cerevisiae. We show that recruitment of the Swi/Snf chromatin remodeling complex to the induced CHA1 promoter, as well as its association with several constitutively active promoters, depends on the Mediator complex but is independent of Mediator at the induced MET2 and MET6 genes. Although transcriptional activation and histone eviction at CHA1 depends on Swi/Snf, Swi/Snf recruitment is not sufficient for histone eviction at the induced CHA1 promoter. Loss of Swi/Snf activity does not affect histone occupancy of several constitutively active promoters; in contrast, higher histone occupancy is seen at these promoters in Mediator and PIC component mutants. We propose that an initial activator-dependent, nucleosome remodeling step allows PIC components to outcompete histones for occupancy of promoter sequences. We also observe reduced promoter association of Mediator and TATA-binding protein in a Pol II (rpb1-1) mutant, indicating mutually cooperative binding of these components of the transcription machinery and indicating that it is the PIC as a whole whose binding results in stable histone eviction.

Functions of protosilencers in the formation and maintenance of heterochromatin in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, transcriptionally silent heterochromatin at HML and HMR loci is established by silencers that recruit SIR complex and promote its propagation along chromatin. Silencers consist of various combinations of two or three binding sites for origin recognition complex (ORC), Abf1 and Rap1. A single ORC, Abf1 or Rap1 site cannot promote silencing, but can enhance silencing by a distant silencer, and is called a protosilencer. The mechanism of protosilencer function is not known. We examine the functions of ORC, Abf1 and Rap1 sites as components of the HMR-E silencer, and as protosilencers. We find that the Rap1 site makes a larger and unique contribution to HMR-E function compared to ORC and Abf1 sites. On the other hand, Rap1 site does not act as a protosilencer to assist HML-E silencer in forming heterochromatin, whereas ORC and Abf1 sites do. Therefore, different mechanisms may be involved in the roles of Rap1 site as a component of HMR-E and as a protosilencer. Heterochromatin formed by ORC or Abf1 site in collaboration with HML-E is not as stable as that formed by HMR-E and HML-E, but increasing the copy number of Abf1 site enhances heterochromatin stability. ORC and Abf1 sites acting as protosilencers do not modulate chromatin structure in the absence of SIR complex, which argues against the hypothesis that protosilencers serve to create a chromatin structure favorable for SIR complex propagation. We also investigate the function of ARS1 containing an ORC site and an Abf1 site as a protosilencer. We find that ARS1 inserted at HML enhances heterochromatin stability, and promotes de novo formation of a chromatin structure that partially resembles heterochromatin in an S phase dependent manner. Taken together, our results indicate that protosilencers aid in the formation and maintenance of heterochromatin structure.

Differential contributions of histone H3 and H4 residues to heterochromatin structure

Transcriptional silencing in Saccharomyces cerevisiae is mediated by heterochromatin. There is a plethora of information regarding the roles of histone residues in transcriptional silencing, but exactly how histone residues contribute to heterochromatin structure is not resolved. We address this question by testing the effects of a series of histone H3 and H4 mutations involving residues in their aminoterminal tails, on the solvent-accessible and lateral surfaces of the nucleosome, and at the interface of the H3/H4 tetramer and H2A/H2B dimer on heterochromatin structure and transcriptional silencing. The general state, stability, and conformational heterogeneity of chromatin are examined with a DNA topology-based assay, and the primary chromatin structure is probed by micrococcal nuclease. We demonstrate that the histone mutations differentially affect heterochromatin. Mutations of lysine 16 of histone H4 (H4-K16) and residues in the LRS (loss of rDNA silencing) domain of nucleosome surface markedly alter heterochromatin structure, supporting the notion that H4-K16 and LRS play key roles in heterochromatin formation. Deletion of the aminoterminal tail of H3 moderately alters heterochromatin structure. Interestingly, a group of mutations in the globular domains of H3 and H4 that abrogate or greatly reduce transcriptional silencing increase the conformational heterogeneity and/or reduce the stability of heterochromatin without affecting its overall structure. Surprisingly, yet another series of mutations abolish or reduce silencing without significantly affecting the structure, stability, or conformational heterogeneity of heterochromatin. Therefore, histone residues may contribute to the structure, stability, conformational heterogeneity, or other yet-to-be-characterized features of heterochromatin important for transcriptional silencing.

