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Histone phosphorylation influences transcription, chromosome condensation, DNA repair and apoptosis. Previously, we showed that histone H3 Ser10 phosphorylation (pSer10) by the yeast Snf1 kinase regulates INO1 gene activation in part via Gcn5/SAGA complex-mediated Lys14 acetylation (acLys14). How such chromatin modification patterns develop is largely unexplored. Here we examine the mechanisms surrounding pSer10 at INO1, and at GAL1, which herein is identified as a new regulatory target of Snf1/pSer10. Snf1 behaves as a classic coactivator in its recruitment by DNA-bound activators, and in its role in modifying histones and recruiting TATA-binding protein (TBP). However, one important difference in Snf1 function in vivo at these promoters is that SAGA recruitment at INO1 requires histone phosphorylation via Snf1, whereas at GAL1, SAGA recruitment is independent of histone phosphorylation. In addition, the GAL1 activator physically interacts with both Snf1 and SAGA, whereas the INO1 activator interacts only with Snf1. Thus, at INO1, pSer10's role in recruiting SAGA may substitute for recruitment by DNA-bound activator. Our results emphasize that histone modifications share general functions between promoters, but also acquire distinct roles tailored for promoter-specific requirements.

Gene activation and repression regulated by acetylation and deacetylation represent a paradigm for the function of histone modifications. We provide evidence that, in contrast, histone H2B monoubiquitylation and its deubiquitylation are both involved in gene activation. Substitution of the H2B ubiquitylation site at Lys 123 (K123) lowered transcription of certain genes regulated by the acetylation complex SAGA. Gene-associated H2B ubiquitylation was transient, increasing early during activation, and then decreasing coincident with significant RNA accumulation. We show that Ubp8, a component of the SAGA acetylation complex, is required for SAGA-mediated deubiquitylation of histone H2B in vitro. Loss of Ubp8 in vivo increased both gene-associated and overall cellular levels of ubiquitylated H2B. Deletion of Ubp8 lowered transcription of SAGA-regulated genes, and the severity of this defect was exacerbated by codeletion of the Gcn5 acetyltransferase within SAGA. In addition, disruption of either ubiquitylation or Ubp8-mediated deubiquitylation of H2B resulted in altered levels of gene-associated H3 Lys 4 methylation and Lys 36 methylation, which have both been linked to transcription. These results suggest that the histone H2B ubiquitylation state is dynamic during transcription, and that the sequence of histone modifications helps to control transcription.

Distinct posttranslational modifications on histones occur in specific patterns to mediate certain chromosomal events. For example, on histone H3, phosphorylation at Ser10 can enhance GCN5-mediated Lys14 acetylation to promote transcription. To gain insight into the mechanism underlying this synergism, we determined the structure of Tetrahymena GCN5 (tGCN5) and coenzyme A (CoA) bound to unmodified and Ser10-phosphorylated 19 residue histone H3 peptides (H3p19 and H3p19Pi, respectively). The tGCN5/CoA/H3p19 structure reveals that a 12 amino acid core sequence mediates extensive contacts with the protein, providing the structural basis for substrate specificity by the GCN5/PCAF family of histone acetyltransferases. Comparison with the tGCN5/CoA/H3p19Pi structure reveals that phospho-Ser10 and Thr11 mediate significant histone-protein interactions, and nucleate additional interactions distal to the phosphorylation site. Functional studies show that histone H3 Thr11 is necessary for optimal transcription at yGcn5-dependent promoters requiring Ser10 phosphorylation. Together, these studies reveal how one histone modification can modulate another to affect distinct transcriptional signals.