KIN28 ENCODES A C-TERMINAL DOMAIN KINASE THAT CONTROLS MESSENGER-RNA TRANSCRIPTION IN SACCHAROMYCES-CEREVISIAE BUT LACKS CYCLIN-DEPENDENT KINASE-ACTIVATING KINASE (CAK) ACTIVITY

The Saccharomyces cerevisiae gene KIN28 is a member of the cyclin-depe ndent kinase (CDK) family. The Kin28 protein shares extensive sequence identity with the vertebrate CDK-activating kinase MO15 (Cdk7), which phosphorylates CDKs in vitro on a critical threonine residue. Kin28 a nd MO15 have recently been found to copurify with the transcription fa ctor IIH (TFIIH) holoenzyme of yeast and human cells, respectively. Al though TFIIH is capable of phosphorylating the C-terminal domain (CTD) of RNA polymerase II, it has been unclear whether Kin28 is the physio logically relevant CTD kinase or what role CTD phosphorylation plays i n transcription. In this study, we used a thermosensitive allele of KI N28 and a hemagglutinin epitope-tagged Kin28 protein to investigate Ki n28 function in transcription and in the cell cycle. We show that Kin2 8 acts as a positive regulator of mRNA transcription in vivo and posse sses CTD kinase activity in vitro. However, Kin28 neither regulates th e phosphorylation state of the yeast cell cycle CDK, Cdc28, nor posses ses CDK-activating kinase activity in vitro. We conclude that Kin28 is a strong candidate for the physiological CTD kinase of S. cerevisiae and that Kin28 function is required for mRNA transcription.