Roles of chromatin remodeling factors in the formation and maintenance of heterochromatin structure

Heterochromatin consists of highly ordered nucleosomes with characteristic histone modifications. There is evidence implicating chromatin remodeling proteins in heterochromatin formation, but their exact roles are not clear. We demonstrate in Saccharomyces cerevisiae that the Fun30p and Isw1p chromatin remodeling factors are similarly required for transcriptional silencing at the HML locus, but they differentially contribute to the structure and stability of HML heterochromatin. In the absence of Fun30p, only a partially silenced structure is established at HML. Such a structure resembles fully silenced heterochromatin in histone modifications but differs markedly from both fully silenced and derepressed chromatin structures regarding nucleosome arrangement. This structure likely represents an intermediate state of heterochromatin that can be converted by Fun30p to the mature state. Moreover, Fun30p removal reduces the rate of de novo establishment of heterochromatin, suggesting that Fun30p assists the silencing machinery in forming heterochromatin. We also find evidence suggesting that Fun30p functions together with, or after, the action of the silencing machinery. On the other hand, Isw1p is dispensable for the formation of heterochromatin structure but is instead critically required for maintaining its stability. Therefore, chromatin remodeling proteins may rearrange nucleosomes during the formation of heterochromatin or serve to stabilize/maintain heterochromatin structure.

The effect of micrococcal nuclease digestion on nucleosome positioning data

Eukaryotic genomes are packed into chromatin, whose basic repeating unit is the nucleosome. Nucleosome positioning is a widely researched area. A common experimental procedure to determine nucleosome positions involves the use of micrococcal nuclease (MNase). Here, we show that the cutting preference of MNase in combination with size selection generates a sequence-dependent bias in the resulting fragments. This strongly affects nucleosome positioning data and especially sequence-dependent models for nucleosome positioning. As a consequence we see a need to re-evaluate whether the DNA sequence is a major determinant of nucleosome positioning in vivo. More generally, our results show that data generated after MNase digestion of chromatin requires a matched control experiment in order to determine nucleosome positions.

Isolation of highly purified yeast nuclei for nuclease mapping of chromatin structure

Probing chromatin structure with nucleases is a well-established method for determining the accessibility of DNA to gene regulatory proteins and measuring competency for transcription. A hallmark of many silent genes is the presence of translationally positioned nucleosomes over their promoter regions, which can be inferred by the sensitivity of the underlying DNA to nucleases, particularly micrococcal nuclease. The quality of this data is highly dependent upon the nuclear preparation, especially if the digestion products are analyzed by high-resolution detection methods such as reiterative primer extension. Here we describe a method to isolate highly purified nuclei from the budding yeast Saccharomyces cerevisiae and the use of micrococcal nuclease to map the positions of nucleosomes at the RNR3 gene. Nuclei isolated by this procedure are competent for many of the commonly used chromatin mapping and detection procedures.

The promoter and transcribed regions of the Leishmania tarentolae spliced leader RNA gene array are devoid of nucleosomes

BACKGROUND

The spliced leader (SL) RNA provides the 5' m7G cap and first 39 nt for all nuclear mRNAs in kinetoplastids. This small nuclear RNA is transcribed by RNA polymerase II from individual promoters. In Leishmania tarentolae the SL RNA genes reside in two multi-copy tandem arrays designated MINA and MINB. The transcript accumulation from the SL promoter on the drug-selected, episomal SL RNA gene cassette pX-tSL is ~10% that of the genomic array in uncloned L. tarentolae transfectants. This disparity is neither sequence- nor copy-number related, and thus may be due to interference of SL promoter function by epigenetic factors. To explore these possibilities we examined the nucleoplasmic localization of the SL RNA genes as well as their nucleosomal architecture.

RESULTS

The genomic SL RNA genes and the episome did not co-localize within the nucleus. Each genomic repeat contains one nucleosome regularly positioned within the non-transcribed intergenic region. The 363-bp MINA array was resistant to micrococcal nuclease digestion between the -258 and -72 positions relative to the transcription start point due to nucleosome association, leaving the promoter elements and the entire transcribed region exposed for protein interactions. A pattern of ~164-bp protected segments was observed, corresponding to the amount of DNA typically bound by a nucleosome. By contrast, nucleosomes on the pX-tSL episome were randomly distributed over the episomal SL cassette, reducing transcription factor access to the episomal promoter by approximately 74%. Cloning of the episome transfectants revealed a range of transcriptional activities, implicating a mechanism of epigenetic heredity.

