The yeast histone deacetylase Rpd3 can be recruited to promoters to repress transcription initiation. Biochemical, genetic, and gene-expression analyses show that Rpd3 exists in two distinct complexes. The smaller complex, Rpd3C(S), shares Sin3 and Ume1 with Rpd3C(L) but contains the unique subunits Rco1 and Eaf3. Rpd3C(S) mutants exhibit phenotypes remarkably similar to those of Set2, a histone methyltransferase associated with elongating RNA polymerase II. Chromatin immunoprecipitation and biochemical experiments indicate that the chromodomain of Eaf3 recruits Rpd3C(S) to nucleosomes methylated by Set2 on histone H3 lysine 36, leading to deacetylation of transcribed regions. This pathway apparently acts to negatively regulate transcription because deleting the genes for Set2 or Rpd3C(S) bypasses the requirement for the positive elongation factor Bur1/Bur2.

The histone H2A variant H2A.Z (Saccharomyces cerevisiae Htz1) plays roles in transcription, DNA repair, chromosome stability, and limiting telomeric silencing. The Swr1-Complex (SWR-C) inserts Htz1 into chromatin and shares several subunits with the NuA4 histone acetyltransferase. Furthermore, mutants of these two complexes share several phenotypes, suggesting they may work together. Here we show that NuA4 acetylates Htz1 Lys 14 (K14) after the histone is assembled into chromatin by the SWR-C. K14 mutants exhibit specific defects in chromosome transmission without affecting transcription, telomeric silencing, or DNA repair. Functionspecific modifications may help explain how the same component of chromatin can function in diverse pathways. Two classes of enzymes have been implicated in regulating chromatin structure and access to the underlying DNA template. The ATP-dependent chromatin remodeling enzymes use ATP hydrolysis to induce nucleosome mobility or disrupt histone-DNA interactions. The second class of enzymes covalently modify (e.g., lysine acetylation, serine phosphorylation, lysine and arginine methylation, ubiquitylation, or ADP ribosylation) various histones, usually on their N-terminal tails (Strahl and Allis 2000; Jenuwein and Allis 2001). Acetylation is carried out by histone acetyltransferases (HATs), which in Saccharomyces cerevisiae include the Gcn5-containing ADA and SAGA complexes, Hat1, Elongator, NuA3, and NuA4 (for review, see Bottomley 2004). These typically have specificity for distinct lysine residues on certain histone N-terminal tails. The acetylation of lysine residues on the N-terminal tails of histones H3 and H4 neutralizes their positive charge, possibly decreasing their affinity for DNA and facilitating chromatin decompaction and disassembly (Eberharter and Becker 2002). Perhaps more important than simple charge neutralization is the specific pattern of acetylation at individual lysine residues, at least some of which recruit bromodomain-containing proteins (Matangkasombut and Buratowski 2003, and references therein).Further chromatin specialization can be introduced by incorporation of variant histones. The major histones are assembled during DNA replication, but can be replaced by variants at specific locations (for review, see The SWR-C shares several subunits with the NuA4 HAT complex (Kobor et al. 2004;Krogan et al. 2004;Mizuguchi et al. 2004), and expression microarray analysis shows the two complexes have common regulatory targets (Krogan et al. 2004). NuA4 is required for the majority of histone H4 acetylation on Lys 5 (K5), K8, and K12 and some on histone H2A K7 Allard et al. 1999). Htz1, SWR-C, and NuA4 have each been implicated in the maintenance of chromosome stability (Krogan et al. 2004). This function for H2A.Z is conserved in the fission yeast Schizosaccharomyces pombe (Carr et al. 1994) and metazoans (Rangasamy et al. 2004).How NuA4 and SWR-C are functionally connected remains unclear. Htz1 incorporation into chromatin is dependent on SWR-C, but independent of NuA...

The Saccharomyces cerevisiae cyclin-dependent kinase (CDK) Bur1 (Sgv1) may be homologous to mammalian Cdk9, which functions in transcriptional elongation. Although Bur1 can phosphorylate the Rpb1 carboxyterminal domain (CTD) kinase in vitro, it has no strong specificity within the consensus heptapeptide YSPTSPS for Ser2 or Ser5. BUR1 mutants are sensitive to the drugs 6-azauracil and mycophenolic acid and interact genetically with the elongation factors Ctk1 and Spt5. Chromatin immunoprecipitation experiments show that Bur1 and its cyclin partner Bur2 are recruited to transcription elongation complexes, cross-linking to actively transcribing genes. Interestingly, Bur1 shows reduced cross-linking to transcribed regions downstream of polyadenylation sites. In addition, bur1 mutant strains have a reduced cross-linking ratio of RNA polymerase II at the 3 end of genes relative to promoter regions. Phosphorylation of CTD serines 2 and 5 appears normal in mutant cells, suggesting that Bur1 is not a significant source of cotranscriptional Rpb1 phosphorylation. These results show that Bur1 functions in transcription elongation but may phosphorylate a substrate other than the CTD.

