Multiple histone modifications in euchromatin promote heterochromatin formation by redundant mechanisms in Saccharomyces cerevisiae

SESAME-catalyzed H3T11 phosphorylation inhibits Dot1-catalyzed H3K79me3 to regulate autophagy and telomere silencing

The glycolytic enzyme, pyruvate kinase Pyk1 maintains telomere heterochromatin by phosphorylating histone H3T11 (H3pT11), which promotes SIR (silent information regulator) complex binding at telomeres and prevents autophagy-mediated Sir2 degradation. However, the exact mechanism of action for H3pT11 is poorly understood. Here, we report that H3pT11 directly inhibits Dot1-catalyzed H3K79 tri-methylation (H3K79me3) and uncover how this histone crosstalk regulates autophagy and telomere silencing. Mechanistically, Pyk1-catalyzed H3pT11 directly reduces the binding of Dot1 to chromatin and inhibits Dot1-catalyzed H3K79me3, which leads to transcriptional repression of autophagy genes and reduced autophagy. Despite the antagonism between H3pT11 and H3K79me3, they work together to promote the binding of SIR complex at telomeres to maintain telomere silencing. Furthermore, we identify Reb1 as a telomere-associated factor that recruits Pyk1-containing SESAME (Serine-responsive SAM-containing Metabolic Enzyme) complex to telomere regions to phosphorylate H3T11 and prevent the invasion of H3K79me3 from euchromatin into heterochromatin to maintain telomere silencing. Together, these results uncover a histone crosstalk and provide insights into dynamic regulation of silent heterochromatin and autophagy in response to cell metabolism.

BAP1 constrains pervasive H2AK119ub1 to control the transcriptional potential of the genome

Histone-modifying systems play fundamental roles in gene regulation and the development of multicellular organisms. Histone modifications that are enriched at gene regulatory elements have been heavily studied, but the function of modifications found more broadly throughout the genome remains poorly understood. This is exemplified by histone H2A monoubiquitylation (H2AK119ub1), which is enriched at Polycomb-repressed gene promoters but also covers the genome at lower levels. Here, using inducible genetic perturbations and quantitative genomics, we found that the BAP1 deubiquitylase plays an essential role in constraining H2AK119ub1 throughout the genome. Removal of BAP1 leads to pervasive genome-wide accumulation of H2AK119ub1, which causes widespread reductions in gene expression. We show that elevated H2AK119ub1 preferentially counteracts Ser5 phosphorylation on the C-terminal domain of RNA polymerase II at gene regulatory elements and causes reductions in transcription and transcription-associated histone modifications. Furthermore, failure to constrain pervasive H2AK119ub1 compromises Polycomb complex occupancy at a subset of Polycomb target genes, which leads to their derepression, providing a potential molecular rationale for why the BAP1 ortholog in Drosophila has been characterized as a Polycomb group gene. Together, these observations reveal that the transcriptional potential of the genome can be modulated by regulating the levels of a pervasive histone modification.

Metabolic regulation of telomere silencing by SESAME complex-catalyzed H3T11 phosphorylation

Telomeres are organized into a heterochromatin structure and maintenance of silent heterochromatin is required for chromosome stability. How telomere heterochromatin is dynamically regulated in response to stimuli remains unknown. Pyruvate kinase Pyk1 forms a complex named SESAME (Serine-responsive SAM-containing Metabolic Enzyme complex) to regulate gene expression by phosphorylating histone H3T11 (H3pT11). Here, we identify a function of SESAME in regulating telomere heterochromatin structure. SESAME phosphorylates H3T11 at telomeres, which maintains SIR (silent information regulator) complex occupancy at telomeres and protects Sir2 from degradation by autophagy. Moreover, SESAME-catalyzed H3pT11 directly represses autophagy-related gene expression to further prevent autophagy-mediated Sir2 degradation. By promoting H3pT11, serine increases Sir2 protein levels and enhances telomere silencing. Loss of H3pT11 leads to reduced Sir2 and compromised telomere silencing during chronological aging. Together, our study provides insights into dynamic regulation of silent heterochromatin by histone modifications and autophagy in response to cell metabolism and aging.

