Promoter regulation by distinct mechanisms of functional interplay between lysine acetylase Rtt109 and histone chaperone Asf1.

The promoter activity of yeast genes can depend on lysine 56 (K56) acetylation of histone H3. This modification of H3 is performed by lysine acetylase Rtt109 acting in concert with histone chaperone Asf1. We have examined the contributions of Rtt109, Asf1, and H3 K56 acetylation to nutrient regulation of a well-studied metabolic gene, ARG1. As expected, Rtt109, Asf1, and H3 K56 acetylation are required for maximal transcription of ARG1 under inducing conditions. However, Rtt109 and Asf1 also inhibit ARG1 under repressing conditions. This inhibition requires Asf1 binding to H3-H4 and Rtt109 KAT activity, but not tail acetylation of H3-H4 or K56 acetylation of H3. These observations suggest the existence of a unique mechanism of transcriptional regulation by Rtt109. Indeed, chromatin immunoprecipitation and genetic interaction studies support a model in which promoter-targeted Rtt109 represses ARG1 by silencing a pathway of transcriptional activation that depends on ASF1. Collectively, our results show that ARG1 transcription intensity at its induced and repressed set points is controlled by different mechanisms of functional interplay between Rtt109 and Asf1.

SWI/SNF and Asf1 independently promote derepression of the DNA damage response genes under conditions of replication stress.

The histone chaperone Asf1 and the chromatin remodeler SWI/SNF have been separately implicated in derepression of the DNA damage response (DDR) genes in yeast cells treated with genotoxins that cause replication interference. Using genetic and biochemical approaches, we have tested if derepression of the DDR genes in budding yeast involves functional interplay between Asf1 and SWI/SNF. We find that Asf1 and SWI/SNF are both recruited to DDR genes under replication stress triggered by hydroxyurea, and have detected a soluble complex that contains Asf1 and the Snf2 subunit of SWI/SNF. SWI/SNF recruitment to DDR genes however does not require Asf1, and deletion of Snf2 does not affect Asf1 occupancy of DDR gene promoters. A checkpoint engagement defect is sufficient to explain the synthetic effect of deletion of ASF1 and SNF2 on derepression of the DDR genes in hydroxyurea-treated cells. Collectively, our results show that the DDR genes fall into a class in which Asf1 and SWI/SNF independently control transcriptional induction.

Asf1 can promote trimethylation of H3 K36 by Set2.

Asf1 is a conserved histone H3/H4 chaperone that can assemble and disassemble nucleosomes and promote histone acetylation. Set2 is an H3 K36 methyltransferase. The functions of these proteins intersect in the context of transcription elongation by RNA polymerase II: both contribute to the establishment of repressive chromatin structures that inhibit spurious intragenic transcription. Here we characterize further interactions between budding yeast (Saccharomyces cerevisiae) Asf1 and Set2 using assays of intragenic transcription, H3/H4 posttranslational modification, coding region cross-linking of Asf1 and Set2, and cooccurrence of Asf1 and Set2 in protein complexes. We find that at some genes Asf1 and Set2 control chromatin metabolism as components of separate pathways. However, the existence of a low-abundance complex containing both proteins suggests that Asf1 and Set2 can more directly collaborate in chromatin regulation. Consistent with this possibility, we show that Asf1 stimulates Set2 occupancy of the coding region of a highly transcribed gene by a mechanism that depends on Asf1 binding to H3/H4. This function of Asf1 promotes the switch from di- to trimethylation of H3 K36 at that gene. These results support the view that Set2 function in chromatin metabolism can intimately involve histone chaperone Asf1.