Substitutions in the amino-terminal tail of neurospora histone H3 have varied effects on DNA methylation

H3T11 phosphorylation by CKII is required for heterochromatin formation in Neurospora

Heterochromatin is a key feature of eukaryotic genomes and is crucial for maintaining genomic stability. In fission yeast, heterochromatin nucleation is mainly mediated by DNA-binding proteins or the RNA interference (RNAi) pathway. In the filamentous fungus Neurospora crassa, however, the mechanism that causes the initiation of heterochromatin at the relics of repeat-induced point mutation is unknown and independent of the classical RNAi pathway. Here, we show that casein kinase II (CKII) and its kinase activity are required for heterochromatin formation at the well-defined 5-kb heterochromatin of the 5H-cat-3 region and transcriptional repression of its adjacent cat-3 gene. Similarly, mutation of the histone H3 phosphorylation site T11 also impairs heterochromatin formation at the same locus. The catalytic subunit CKA colocalizes with H3T11 phosphorylation (H3pT11) within the 5H-cat-3 domain and the deletion of cka results in a significant decrease in H3T11 phosphorylation. Furthermore, the loss of kinase activity of CKII results in a significant reduction of H3pT11, H3K9me3 (histone H3 lysine 9 trimethylation) and DNA methylation levels, suggesting that CKII regulates heterochromatin formation by promoting H3T11 phosphorylation. Together, our results establish that histone H3 phosphorylation by CKII is a critical event required for heterochromatin formation.

Acetylation of WCC is dispensable for the core circadian clock but differentially regulates acute light responses in Neurospora

In the Neurospora circadian system, the White Collar Complex (WCC) formed by WC-1 and WC-2 drives expression of the frequency (frq) gene whose product FRQ feedbacks to inhibit transcriptional activity of WCC. Phosphorylation of WCC has been extensively studied, but the extent and significance of other post-translational modifications (PTM) have been poorly studied. To this end, we used mass-spectrometry to study alkylation sites on WCC, resulting in discovery of nine acetylation sites. Mutagenesis analysis showed most of the acetylation events individually do not play important roles in period determination. Moreover, mutating all the lysines falling in either half of WC-1 or all the lysine residues in WC-2 to arginines did not abolish circadian rhythms. In addition, we also found nine mono-methylation sites on WC-1, but like acetylation, individual ablation of most of the mono-methylation events did not result in a significant period change. Taken together, the data here suggest that acetylation or mono-methylation on WCC is not a determinant of the pace of the circadian feedback loop. The finding is consistent with a model in which repression of WCC's circadian activity is mainly controlled by phosphorylation. Interestingly, light-induced expression of some light-responsive genes has been modulated in certain wc-1 acetylation mutants, suggesting that WC-1 acetylation events differentially regulate light responses.

Histone 3.3-related chromatinopathy: missense variants throughout H3-3A and H3-3B cause a range of functional consequences across species

There has been considerable recent interest in the role that germline variants in histone genes play in Mendelian syndromes. Specifically, missense variants in H3-3A and H3-3B, which both encode Histone 3.3, were discovered to cause a novel neurodevelopmental disorder, Bryant-Li-Bhoj syndrome. Most of the causative variants are private and scattered throughout the protein, but all seem to have either a gain-of-function or dominant negative effect on protein function. This is highly unusual and not well understood. However, there is extensive literature about the effects of Histone 3.3 mutations in model organisms. Here, we collate the previous data to provide insight into the elusive pathogenesis of missense variants in Histone 3.3.

Acetylation of WCC is dispensable for the core circadian clock but differentially regulates acute light responses in Neurospora

In the Neurospora circadian system, the White Collar Complex (WCC) formed by WC-1 and WC-2 drives expression of the frequency ( frq ) gene whose product FRQ feedbacks to inhibit transcriptional activity of WCC. Phosphorylation of WCC has been extensively studied, but the extent and significance of other post-translational modifications (PTM) has been poorly studied. To this end, we used mass-spectrometry to study alkylation sites on WCC, resulting in discovery of nine acetylation sites. Mutagenesis analysis showed most of the acetylation events individually do not play important roles in period determination. Moreover, mutating all the lysines falling in either half of WC-1 or all the lysine residues in WC-2 to arginines did not abolish circadian rhythms. In addition, we also found nine mono-methylation sites on WC-1, but like acetylation, individual ablation of most of the mono-methylation events did not result in a significant period change. Taken together, the data here suggest that acetylation or mono-methylation on WCC is not a determinant of the pace of the circadian feedback loop. The finding is consistent with a model in which repression of WCC's circadian activity is controlled mainly by phosphorylation. Interestingly, light-induced expression of some light-responsive genes has been modulated in certain wc-1 acetylation mutants, suggesting that WC-1 acetylation events differentially regulate light responses.