Antisense transcriptional interference mediates condition-specific gene repression in budding yeast

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

Yeast epigenetics: the inheritance of histone modification states

Saccharomyces cerevisiae (budding yeast) and Schizosaccharomyces pombe (fission yeast) are two of the most recognised and well-studied model systems for epigenetic regulation and the inheritance of chromatin states. Their silent loci serve as a proxy for heterochromatic chromatin in higher eukaryotes, and as such both species have provided a wealth of information on the mechanisms behind the establishment and maintenance of epigenetic states, not only in yeast, but in higher eukaryotes. This review focuses specifically on the role of histone modifications in governing telomeric silencing in S. cerevisiae and centromeric silencing in S. pombe as examples of genetic loci that exemplify epigenetic inheritance. We discuss the recent advancements that for the first time provide a mechanistic understanding of how heterochromatin, dictated by histone modifications specifically, is preserved during S-phase. We also discuss the current state of our understanding of yeast nucleosome dynamics during DNA replication, an essential component in delineating the contribution of histone modifications to epigenetic inheritance.

Ten principles of heterochromatin formation and function

Heterochromatin is a key architectural feature of eukaryotic chromosomes, which endows particular genomic domains with specific functional properties. The capacity of heterochromatin to restrain the activity of mobile elements, isolate DNA repair in repetitive regions and ensure accurate chromosome segregation is crucial for maintaining genomic stability. Nucleosomes at heterochromatin regions display histone post-translational modifications that contribute to developmental regulation by restricting lineage-specific gene expression. The mechanisms of heterochromatin establishment and of heterochromatin maintenance are separable and involve the ability of sequence-specific factors bound to nascent transcripts to recruit chromatin-modifying enzymes. Heterochromatin can spread along the chromatin from nucleation sites. The propensity of heterochromatin to promote its own spreading and inheritance is counteracted by inhibitory factors. Because of its importance for chromosome function, heterochromatin has key roles in the pathogenesis of various human diseases. In this Review, we discuss conserved principles of heterochromatin formation and function using selected examples from studies of a range of eukaryotes, from yeast to human, with an emphasis on insights obtained from unicellular model organisms.

Spurious transcription factor binding: non-functional or genetically redundant?

Transcription factor binding sites (TFBSs) on the DNA are generally accepted as the key nodes of gene control. However, the multitudes of TFBSs identified in genome-wide studies, some of them seemingly unconstrained in evolution, have prompted the view that in many cases TF binding may serve no biological function. Yet, insights from transcriptional biochemistry, population genetics and functional genomics suggest that rather than segregating into 'functional' or 'non-functional', TFBS inputs to their target genes may be generally cumulative, with varying degrees of potency and redundancy. As TFBS redundancy can be diminished by mutations and environmental stress, some of the apparently 'spurious' sites may turn out to be important for maintaining adequate transcriptional regulation under these conditions. This has significant implications for interpreting the phenotypic effects of TFBS mutations, particularly in the context of genome-wide association studies for complex traits.

Maintenance of heterochromatin boundary and nucleosome composition at promoters by the Asf1 histone chaperone and SWR1-C chromatin remodeler in Saccharomyces cerevisiae

Chromatin remodeling complexes cooperate to regulate gene promoters and to define chromatin neighborhoods. Here, we identified genetic and functional connections between two silencing-related chromatin factors in the maintenance of native heterochromatic structures and nucleosome composition at promoters. Building on a previously reported link between the histone chaperone Asf1 and the Yaf9 subunit of the SWR1-C chromatin remodeler, we found that ASF1 broadly interacted with genes encoding for SWR1-C subunits. Asf1 and Yaf9 were required for maintaining expression of heterochromatin-proximal genes and they worked cooperatively to prevent repression of telomere-proximal genes by limiting the spread of SIR complexes into nearby regions. Genome-wide Sir2 profiling, however, revealed that the cooperative heterochromatin regulation of Asf1 and SWR1-C occurred only on a subset of yeast telomeres. Extensive analyses demonstrated that formation of aberrant heterochromatin structures in the absence of ASF1 and YAF9 was not causal for the pronounced growth and transcriptional defects in cells lacking both these factors. Instead, genetic and molecular analysis revealed that H3K56 acetylation was required for efficient deposition of H2A.Z at subtelomeric and euchromatic gene promoters, pointing to a role for Asf1-dependent H3K56 acetylation in SWR1-C biology.

