Abstract
TFIIS, an elongation factor encoded by DST1 in Saccharomyces cerevisiae, stimulates transcript cleavage in arrested RNA polymerase II. Two components of the RNA polymerase II machinery, Med13 (Srb9) and Spt8, were isolated as two-hybrid partners of the conserved TFIIS N-terminal domain. They belong to the Cdk8 module of the Mediator and to a subform of the SAGA co-activator, respectively. Co-immunoprecipitation experiments showed that TFIIS can bind the Cdk8 module and SAGA in cell-free extracts. spt8Delta and dst1Delta mutants were sensitive to nucleotide-depleting drugs and epistatic to null mutants of the RNA polymerase II subunit Rpb9, suggesting that their elongation defects are mediated by Rpb9. rpb9Delta, spt8Delta and dst1Delta were lethal in cells lacking the Rpb4 subunit. The TFIIS N-terminal domain is also strictly required for viability in rpb4Delta, although it is not needed for binding to RNA polymerase II or for transcript cleavage. It is proposed that TFIIS and the Spt8-containing form of SAGA co-operate to rescue RNA polymerase II from unproductive elongation complexes, and that the Cdk8 module temporarily blocks transcription during transcript cleavage.