The effect of SiNPs on DNA methylation of genome in mouse spermatocytes

Silica nanoparticles (SiNPs), which are the main inorganic components of atmospheric particulate matter, have been proved to have certain male reproductive toxicity in previous studies. Spermatogenesis involves complex epigenetic regulation, but it is still unclear if SiNPs exposure will interfere with the DNA methylation patterns in mouse spermatocytes. The present study was designed to investigate the effects of SiNPs on DNA methylation in the mouse spermatocyte GC-2spd(ts). GC-2 cells were treated with 0 and 20 μg/mL SiNPs for 24 h. MeDIP-seq assay was then performed to analyze the differentially methylated genes related to spermatogenesis. The results showed that SiNPs induced extensive methylation changes in the genome of GC-2 cells, and 24a total of 428 hyper-methylated genes and 398 hypo-methylated genes were identified. Gene Ontology and pathway analysis showed that differential DNA methylation induced by SiNPs was probably involved with abnormal transcription and translation, mitochondrial damage, and cell apoptosis. Results from qRT-PCR verification showed that the expression of spermatogenesis-related genes Akap1, Crem, Spz1, and Tex11 were dysregulated by SiNPs exposure, which was consistent with the MeDIP-seq assay. In general, this study suggested that SiNPs caused genome-wide DNA methylation changes in GC-2 cells, providing valuable reference for the future epigenetic studies in SiNPs-induced male reproductive toxicity.

Histone Variant H3.3 Mutations in Defining the Chromatin Function in Mammals

The systematic mutation of histone 3 (H3) genes in model organisms has proven to be a valuable tool to distinguish the functional role of histone residues. No system exists in mammalian cells to directly manipulate canonical histone H3 due to a large number of clustered and multi-loci histone genes. Over the years, oncogenic histone mutations in a subset of H3 have been identified in humans, and have advanced our understanding of the function of histone residues in health and disease. The oncogenic mutations are often found in one allele of the histone variant H3.3 genes, but they prompt severe changes in the epigenetic landscape of cells, and contribute to cancer development. Therefore, mutation approaches using H3.3 genes could be relevant to the determination of the functional role of histone residues in mammalian development without the replacement of canonical H3 genes. In this review, we describe the key findings from the H3 mutation studies in model organisms wherein the genetic replacement of canonical H3 is possible. We then turn our attention to H3.3 mutations in human cancers, and discuss H3.3 substitutions in the N-terminus, which were generated in order to explore the specific residue or associated post-translational modification.

Selection and Characterization of Mutants Defective in DNA Methylation in Neurospora crassa

DNA methylation, a prototypical epigenetic modification implicated in gene silencing, occurs in many eukaryotes and plays a significant role in the etiology of diseases such as cancer. The filamentous fungus Neurospora crassa places DNA methylation at regions of constitutive heterochromatin such as in centromeres and in other A:T-rich regions of the genome, but this modification is dispensable for normal growth and development. This and other features render N. crassa an excellent model to genetically dissect elements of the DNA methylation pathway. We implemented a forward genetic selection on a massive scale, utilizing two engineered antibiotic-resistance genes silenced by DNA methylation, to isolate mutants d efective i n m ethylation (dim). Hundreds of potential mutants were characterized, yielding a rich collection of informative alleles of 11 genes important for DNA methylation, most of which were already reported. In parallel, we characterized the pairwise interactions in nuclei of the DCDC, the only histone H3 lysine 9 methyltransferase complex in Neurospora, including those between the DIM-5 catalytic subunit and other complex members. We also dissected the N- and C-termini of the key protein DIM-7, required for DIM-5 histone methyltransferase localization and activation. Lastly, we identified two alleles of a novel gene, dim-10 - a homolog of Clr5 in Schizosaccharomyces pombe - that is not essential for DNA methylation, but is necessary for repression of the antibiotic-resistance genes used in the selection, which suggests that both DIM-10 and DNA methylation promote silencing of constitutive heterochromatin.