CONCLUSION

The disorganized nucleosomes on the pX episome are in a permissive conformation for transcription of the SL RNA cassette approximately 25% of the time within a given parasite. Nucleosome interference is likely the major factor in the apparent transcriptional repression of the SL RNA gene cassette. Coupled with the requirement for run-around transcription that drives expression of the selectable drug marker, transcription of the episomal SL may be reduced even further due to sub-optimal nucleoplasmic localization and initiation complex disruption.

Asymmetric positioning of nucleosomes and directional establishment of transcriptionally silent chromatin by Saccharomyces cerevisiae silencers

In Saccharomyces cerevisiae, silencers flanking the HML and HMR loci consist of various combinations of binding sites for Abf1p, Rap1p, and the origin recognition complex (ORC) that serve to recruit the Sir silencing complex, thereby initiating the establishment of transcriptionally silent chromatin. There have been seemingly conflicting reports concerning whether silencers function in an orientation-dependent or -independent manner, and what determines the directionality of a silencer has not been explored. We demonstrate that chromatin plays a key role in determining the potency and directionality of silencers. We show that nucleosomes are asymmetrically distributed around the HML-I or HMR-E silencer so that a nucleosome is positioned close to the Abf1p side but not the ORC side of the silencer. This coincides with preferential association of Sir proteins and transcriptional silencing on the Abf1p side of the silencer. Elimination of the asymmetry in nucleosome positioning at a silencer leads to comparable silencing on both sides. Asymmetric nucleosome positioning in the immediate vicinity of a silencer is independent of its orientation and genomic context, indicating that it is the inherent property of the silencer. Moreover, it is also independent of the Sir complex and thus precedes the formation of silent chromatin. Finally, we demonstrate that asymmetric positioning of nucleosomes and directional silencing by a silencer depend on ORC and Abf1p. We conclude that the HML-I and HMR-E silencers promote asymmetric positioning of nucleosomes, leading to unequal potentials of transcriptional silencing on their sides and, hence, directional silencing.

Molecular genetic analysis of the yeast repressor Rfx1/Crt1 reveals a novel two-step regulatory mechanism

In Saccharomyces cerevisiae, the repressor Crt1 and the global corepressor Ssn6-Tup1 repress the DNA damage-inducible ribonucleotide reductase (RNR) genes. Initiation of DNA damage signals causes the release of Crt1 and Ssn6-Tup1 from the promoter, coactivator recruitment, and derepression of transcription, indicating that Crt1 plays a crucial role in the switch between gene repression and activation. Here we have mapped the functional domains of Crt1 and identified two independent repression domains and a region required for gene activation. The N terminus of Crt1 is the major repression domain, it directly binds to the Ssn6-Tup1 complex, and its repression activities are dependent upon Ssn6-Tup1 and histone deacetylases (HDACs). In addition, we identified a C-terminal repression domain, which is independent of Ssn6-Tup1 and HDACs and functions at native genes in vivo. Furthermore, we show that TFIID and SWI/SNF bind to a region within the N terminus of Crt1, overlapping with but distinct from the Ssn6-Tup1 binding and repression domain, suggesting that Crt1 may have activator functions. Crt1 mutants were constructed to dissect its activator and repressor functions. All of the mutants were competent for repression of the DNA damage-inducible genes, but a majority were "derepression-defective" mutants. Further characterization of these mutants indicated that they are capable of receiving DNA damage signals and releasing the Ssn6-Tup1 complex from the promoter but are selectively impaired for TFIID and SWI/SNF recruitment. These results imply a two-step activation model of the DNA damage-inducible genes and that Crt1 functions as a signal-dependent dual-transcription activator and repressor that acts in a transient manner.

