A histone H3 Ser10 phosphorylation‐independent function of Snf1 and Reg1 proteins rescues a gcn5¬C mutant in HIS3 expression

Gcn5 serves as the catalytic subunit of the SAGA histone acetyltransferase (HAT) complex and is critical for HIS3 activation in Saccharomyces cerevisiae. A truncated Reg1 protein, Reg1(1–740), was found to be a dominant suppressor rescuing the HIS3 transcriptional defects of a gcn5 mutant (E173H). The function of Reg1(1–740) protein requires an intact cis‐acting element for the Gcn4 transcriptional activator, and another SAGA component, Spt3, suggesting that the observed suppression was achieved via the normal SAGA regulatory pathway. Reg1 protein functionally and physically interacts with a histone H3 kinase, Snf1. Indeed, Snf1 plays an important role for normal and Reg1(1–740)‐dependent HIS3 activation. However, substituting the phosphorylation residue in H3, i.e. Ser10, with alanine or glutamate neither attenuates nor augments the suppression phenotypes. These results argue against an essential role of H3 phosphorylation in HIS3 expression. Both Reg1(1–740) and overexpressed Snf1 rescue the E173H allele of gcn5 selectively, suggesting physical interactions between Gcn5 and these two proteins. In vivo co‐purification experiments confirmed this notion, and importantly, in vitro data showed that Gcn5 can be phosphorylated by Snf1 protein. Together, these data suggest that Reg1 and Snf1 proteins activate HIS3 in a histone H3 phosphorylation‐independent manner that may also involve a non‐catalytic function or a novel HAT activity of the Gcn5 protein. This work was supported by NIH grant GM 62282