Yeast Rpd3 histone deacetylase plays an important role at actively transcribed genes. We characterized two distinct Rpd3 complexes, Rpd3L and Rpd3S, by MudPIT analysis. Both complexes shared a three subunit core and Rpd3L contains unique subunits consistent with being a promoter targeted corepressor. Rco1 and Eaf3 were subunits specific to Rpd3S. Mutants of RCO1 and EAF3 exhibited increased acetylation in the FLO8 and STE11 open reading frames (ORFs) and the appearance of aberrant transcripts initiating within the body of these ORFs. Mutants in the RNA polymerase II-associated SET2 histone methyltransferase also displayed these defects. Set2 functioned upstream of Rpd3S and the Eaf3 methyl-histone binding chromodomain was important for recruitment of Rpd3S and for deacetylation within the STE11 ORF. These data indicate that Pol II-associated Set2 methylates H3 providing a transcriptional memory which signals for deacetylation of ORFs by Rpd3S. This erases transcription elongation-associated acetylation to suppress intragenic transcription initiation.

Alterations of chromatin structure have been shown to be crucial for response to cell signaling and for programmed gene expression in development. Posttranslational histone modifications influence changes in chromatin structure both directly and by targeting or activating chromatin-remodeling complexes. Histone modifications intersect with cell signaling pathways to control gene expression and can act combinatorially to enforce or reverse epigenetic marks in chromatin. Through their recognition by protein complexes with enzymatic activities cross talk is established between different modifications and with other epigenetic pathways, including noncoding RNAs (ncRNAs) and DNA methylation. Here, we review the functions of histone modifications and their exploitation in the programming of gene expression during several events in development.

Histone modifications play a complex role in the regulation of transcription. Recent studies (Duncan et al., 2008; Lee et al., 2007; Li et al., 2008) reveal that regulation of histone modifications can be functionally linked to reinforce the activation or repression of gene expression.

The Ada2a-containing (ATAC) complex is an essential Drosophila melanogaster histone acetyltransferase (HAT) complex that contains the transcriptional cofactors Gcn5 (KAT2), Ada3, Ada2a, Atac1 and Hcf. We have analyzed the complex by MudPIT (multidimensional protein identification technology) and found eight previously unidentified subunits. These include the WD40 repeat protein WDS, the PHD and HAT domain protein CG10414 (herein renamed Atac2/KAT14), the YEATS family member D12, the histone fold proteins CHRAC14 and NC2beta, CG30390, CG32343 (Atac3) and CG10238. The presence of CG10414 (Atac2) suggests that it acts as a second acetyltransferase enzyme in ATAC in addition to Gcn5. Indeed, recombinant Atac2 displays HAT activity in vitro with a preference for acetylating histone H4, and mutation of Atac2 abrogated H4 lysine 16 acetylation in D. melanogaster embryos. Furthermore, although ATAC does not show nucleosome-remodeling activity itself, it stimulates nucleosome sliding by the ISWI, SWI-SNF and RSC complexes.

Pyruvate kinase M2 (PKM2) is a key enzyme for glycolysis and catalyzes the conversion of phosphoenolpyruvate (PEP) to pyruvate, which supplies cellular energy. PKM2 also phosphorylates histone H3 threonine 11 (H3T11); however, it is largely unknown how PKM2 links cellular metabolism to chromatin regulation. Here, we show that the yeast PKM2 homolog, Pyk1, is a part of a novel protein complex named SESAME (Serine-responsive SAM-containing Metabolic Enzyme complex), which contains serine metabolic enzymes, SAM (S-adenosylmethionine) synthetases, and an acetyl-CoA synthetase. SESAME interacts with the Set1 H3K4 methyltransferase complex, which requires SAM synthesized from SESAME, and recruits SESAME to target genes, resulting in phosphorylation of H3T11. SESAME regulates the crosstalk between H3K4 methylation and H3T11 phosphorylation by sensing glycolysis and glucose-derived serine metabolism. This leads to auto-regulation of PYK1 expression. Thus, our study provides insights into the mechanism of regulating gene expression, responding to cellular metabolism via chromatin modifications.