Set5 and Set1 cooperate to repress gene expression at telomeres and retrotransposons

A complex interplay between multiple chromatin modifiers is critical for cells to regulate chromatin structure and accessibility during essential DNA-templated processes such as transcription. However, the coordinated activities of these chromatin modifiers in the regulation of gene expression are not fully understood. We previously determined that the budding yeast histone H4 methyltransferase Set5 functions together with Set1, the H3K4 methyltransferase, in specific cellular contexts. Here, we sought to understand the relationship between these evolutionarily conserved enzymes in the regulation of gene expression. We generated a comprehensive genetic interaction map of the functionally uncharacterized Set5 methyltransferase and expanded the existing genetic interactome of the global chromatin modifier Set1, revealing functional overlap of the two enzymes in chromatin-related networks, such as transcription. Furthermore, gene expression profiling via RNA-Seq revealed an unexpected synergistic role of Set1 and Set5 in repressing transcription of Ty transposable elements and genes located in subtelomeric regions. This study uncovers novel pathways in which the methyltransferase Set5 participates and, more importantly, reveals a partnership between Set1 and Set5 in transcriptional repression near repetitive DNA elements in budding yeast. Together, our results define a new functional relationship between histone H3 and H4 methyltransferases, whose combined activity may be implicated in preserving genomic integrity.

SIR proteins and the assembly of silent chromatin in budding yeast

Saccharomyces cerevisiae provides a well-studied model system for heritable silent chromatin in which a histone-binding protein complex [the SIR (silent information regulator) complex] represses gene transcription in a sequence-independent manner by spreading along nucleosomes, much like heterochromatin in higher eukaryotes. Recent advances in the biochemistry and structural biology of the SIR-chromatin system bring us much closer to a molecular understanding of yeast silent chromatin. Simultaneously, genome-wide approaches have shed light on the biological importance of this form of epigenetic repression. Here, we integrate genetic, structural, and cell biological data into an updated overview of yeast silent chromatin assembly.

Sirt1 suppresses RNA synthesis after UV irradiation in combined xeroderma pigmentosum group D/Cockayne syndrome (XP-D/CS) cells

Specific mutations in the XPD subunit of transcription factor IIH result in combined xeroderma pigmentosum (XP)/Cockayne syndrome (CS), a severe DNA repair disorder characterized at the cellular level by a transcriptional arrest following UV irradiation. This transcriptional arrest has always been thought to be the result of faulty transcription-coupled repair. In the present study, we showed that, following UV irradiation, XP-D/CS cells displayed a gross transcriptional dysregulation compared with "pure" XP-D cells or WT cells. Furthermore, global RNA-sequencing analysis showed that XP-D/CS cells repressed the majority of genes after UV, whereas pure XP-D cells did not. By using housekeeping genes as a model, we demonstrated that XP-D/CS cells were unable to reassemble these gene promoters and thus to restart transcription after UV irradiation. Furthermore, we found that the repression of these promoters in XP-D/CS cells was not a simple consequence of deficient repair but rather an active heterochromatinization process mediated by the histone deacetylase Sirt1. Indeed, RNA-sequencing analysis showed that inhibition of and/or silencing of Sirt1 changed the chromatin environment at these promoters and restored the transcription of a large portion of the repressed genes in XP-D/CS cells after UV irradiation. Our work demonstrates that a significant part of the transcriptional arrest displayed by XP-D/CS cells arises as a result of an active repression process and not simply as a result of a DNA repair deficiency. This dysregulation of Sirt1 function that results in transcriptional repression may be the cause of various severe clinical features in patients with XP-D/CS that cannot be explained by a DNA repair defect.

Mechanism for epigenetic variegation of gene expression at yeast telomeric heterochromatin

Yeast contains heterochromatin at telomeres and the silent mating-type loci (HML/HMR). Genes positioned within the telomeric heterochromatin of Saccharomyces cerevisiae switch stochastically between epigenetically bistable ON and OFF expression states. Important aspects of the mechanism of variegated gene expression, including the chromatin structure of the natural ON state and the mechanism by which it is maintained, are unknown. To address this issue, we developed approaches to select cells in the ON and OFF states. We found by chromatin immunoprecipitation (ChIP) that natural ON telomeres are associated with Rap1 binding and, surprisingly, also contain known characteristics of OFF telomeres, including significant amounts of Sir3 and H4K16 deacetylated nucleosomes. Moreover, we found that H3K79 methylation (H3K79me), H3K4me, and H3K36me, which are depleted from OFF telomeres, are enriched at ON telomeres. We demonstrate in vitro that H3K79me, but not H3K4me or H3K36me, disrupts transcriptional silencing. Importantly, H3K79me does not significantly reduce Sir complex binding in vivo or in vitro. Finally, we show that maintenance of H3K79me at ON telomeres is dependent on transcription. Therefore, although Sir proteins are required for silencing, we propose that epigenetic variegation of telomeric gene expression is due to the bistable enrichment/depletion of H3K79me and not the fluctuation in the amount of Sir protein binding to nucleosomes.

