GAL4 is regulated by the RNA polymerase II holoenzyme-associated cyclin-dependent protein kinase SRB10/CDK8

Phosphorylation of the yeast transcription factor GAL4 at S699 is required for efficient galactose-inducible transcription. We demonstrate that this s ite is a substrate for the RNA polymerase holoenzyme-associated CDK SRB10. S699 phosphorylation requires SRB10 in vivo, and this site is phosphorylate d by purified SRB10/ SRB11 CDK/cyclin in vitro. RNA pol II holoenzymes puri fied from WT yeast phosphorylate GAL4 at sites observed in vivo whereas hol oenzymes from srb10 yeast are incapable of phosphorylating GAL4 at S699. Mu tations at GAL4 S699 and srb10 are epistatic for GAL induction, demonstrati ng that SRB10 regulates GAL4 activity through this phosphorylation in vivo. These results demonstrate a function for the SRB10/ CDK8 holoenzyme-associ ated CDK that involves regulation of transactivators by phosphorylation dur ing transcriptional activation.