Nucleosome Positioning by an Evolutionarily Conserved Chromatin Remodeler Prevents Aberrant DNA Methylation in Neurospora

In the filamentous fungus Neurospora crassa, constitutive heterochromatin is marked by tri-methylation of histone H3 lysine 9 (H3K9me3) and DNA methylation. We identified mutations in the Neurospora defective in methylation-1 (dim-1) gene that cause defects in cytosine methylation and implicate a putative AAA-ATPase chromatin remodeler. Although it was well-established that chromatin remodelers can affect transcription by influencing DNA accessibility with nucleosomes, little was known about the role of remodelers on chromatin that is normally not transcribed, including regions of constitutive heterochromatin. We found that dim-1 mutants display both reduced DNA methylation in heterochromatic regions as well as increased DNA methylation and H3K9me3 in some intergenic regions associated with highly expressed genes. Deletion of dim-1 leads to atypically spaced nucleosomes throughout the genome and numerous changes in gene expression. DIM-1 localizes to both heterochromatin and intergenic regions that become hyper-methylated in dim-1 strains. Our findings indicate that DIM-1 normally positions nucleosomes in both heterochromatin and euchromatin and that the standard arrangement and density of nucleosomes is required for the proper function of heterochromatin machinery.

Induction of H3K9me3 and DNA methylation by tethered heterochromatin factors in Neurospora crassa

Functionally different chromatin domains display distinct chemical marks. Constitutive heterochromatin is commonly associated with trimethylation of lysine 9 on histone H3 (H3K9me3), hypoacetylated histones, and DNA methylation, but the contributions of and interplay among these features are not fully understood. To dissect the establishment of heterochromatin, we investigated the relationships among these features using an in vivo tethering system in Neurospora crassa Artificial recruitment of the H3K9 methyltransferase DIM-5 (defective in methylation-5) induced H3K9me3 and DNA methylation at a normally active, euchromatic locus but did not bypass the requirement of DIM-7, previously implicated in the localization of DIM-5, indicating additional DIM-7 functionality. Tethered heterochromatin protein 1 (HP1) induced H3K9me3, DNA methylation, and gene silencing. The induced heterochromatin required histone deacetylase 1 (HDA-1), with an intact catalytic domain, but HDA-1 was not essential for de novo heterochromatin formation at native heterochromatic regions. Silencing did not require H3K9me3 or DNA methylation. However, DNA methylation contributed to establishment of H3K9me3 induced by tethered HP1. Our analyses also revealed evidence of regulatory mechanisms, dependent on HDA-1 and DIM-5, to control the localization and catalytic activity of the DNA methyltransferase DIM-2. Our study clarifies the interrelationships among canonical aspects of heterochromatin and supports a central role of HDA-1-mediated histone deacetylation in heterochromatin spreading and gene silencing.

Regulation of Neurospora Catalase-3 by global heterochromatin formation and its proximal heterochromatin region

Catalase-3 (CAT-3) constitutes the main catalase activity in growing hyphae of Neurospora crassa, and its activity increases during exponential growth or is induced under different stress conditions. Although extensive progress has been made to identify catalase regulators, the regulation mechanism of CAT-3 at the chromatin level still remains unclear. Here, we aim at investigating the molecular regulation mechanisms of cat-3 at the chromatin level. We found that CAT-3 protein levels increased in mutants defective in proper global heterochromatin formation. Bioinformatics analysis identified a 5-kb AT-rich sequence adjacent to the cat-3 promoter as a heterochromatin region because of its enrichment of H3K9me3 and HP1. Expression of CAT-3 was induced by H₂O₂ treatment in wild-type and such change occurred along with the accumulation of histone H3 acetylation at 5-kb heterochromatin boundaries and cat-3 locus, but without alteration of its H3K9me3 repressive modification. Moreover, disruption of 5-kb heterochromatin region results in elevated cat-3 expression, and higher levels of cat-3 expression were promoted by the combination with global heterochromatin defective mutants. Interestingly, the molecular weight and activity bands of CAT-3 protein are different in heterochromatin defective mutants compared with those in wild-type, suggesting that its N-terminal processing and modification may be altered. Our study indicates that the local chromatin structure creates a heterochromatin repressive environment to repress nearby gene expression.