Deficiency of H3K79 histone methyltransferase Dot1-like protein (DOT1L) inhibits cell proliferation

Dot1-like protein (DOT1L) is an evolutionarily conserved histone methyltransferase that methylates lysine 79 of histone H3 (H3K79). Mammalian DOT1L participates in the regulation of transcription, development, erythropoiesis, differentiation, and proliferation of normal cells. However, the role of DOT1L in cancer cell proliferation has not been fully elucidated. DOT1L siRNA-transfected A549 or NCI-H1299 lung cancer cells displayed a nonproliferating multinucleated phenotype. DOT1L-deficient cells also showed abnormal mitotic spindle formation and centrosome number, suggesting that DOT1L deficiency leads to chromosomal missegregation. This chromosomal instability in DOT1L-deficient cells led to cell cycle arrest at the G(1) phase and induced senescence as determined by enhanced activity of senescence-associated β-galactosidase activity. Meanwhile, overexpression of a catalytically active DOT1L, not an inactive mutant, restored DOT1L siRNA-induced phenotypes. Overall, these data imply that down-regulation of DOT1L-mediated H3K79 methylation disturbs proliferation of human cells. In addition, although H3K79 methylation is down-regulated in aged tissues, it is up-regulated in lung cancer cell lines and tumor tissues of lung cancer patients. Therefore, H3K79 methylation is a critical histone modification that regulates cell proliferation and would be a novel histone mark for aging and cancer.

The inheritance of histone modifications depends upon the location in the chromosome in Saccharomyces cerevisiae

Histone modifications are important epigenetic features of chromatin that must be replicated faithfully. However, the molecular mechanisms required to duplicate and maintain histone modification patterns in chromatin remain to be determined. Here, we show that the introduction of histone modifications into newly deposited nucleosomes depends upon their location in the chromosome. In Saccharomyces cerevisiae, newly deposited nucleosomes consisting of newly synthesized histone H3-H4 tetramers are distributed throughout the entire chromosome. Methylation of lysine 4 on histone H3 (H3-K4), a hallmark of euchromatin, is introduced into these newly deposited nucleosomes, regardless of whether the neighboring preexisting nucleosomes harbor the K4 mutation in histone H3. Furthermore, if the heterochromatin-binding protein Sir3 is unavailable during DNA replication, histone H3-K4 methylation is introduced onto newly deposited nucleosomes in telomeric heterochromatin. Thus, a conservative distribution model most accurately explains the inheritance of histone modifications because the location of histones within euchromatin or heterochromatin determines which histone modifications are introduced.

Chromatin structure following UV-induced DNA damage-repair or death?

In eukaryotes, DNA is compacted into a complex structure known as chromatin. The unravelling of DNA is a crucial step in DNA repair, replication, transcription and recombination as this allows access to DNA for these processes. Failure to package DNA into the nucleosome, the individual unit of chromatin, can lead to genomic instability, driving a cell into apoptosis, senescence, or cellular proliferation. Ultraviolet (UV) radiation damage causes destabilisation of chromatin integrity. UV irradiation induces DNA damage such as photolesions and subjects the chromatin to substantial rearrangements, causing the arrest of transcription forks and cell cycle arrest. Highly conserved processes known as nucleotide and base excision repair (NER and BER) then begin to repair these lesions. However, if DNA repair fails, the cell may be forced into apoptosis. The modification of various histones as well as nucleosome remodelling via ATP-dependent chromatin remodelling complexes are required not only to repair these UV-induced DNA lesions, but also for apoptosis signalling. Histone modifications and nucleosome remodelling in response to UV also lead to the recruitment of various repair and pro-apoptotic proteins. Thus, the way in which a cell responds to UV irradiation via these modifications is important in determining its fate. Failure of these DNA damage response steps can lead to cellular proliferation and oncogenic development, causing skin cancer, hence these chromatin changes are critical for a proper response to UV-induced injury.