Cell cycle regulation of chromatin at an origin of DNA replication

Selection and licensing of mammalian DNA replication origins may be regulated by epigenetic changes in chromatin structure. The Epstein-Barr virus (EBV) origin of plasmid replication (OriP) uses the cellular licensing machinery to regulate replication during latent infection of human cells. We found that the minimal replicator sequence of OriP, referred to as the dyad symmetry (DS), is flanked by nucleosomes. These nucleosomes were subject to cell cycle-dependent chromatin remodeling and histone modifications. Restriction enzyme accessibility assay indicated that the DS-bounded nucleosomes were remodeled in late G1. Remarkably, histone H3 acetylation of DS-bounded nucleosomes decreased during late G1, coinciding with nucleosome remodeling and MCM3 loading, and preceding the onset of DNA replication. The ATP-dependent chromatin-remodeling factor SNF2h was also recruited to DS in late G1, and formed a stable complex with HDAC2 at DS. siRNA depletion of SNF2h reduced G1-specific nucleosome remodeling, histone deacetylation, and MCM3 loading at DS. We conclude that an SNF2h-HDAC1/2 complex coordinates G1-specific chromatin remodeling and histone deacetylation with the DNA replication initiation process at OriP.

ORC, MCM, and histone hyperacetylation at the Kaposi's sarcoma-associated herpesvirus latent replication origin

The viral genome of Kaposi's sarcoma-associated herpesvirus (KSHV) persists as an extrachromosomal plasmid in latently infected cells. The KSHV latency-associated nuclear antigen (LANA) stimulates plasmid maintenance and DNA replication by binding to an approximately 150-bp region within the viral terminal repeats (TR). We have used chromatin immunoprecipitation assays to demonstrate that LANA binds specifically to the replication origin sequence within the KSHV TR in latently infected cells. The latent replication origin within the TR was also bound by LANA-associated proteins CBP, double-bromodomain-containing protein 2 (BRD2), and the origin recognition complex 2 protein (ORC2) and was enriched in hyperacetylated histones H3 and H4 relative to other regions of the latent genome. Cell cycle analysis indicated that the minichromosome maintenance complex protein, MCM3, bound TR in late-G(1)/S-arrested cells, which coincided with the loss of histone H3 K4 methylation. Micrococcal nuclease studies revealed that TRs are embedded in a highly ordered nucleosome array that becomes disorganized in late G(1)/S phase. ORC binding to TR was LANA dependent when reconstituted in transfected plasmids. DNA affinity purification confirmed that LANA, CBP, BRD2, and ORC2 bound TR specifically and identified the histone acetyltransferase HBO1 (histone acetyltransferase binding to ORC1) as a potential TR binding protein. Disruption of ORC2, MCM5, and HBO1 expression by small interfering RNA reduced LANA-dependent DNA replication of TR-containing plasmids. These findings are the first demonstration that cellular replication and origin licensing factors are required for KSHV latent cycle replication. These results also suggest that the KSHV latent origin of replication is a unique chromatin environment containing histone H3 hyperacetylation within heterochromatic tandem repeats.

Redundant mechanisms are used by Ssn6-Tup1 in repressing chromosomal gene transcription in Saccharomyces cerevisiae

The Ssn6-Tup1 corepressor complex regulates many genes in Saccharomyces cerevisiae. Three mechanisms have been proposed to explain its repression functions: 1) nucleosome positioning by binding histone tails; 2) recruitment of histone deacetylases; and 3) direct interference with the general transcription machinery or activators. It is unclear if Ssn6-Tup1 utilizes each of these mechanisms at a single gene in a redundant manner or each individually at different loci. A systematic analysis of the contribution of each mechanism at a native promoter has not been reported. Here we employed a genetic strategy to analyze the contributions of nucleosome positioning, histone deacetylation, and Mediator interference in the repression of chromosomal Tup1 target genes in vivo. We exploited the fact that Ssn6-Tup1 requires the ISW2 chromatin remodeling complex to establish nucleosome positioning in vivo to disrupt chromatin structure without affecting other Tup1 repression functions. Deleting ISW2, the histone deacetylase gene HDA1, or genes encoding Mediator subunits individually caused slight or no derepression of RNR3 and HUG1. However, when Mediator mutations were combined with Deltaisw2 or Deltahda1 mutations, enhanced transcription was observed, and the strongest level of derepression was observed in triple Deltaisw2/Deltahda1/Mediator mutants. The increased transcription in the mutants was not due to the loss of Tup1 at the promoter and correlated with increased TBP cross-linking to promoters. Thus, Tup1 utilizes multiple redundant mechanisms to repress transcription of native genes, which may be important for it to act as a global corepressor at a wide variety of promoters.

