Transcriptional activation independent of TFIIH kinase and the RNA polymerase II mediator in vivo.

The carboxy-terminal domain (CTD) of the largest subunit of RNA polymerase II becomes multiply phosphorylated by protein kinases during early steps in the gene transcription cycle both in vivo and in vitro. In yeast, the major CTD kinase is a subunit of the general transcription factor TFIIH, and is encoded by an essential gene, KIN28. Although the CTD and its phosphorylation are important for transcription, in vitro studies have challenged whether CTD phosphorylation is an absolutely required step. The general importance of CTD phosphorylation by Kin28 for transcription in yeast has been suggested because, for all genes tested, transcription is inhibited at the non-permissive temperature in temperature-sensitive kin28 mutants. However, using such a mutant and a copper-inducible targeted destruction method, we show here that transcription of certain genes can be highly induced even when cells lack Kin28. We also show that transcription of these Kin28-independent genes is independent of Srb4 and Srb6, critical components of the CTD-associated transcriptional mediator complex. These results indicate that there are at least two distinct pathways for transcriptional activation: one is dependent on Kin28 and the mediator complex, and the other is not.