A dual role of H4K16 acetylation in the establishment of yeast silent chromatin

Discrete regions of the eukaryotic genome assume heritable chromatin structure that is refractory to transcription. In budding yeast, silent chromatin is characterized by the binding of the Silent Information Regulatory (Sir) proteins to unmodified nucleosomes. Using an in vitro reconstitution assay, which allows us to load Sir proteins onto arrays of regularly spaced nucleosomes, we have examined the impact of specific histone modifications on Sir protein binding and linker DNA accessibility. Two typical marks for active chromatin, H3K79(me) and H4K16(ac) decrease the affinity of Sir3 for chromatin, yet only H4K16(ac) affects chromatin structure, as measured by nuclease accessibility. Surprisingly, we found that the Sir2-4 subcomplex, unlike Sir3, has higher affinity for chromatin carrying H4K16(ac). NAD-dependent deacetylation of H4K16(ac) promotes binding of the SIR holocomplex but not of the Sir2-4 heterodimer. This function of H4K16(ac) cannot be substituted by H3K56(ac). We conclude that acetylated H4K16 has a dual role in silencing: it recruits Sir2-4 and repels Sir3. Moreover, the deacetylation of H4K16(ac) by Sir2 actively promotes the high-affinity binding of the SIR holocomplex.

Regulation of the DNA damage response and gene expression by the Dot1L histone methyltransferase and the 53Bp1 tumour suppressor

Background

Dot1L, a histone methyltransferase that targets histone H3 lysine 79 (H3K79), has been implicated in gene regulation and the DNA damage response although its functions in these processes remain poorly defined.

Methodology/Principal Findings

Using the chicken DT40 model system, we generated cells in which the Dot1L gene is disrupted to examine the function and focal recruitment of the 53Bp1 DNA damage response protein. Detailed kinetic and dose response assays demonstrate that, despite the absence of H3K79 methylation demonstrated by mass spectrometry, 53Bp1 focal recruitment is not compromised in these cells. We also describe, for the first time, the phenotypes of a cell line lacking both Dot1L and 53Bp1. Dot1L^−/− and wild type cells are equally resistant to ionising radiation, whereas 53Bp1^−/−/Dot1L^−/− cells display a striking DNA damage resistance phenotype. Dot1L and 53Bp1 also affect the expression of many genes. Loss of Dot1L activity dramatically alters the mRNA levels of over 1200 genes involved in diverse biological functions. These results, combined with the previously reported list of differentially expressed genes in mouse ES cells knocked down for Dot1L, demonstrates surprising cell type and species conservation of Dot1L-dependent gene expression. In 53Bp1^−/− cells, over 300 genes, many with functions in immune responses and apoptosis, were differentially expressed. To date, this is the first global analysis of gene expression in a 53Bp1-deficient cell line.

Conclusions/Significance

Taken together, our results uncover a negative role for Dot1L and H3K79 methylation in the DNA damage response in the absence of 53Bp1. They also enlighten the roles of Dot1L and 53Bp1 in gene expression and the control of DNA double-strand repair pathways in the context of chromatin.

Dot1 binding induces chromatin rearrangements by histone methylation-dependent and -independent mechanisms

Background

Methylation of histone H3 lysine 79 (H3K79) by Dot1 is highly conserved among species and has been associated with both gene repression and activation. To eliminate indirect effects and examine the direct consequences of Dot1 binding and H3K79 methylation, we investigated the effects of targeting Dot1 to different positions in the yeast genome.

Results

Targeting Dot1 did not activate transcription at a euchromatic locus. However, chromatin-bound Dot1 derepressed heterochromatin-mediated gene silencing over a considerable distance. Unexpectedly, Dot1-mediated derepression was established by both a H3K79 methylation-dependent and a methylation-independent mechanism; the latter required the histone acetyltransferase Gcn5. By monitoring the localization of a fluorescently tagged telomere in living cells, we found that the targeting of Dot1, but not its methylation activity, led to the release of a telomere from the repressive environment at the nuclear periphery. This probably contributes to the activity-independent derepression effect of Dot1.

Conclusions

Targeting of Dot1 promoted gene expression by antagonizing gene repression through both histone methylation and chromatin relocalization. Our findings show that binding of Dot1 to chromatin can positively affect local gene expression by chromatin rearrangements over a considerable distance.