The RNA polymerase II enzyme from the yeast Saccharomyces cerevisiae is a complex of 12 subunits, Rpb1 to Rpb12. Crystal structures of the full complex show that the polymerase consists of two separable components, a 10-subunit core including the catalytic active site and a heterodimer of the Rpb4 and Rpb7 subunits. To characterize the role of the Rpb4/7 heterodimer during transcription in vivo, chromatin immunoprecipitation was used to examine an rpb4⌬ strain for effects on the behavior of the core polymerase as well as recruitment of other protein factors involved in transcription. Rpb4/7 cross-links throughout transcribed regions. Loss of Rpb4 results in a reduction of RNA polymerase II levels near 3 ends of multiple mRNA genes as well as a decreased association of 3-end processing factors. Furthermore, loss of Rpb4 results in altered polyadenylation site usage at the RNA14 gene. Together, these results indicate that Rpb4 contributes to proper cotranscriptional 3-end processing in vivo.The synthesis of eukaryotic mRNA by RNA polymerase II (RNApII) is a multistep process involving initiation, elongation, and termination. During the transcription cycle, RNApII associates with many proteins involved in the regulation of these processes, including the basal transcription factors, coactivators, elongation factors, and factors involved in 3Ј-end formation and termination (6). This regulation of transcription by RNApII and its associated proteins is critical for gene expression.The RNApII enzyme from the yeast Saccharomyces cerevisiae is a complex of 12 subunits, Rpb1 to Rpb12, that can dissociate into a 10-subunit core and a heterodimer consisting of Rpb4 and Rpb7 (6). Rpb4 is not essential during optimal growth conditions, although the deletion strain grows very slowly (32). RNApII purified from an rpb4⌬ strain lacks Rpb4 but also contains no detectable level of Rpb7 (8). It has been shown that Rpb7, a subunit that is essential for cell viability (22), can interact with polymerase independently of Rpb4, but this interaction is weak and can easily be detected only when Rpb7 is overexpressed (28). RNApII lacking Rpb4/7 is catalytically active for polymerization, but the heterodimer is required for promoter-dependent transcription in vitro (8).Crystal structures show the location of the Rpb4/7 heterodimer in the context of the complete RNApII complex (2, 3). Rpb4 makes very little contact with the core subunits; the dimer is held primarily through contacts between Rpb7 and core subunits Rpb1 and Rpb6. Rpb4/7 is found near both the transcript-exit groove and the linker to the C-terminal domain (CTD) of Rpb1, a location that would allow for interactions with the nascent RNA transcript as well as protein factors involved in transcription regulation. In fact, Rpb7 contains a potential oligonucleotide-binding domain that faces the presumed RNA exit site (6) and has recently been shown to cross-link to the emerging RNA transcript (31).Recent reports also indicate that the heterodimer can interact with several transcription...

Basal transcription factor TFIIH phosphorylates the RNA polymerase II (RNApII) carboxy-terminal domain (CTD) within the transcription initiation complex. The catalytic kinase subunit of TFIIH is a member of the cyclin-dependent kinase (Cdk) family, designated Kin28 in Saccharomyces cerevisiae and Cdk7 in higher eukaryotes. Together with TFIIH subunits cyclin H and Mat1, Cdk7 kinase is also found in a trimer complex known as Cdk activating kinase (CAK). A yeast trimer complex has not previously been identified, although a Kin28-Ccl1 dimer called TFIIK has been isolated as a breakdown product of TFIIH. Here we show that a trimeric complex of Kin28-Ccl1-Tfb3 exists in yeast extracts. Several Kin28 point mutants that are defective in CTD phosphorylation were created. Consistent with earlier studies, these mutants have no transcriptional defect in vitro. Like other Cdks, Kin28 is activated by phosphorylation on T162 of the T loop. Kin28 T162 mutants have no growth defects alone but do demonstrate synthetic phenotypes when combined with mutant versions of the cyclin partner, Ccl1. Surprisingly, these phosphorylation site mutants appear to destabilize the association of the cyclin subunit within the context of TFIIH but not within the trimer complex.