Dual chromatin recognition by the histone deacetylase complex HCHC is required for proper DNA methylation in Neurospora crassa

DNA methylation, heterochromatin protein 1 (HP1), histone H3 lysine 9 (H3K9) methylation, histone deacetylation, and highly repeated sequences are prototypical heterochromatic features, but their interrelationships are not fully understood. Prior work showed that H3K9 methylation directs DNA methylation and histone deacetylation via HP1 in Neurospora crassa and that the histone deacetylase complex HCHC is required for proper DNA methylation. The complex consists of the chromodomain proteins HP1 and chromodomain protein 2 (CDP-2), the histone deacetylase HDA-1, and the AT-hook motif protein CDP-2/HDA-1-associated protein (CHAP). We show that the complex is required for proper chromosome segregation, dissect its function, and characterize interactions among its components. Our analyses revealed the existence of an HP1-based DNA methylation pathway independent of its chromodomain. The pathway partially depends on CHAP but not on the CDP-2 chromodomain. CDP-2 serves as a bridge between the recognition of H3K9 trimethylation (H3K9me3) by HP1 and the histone deacetylase activity of HDA-1. CHAP is also critical for HDA-1 localization to heterochromatin. Specifically, the CHAP zinc finger interacts directly with the HDA-1 argonaute-binding protein 2 (Arb2) domain, and the CHAP AT-hook motifs recognize heterochromatic regions by binding to AT-rich DNA. Our data shed light on the interrelationships among the prototypical heterochromatic features and support a model in which dual recognition by the HP1 chromodomain and the CHAP AT-hooks are required for proper heterochromatin formation.

Loss of HP1 causes depletion of H3K27me3 from facultative heterochromatin and gain of H3K27me2 at constitutive heterochromatin

Methylated lysine 27 on histone H3 (H3K27me) marks repressed “facultative heterochromatin,” including developmentally regulated genes in plants and animals. The mechanisms responsible for localization of H3K27me are largely unknown, perhaps in part because of the complexity of epigenetic regulatory networks. We used a relatively simple model organism bearing both facultative and constitutive heterochromatin, Neurospora crassa, to explore possible interactions between elements of heterochromatin. In higher eukaryotes, reductions of H3K9me3 and DNA methylation in constitutive heterochromatin have been variously reported to cause redistribution of H3K27me3. In Neurospora, we found that elimination of any member of the DCDC H3K9 methylation complex caused massive changes in the distribution of H3K27me; regions of facultative heterochromatin lost H3K27me3, while regions that are normally marked by H3K9me3 became methylated at H3K27. Elimination of DNA methylation had no obvious effect on the distribution of H3K27me. Elimination of HP1, which “reads” H3K9me3, also caused major changes in the distribution of H3K27me, indicating that HP1 is important for normal localization of facultative heterochromatin. Because loss of HP1 caused redistribution of H3K27me2/3, but not H3K9me3, these normally nonoverlapping marks became superimposed. Indeed, mass spectrometry revealed substantial cohabitation of H3K9me3 and H3K27me2 on H3 molecules from an hpo strain. Loss of H3K27me machinery (e.g., the methyltransferase SET-7) did not impact constitutive heterochromatin but partially rescued the slow growth of the DCDC mutants, suggesting that the poor growth of these mutants is partly attributable to ectopic H3K27me. Altogether, our findings with Neurospora clarify interactions of facultative and constitutive heterochromatin in eukaryotes.

Neurospora importin α is required for normal heterochromatic formation and DNA methylation

Heterochromatin and associated gene silencing processes play roles in development, genome defense, and chromosome function. In many species, constitutive heterochromatin is decorated with histone H3 tri-methylated at lysine 9 (H3K9me3) and cytosine methylation. In Neurospora crassa, a five-protein complex, DCDC, catalyzes H3K9 methylation, which then directs DNA methylation. Here, we identify and characterize a gene important for DCDC function, dim-3 (defective in methylation-3), which encodes the nuclear import chaperone NUP-6 (Importin α). The critical mutation in dim-3 results in a substitution in an ARM repeat of NUP-6 and causes a substantial loss of H3K9me3 and DNA methylation. Surprisingly, nuclear transport of all known proteins involved in histone and DNA methylation, as well as a canonical transport substrate, appear normal in dim-3 strains. Interactions between DCDC members also appear normal, but the nup-6dim-3 allele causes the DCDC members DIM-5 and DIM-7 to mislocalize from heterochromatin and NUP-6^dim-3 itself is mislocalized from the nuclear envelope, at least in conidia. GCN-5, a member of the SAGA histone acetyltransferase complex, also shows altered localization in dim-3, raising the possibility that NUP-6 is necessary to localize multiple chromatin complexes following nucleocytoplasmic transport.