The Saccharomyces cerevisiae anaphase-promoting complex interacts with multiple histone-modifying enzymes to regulate cell cycle progression

The anaphase-promoting complex (APC), a large evolutionarily conserved ubiquitin ligase complex, regulates cell cycle progression through mitosis and G(1). Here, we present data suggesting that APC-dependent cell cycle progression relies on a specific set of posttranslational histone-modifying enzymes. Multiple APC subunit mutants were impaired in total and modified histone H3 protein content. Acetylated H3K56 (H3K56(Ac)) levels were as reduced as those of total H3, indicating that loading histones with H3K56(Ac) is unaffected in APC mutants. However, under restrictive conditions, H3K9(Ac) and dimethylated H3K79 (H3K79(me2)) levels were more greatly reduced than those of total H3. In a screen for histone acetyltransferase (HAT) and histone deacetylase (HDAC) mutants that genetically interact with the apc5(CA) (chromatin assembly) mutant, we found that deletion of GCN5 or ELP3 severely hampered apc5(CA) temperature-sensitive (ts) growth. Further analyses showed that (i) the elp3Δ gcn5Δ double mutant ts defect was epistatic to that observed in apc5(CA) cells; (ii) gcn5Δ and elp3Δ mutants accumulate in mitosis; and (iii) turnover of the APC substrate Clb2 is not impaired in elp3Δ gcn5Δ cells. Increased expression of ELP3 and GCN5, as well as genes encoding the HAT Rtt109 and the chromatin assembly factors Msi1 and Asf1, suppressed apc5(CA) defects, while increased APC5 expression partially suppressed elp3Δ gcn5Δ growth defects. Finally, we demonstrate that Gcn5 is unstable during G(1) and following G(1) arrest and is stabilized in APC mutants. We present our working model in which Elp3/Gcn5 and the APC work together to facilitate passage through mitosis and G(1). To progress into S, we propose that at least Gcn5 must then be targeted for degradation in an APC-dependent fashion.

The ubiquitin-conjugating enzyme HR6B is required for maintenance of X chromosome silencing in mouse spermatocytes and spermatids

BACKGROUND

The ubiquitin-conjugating enzyme HR6B is required for spermatogenesis in mouse. Loss of HR6B results in aberrant histone modification patterns on the trancriptionally silenced X and Y chromosomes (XY body) and on centromeric chromatin in meiotic prophase. We studied the relationship between these chromatin modifications and their effects on global gene expression patterns, in spermatocytes and spermatids.

RESULTS

HR6B is enriched on the XY body and on centromeric regions in pachytene spermatocytes. Global gene expression analyses revealed that spermatid-specific single- and multicopy X-linked genes are prematurely expressed in Hr6b knockout spermatocytes. Very few other differences in gene expression were observed in these cells, except for upregulation of major satellite repeat transcription. In contrast, in Hr6b knockout spermatids, 7298 genes were differentially expressed; 65% of these genes was downregulated, but we observed a global upregulation of gene transcription from the X chromosome. In wild type spermatids, approximately 20% of the single-copy X-linked genes reach an average expression level that is similar to the average expression from autosomes.

CONCLUSIONS

Spermatids maintain an enrichment of repressive chromatin marks on the X chromosome, originating from meiotic prophase, but this does not interfere with transcription of the single-copy X-linked genes that are reactivated or specifically activated in spermatids. HR6B represses major satellite repeat transcription in spermatocytes, and functions in the maintenance of X chromosome silencing in spermatocytes and spermatids. It is discussed that these functions involve modification of chromatin structure, possibly including H2B ubiquitylation.

Epigenome manipulation as a pathway to new natural product scaffolds and their congeners

The covalent modification of chromatin is an important control mechanism used by fungi to modulate the transcription of genes involved in secondary metabolite production. To date, both molecular-based and chemical approaches targeting histone and DNA posttranslational processes have shown great potential for rationally directing the activation and/or suppression of natural-product-encoding gene clusters. In this Highlight, the organization of the fungal epigenome is summarized and strategies for manipulating chromatin-related targets are presented. Applications of these techniques are illustrated using several recently published accounts in which chemical-epigenetic methods and mutant studies were successfully employed for the de novo or enhanced production of structurally diverse fungal natural products (e.g., anthraquinones, cladochromes, lunalides, mycotoxins, and nygerones).