In vivo interactions of the Acanthamoeba TBP gene promoter

Transcription of the TATA box binding protein (TBP) gene in Acanthamoeba castellanii is regulated by TATA box binding protein promoter binding factor (TPBF), which binds to an upstream TBP promoter element to stimulate transcription, and to a TATA proximal element, where it represses transcription. In order to extend these observations to the in vivo chromatin context, the TBP gene was examined by in situ footprinting and chromatin immunoprecipitation (ChIP). Acanthamoeba DNA is nucleosomal with a repeat of approximately 160 bp, and an intranucleosomal DNA periodicity of 10.5 bp. The TBP gene comprises a 220 bp micrococcal nuclease hypersensitive site corresponding to the promoter regulatory elements previously identified, flanked by protected regions of a size consistent with the presence of nucleosomes. ChIP data indicated that TPBF is associated with the TBP, TPBF and MIL gene promoters, but not to the CSP21, MIIHC, 5SrRNA or 39SrRNA promoters, or to the MIL gene C-terminal region. Binding by TPBF to the TPBF and MIL gene promoters was confirmed by in vitro assays. These results validate the in vitro model for TBP gene regulation and further suggest that TPBF may be autoregulated and may participate in the regulation of the MIL gene.

Chromatin assembly factor I mutants defective for PCNA binding require Asf1/Hir proteins for silencing

Chromatin assembly factor I (CAF-I) is a conserved histone H3/H4 deposition complex. Saccharomyces cerevisiae mutants lacking CAF-I subunit genes (CAC1 to CAC3) display reduced heterochromatic gene silencing. In a screen for silencing-impaired cac1 alleles, we isolated a mutation that reduced binding to the Cac3p subunit and another that impaired binding to the DNA replication protein PCNA. Surprisingly, mutations in Cac1p that abolished PCNA binding resulted in very minor telomeric silencing defects but caused silencing to be largely dependent on Hir proteins and Asf1p, which together comprise an alternative silencing pathway. Consistent with these phenotypes, mutant CAF-I complexes defective for PCNA binding displayed reduced nucleosome assembly activity in vitro but were stimulated by Asf1p-histone complexes. Furthermore, these mutant CAF-I complexes displayed a reduced preference for depositing histones onto newly replicated DNA. We also observed a weak interaction between Asf1p and Cac2p in vitro, and we hypothesize that this interaction underlies the functional synergy between these histone deposition proteins.

Ssn6-Tup1 regulates RNR3 by positioning nucleosomes and affecting the chromatin structure at the upstream repression sequence

The DNA damage inducible gene ribonucleotide reductase (RNR3) is regulated by a transcriptional repression mechanism by the recruitment of the Ssn6-Tup1 corepressor complex to its promoter by the sequence-specific DNA-binding protein Crt1. Ssn6-Tup1 is reported to represses transcription by interfering with transcription factors, recruiting histone deacetylases, and positioning nucleosomes at the promoter of its target genes. Two of the three mechanisms involve effects on chromatin structure, and therefore, we have delineated the nucleosomal structure of RNR3 in the repressed and derepressed state using multiple nuclease mapping strategies. A regular array of positioned nucleosomes is detected over the repressed RNR3 promoter that extends into the coding sequence. Treating cells with DNA damaging agents or deleting CRT1, SSN6, or TUP1 derepresses RNR3 transcription, and causes a dramatic disruption of nucleosome positioning over its promoter. Furthermore, derepression of RNR3 correlated with changes in nuclease sensitivity within the upstream repression sequence (URS) region. Specifically, the loss of a MNase-hypersensitive site, and the appearance of strong DNase I hypersensitivity, was observed over the URS. Interestingly, we find that the binding of Crt1 to the promoter in the absence of Ssn6 or Tup1 is insufficient for nucleosome positioning or regulating chromatin structure at the URS; thus, these two functions are strictly dependent upon Ssn6-Tup1. We propose that RNR3 is regulated by changes in nucleosome positioning and chromatin structure that are mediated by Ssn6, Tup1, and Crt1.