The epigenetic information contained in chromatin is essential for development of higher organisms, and if misregulated, can lead to the unregulated growth associated with human cancers. Chromatin is typically classified into two basic types: gene-rich 'euchromatin', and gene-poor heterochromatin, which is also rich in repeated DNA and 'repressive chromatin marks'. As in humans and eukaryotes generally, heterochromatin in Neurospora crassa is decorated with DNA methylation and histone H3 lysine 9 (H3K9) methylation, but unlike the case in mammals, loss of these epigenetic marks does not compromise viability. In Neurospora, the DCDC, a five-member Cul4-based protein complex, trimethylates H3K9. Little information is available on the regulation of DCDC or similar complexes in other organisms. Using forward genetics, we identified a novel role for Importin α (NUP-6) for the function of DCDC. Although NUP-6 typically functions in nucleocytoplasmic transport, the dim-3 strain, which contains an altered nup-6 gene that reduces DNA methylation and H3K9me3, shows normal nuclear transport of the heterochromatin machinery and a canonical transport substrate. Two DCDC members are mislocalized from heterochromatin in the dim-3 mutant, signifying that NUP-6 may be important for targeting key proteins to incipient heterochromatic DNA. The euchromatic complex SAGA has increased euchromatin localization in dim-3, suggesting that NUP-6 may localize multiple chromatin complexes to sub-nuclear genomic targets.

Heterochromatin controls γH2A localization in Neurospora crassa

In response to genotoxic stress, ATR and ATM kinases phosphorylate H2A in fungi and H2AX in animals on a C-terminal serine. The resulting modified histone, called γH2A, recruits chromatin-binding proteins that stabilize stalled replication forks or promote DNA double-strand-break repair. To identify genomic loci that might be prone to replication fork stalling or DNA breakage in Neurospora crassa, we performed chromatin immunoprecipitation (ChIP) of γH2A followed by next-generation sequencing (ChIP-seq). γH2A-containing nucleosomes are enriched in Neurospora heterochromatin domains. These domains are comprised of A·T-rich repetitive DNA sequences associated with histone H3 methylated at lysine-9 (H3K9me), the H3K9me-binding protein heterochromatin protein 1 (HP1), and DNA cytosine methylation. H3K9 methylation, catalyzed by DIM-5, is required for normal γH2A localization. In contrast, γH2A is not required for H3K9 methylation or DNA methylation. Normal γH2A localization also depends on HP1 and a histone deacetylase, HDA-1, but is independent of the DNA methyltransferase DIM-2. γH2A is globally induced in dim-5 mutants under normal growth conditions, suggesting that the DNA damage response is activated in these mutants in the absence of exogenous DNA damage. Together, these data suggest that heterochromatin formation is essential for normal DNA replication or repair.

A cascade of histone modifications induces chromatin condensation in mitosis

Metaphase chromosomes are visible hallmarks of mitosis, yet our understanding of their structure and of the forces shaping them is rudimentary. Phosphorylation of histone H3 serine 10 (H3 S10) by Aurora B kinase is a signature event of mitosis, but its function in chromatin condensation is unclear. Using genetically encoded ultraviolet light-inducible cross-linkers, we monitored protein-protein interactions with spatiotemporal resolution in living yeast to identify the molecular details of the pathway downstream of H3 S10 phosphorylation. This modification leads to the recruitment of the histone deacetylase Hst2p that subsequently removes an acetyl group from histone H4 lysine 16, freeing the H4 tail to interact with the surface of neighboring nucleosomes and promoting fiber condensation. This cascade of events provides a condensin-independent driving force of chromatin hypercondensation during mitosis.

Effect of dynamic DNA methylation and histone acetylation on cPouV expression in differentiation of chick embryonic germ cells

As a crucial pluripotency-related factor, the epigenetic regulation of Oct4 has been studied intensively in mammalians. However, its dynamic changes of DNA methylation and histone modification in avians remain poorly understood. In the present study, we first described the alterations of DNA methylation and histone acetylation in the promoter of chicken PouV (cPouV; the homologue of Oct4 in avian) during chick embryonic germ (EG) cell differentiation. The epigenetic modification analysis showed that DNA methylation in the cPouV promoter increased obviously, while histone acetylation decreased dramatically detected by chromatin immunoprecipitation assay in the process of differentiation. Gene expression analysis detection indicated that the levels of DNA methyltransferase 3a (Dnmt 3a), Dnmt 3b, and histone deacetylase 3 (HDAC 3) transcripts were significantly high, whereas the relative abundance of Dnmt 1, histone acetyltransferase (HAT), and cPouV mRNA was significantly decreased during the conversion of EG to embryoid body-like structures (EBs), which was correlated with the increased level of methylation and reduced level of H3 acetylation. Moreover, in vitro methylation assay indicated that the reporter gene was remarkably inhibited by the methylated promoter of cPouV. To further understand the effect of epigenetic modifiers on cPouV expression, we performed an analysis of EB cells treated with trichostatin A (TSA), Aza-2'-deoxycytidine (Aza), or TSA plus Aza (TSA/Aza). We observed that the effect of TSA/Aza is more sensitive to the reactivation of cPouV compared with TSA or Aza, indicating that these epigenetic inhibitors can function synergistically to facilitate the reprogramming process. The present study provided evidences that a critical role for cPouV activation/repression by DNA methylation and/or histone modifications is involved in the pluripotency maintenance and differentiation process of chick EG.

Neurospora crassa, a model system for epigenetics research

The filamentous fungus Neurospora crassa has provided a rich source of knowledge on epigenetic phenomena that would have been difficult or impossible to gain from other systems. Neurospora sports features found in higher eukaryotes but absent in both budding and fission yeast, including DNA methylation and H3K27 methylation, and also has distinct RNA interference (RNAi)-based silencing mechanisms operating in mitotic and meiotic cells. This has provided an unexpected wealth of information on gene silencing systems. One silencing mechanism, named repeat-induced point mutation (RIP), has both epigenetic and genetic aspects and provided the first example of a homology-based genome defense system. A second silencing mechanism, named quelling, is an RNAi-based mechanism that results in silencing of transgenes and their native homologs. A third, named meiotic silencing, is also RNAi-based but is distinct from quelling in its time of action, targets, and apparent purpose.

Distinct amino acids of histone H3 control secondary metabolism in Aspergillus nidulans

Chromatin remodelling events play an important role in the secondary metabolism of filamentous fungi. Previously, we showed that a bacterium, Streptomyces rapamycinicus, is able to reprogram the histone-modifying Spt-Ada-Gcn5-acetyltransferase/ADA (SAGA/ADA) complex of the model fungus Aspergillus nidulans. Consequently, the histone H3 amino acids lysine 9 and lysine 14 at distinct secondary metabolism genes were specifically acetylated during the bacterial fungal interaction, which, furthermore, was associated with the activation of the otherwise silent orsellinic acid gene cluster. To investigate the importance of the histone modifications for distinct gene expression profiles in fungal secondary metabolism, we exchanged several amino acids of histone H3 of A. nidulans. These amino acids included lysine residues 9, 14, 18, and 23 as well as serine 10 and threonine 11. Lysine residues were replaced by arginine or glutamine residues, and serine/threonine residues were replaced by alanine. All generated mutant strains were viable, allowing direct analysis of the consequences of missing posttranslational histone modifications. In the mutant strains, major changes in the expression patterns at both the transcriptional and metabolite levels of the penicillin, sterigmatocystin, and orsellinic acid biosynthesis gene clusters were detected. These effects were due mainly to the substitution of the acetylatable lysine 14 of histone H3 and were enhanced in a lysine 14/lysine 9 double mutant of histone H3. Taken together, our findings show a causal linkage between the acetylation of lysine residue 14 of histone H3 and the transcription and product formation of secondary metabolite gene clusters.

Regional control of histone H3 lysine 27 methylation in Neurospora

Trimethylated lysine 27 on histone H3 (H3K27me3) is present in Drosophila, Arabidopsis, worms, and mammals, but is absent from yeasts that have been examined. We identified and analyzed H3K27me3 in the filamentous fungus Neurospora crassa and in other Neurospora species. H3K27me3 covers 6.8% of the N. crassa genome, encompassing 223 domains, including 774 genes, all of which are transcriptionally silent. N. crassa H3K27me3-marked genes are less conserved than unmarked genes and only ∼35% of genes marked by H3K27me3 in N. crassa are also H3K27me3-marked in Neurospora discreta and Neurospora tetrasperma. We found that three components of the Neurospora Polycomb repressive complex 2 (PRC2)--[Su-(var)3-9; E(z); Trithorax] (SET)-7, embryonic ectoderm development (EED), and SU(Z)12 (suppressor of zeste12)--are required for H3K27me3, whereas the fourth component, Neurospora protein 55 (an N. crassa homolog of p55/RbAp48), is critical for H3K27me3 only at subtelomeric domains. Loss of H3K27me3, caused by deletion of the gene encoding the catalytic PRC2 subunit, set-7, resulted in up-regulation of 130 genes, including genes in both H3K27me3-marked and unmarked regions.

Modification of enhancer chromatin: what, how, and why?

Emergence of form and function during embryogenesis arises in large part through cell-type- and cell-state-specific variation in gene expression patterns, mediated by specialized cis-regulatory elements called enhancers. Recent large-scale epigenomic mapping revealed unexpected complexity and dynamics of enhancer utilization patterns, with 400,000 putative human enhancers annotated by the ENCODE project alone. These large-scale efforts were largely enabled through the understanding that enhancers share certain stereotypical chromatin features. However, an important question still lingers: what is the functional significance of enhancer chromatin modification? Here we give an overview of enhancer-associated modifications of histones and DNA and discuss enzymatic activities involved in their dynamic deposition and removal. We describe potential downstream effectors of these marks and propose models for exploring functions of chromatin modification in regulating enhancer activity during development.

A permissive chromatin structure is adopted prior to site-specific DNA demethylation of developmentally expressed genes involved in macronuclear differentiation

Background

DNA methylation and demethylation are important epigenetic regulatory mechanisms in eukaryotic cells and, so far, only partially understood. We exploit the minimalistic biological ciliate system to understand the crosstalk between DNA modification and chromatin structure. In the macronucleus of these cells, the DNA is fragmented into individual short DNA molecules, each representing a functional expression and replication unit. Therefore, long range epigenomic interaction can be excluded in this system.

Results

In the stichotrichous ciliate Stylonychia lemnae, cytosine methylation occurs in a small subset of macronuclear nanochromosomes expressed only during sexual reproduction. Methylation pattern shows similarity to that observed in fungi and Drosophila. Cytosine methylation correlates with gene activity and chromatin structure. Upon gene activation, cytosines become demethylated and a redistribution of histone post-translational modifications (PTMs) takes place. Evidence is presented that the formation of a permissive chromatin structure in the vicinity of the 5meCs precedes cytosine methylation and is probably a necessary prerequisite for their demethylation. Shortly after demethylation of cytosines occurs, the parental macronucleus degenerates, a new macronucleus is formed from a micronuclear derivative and the specific methylation pattern is transmitted from the germline micronucleus to the new macronucleus.

Conclusions

We show that very few, or even only one, discrete methylated cytosines are required to assign regulatory functions at a specific locus. Furthermore, evidence is provided that a permissive chromatin structure is probably a necessary prerequisite for the demethylation of specific cytosines. Our results allow us to propose a mechanistic model for the biological function of cytosine methylation in the ciliate cell and its regulation during the cell cycle.

Is histone acetylation the most important physiological function for CBP and p300?

Protein lysine acetyltransferases (HATs or PATs) acetylate histones and other proteins, and are principally modeled as transcriptional coactivators. CREB binding protein (CBP, CREBBP) and its paralog p300 (EP300) constitute the KAT3 family of HATs in mammals, which has mostly unique sequence identity compared to other HAT families. Although studies in yeast show that many histone mutations cause modest or specific phenotypes, similar studies are impractical in mammals and it remains uncertain if histone acetylation is the primary physiological function for CBP/p300. Nonetheless, CBP and p300 mutations in humans and mice show that these coactivators have important roles in development, physiology, and disease, possibly because CBP and p300 act as network “hubs” with more than 400 described protein interaction partners. Analysis of CBP and p300 mutant mouse fibroblasts reveals CBP/p300 are together chiefly responsible for the global acetylation of histone H3 residues K18 and K27, and contribute to other locus-specific histone acetylation events. CBP/p300 can also be important for transcription, but the recruitment of CBP/p300 and their associated histone acetylation marks do not absolutely correlate with a requirement for gene activation. Rather, it appears that target gene context (e.g. DNA sequence) influences the extent to which CBP and p300 are necessary for transcription.