Evidence that the elongation factor TFIIS plays a role in transcription initiation at GAL1 in Saccharomyces cerevisiae

TFIIS is a transcription elongation factor that has been extensively studied biochemically. Although the in vitro mechanisms by which TFIIS stimulates RNA transcript cleavage and polymerase read-through have been well characterized, its in vivo roles remain unclear. To better understand TFIIS function in vivo, we have examined its role during Gal4-mediated activation of the Saccharomyces cerevisiae GAL1 gene. Surprisingly, TFIIS is strongly associated with the GAL1 upstream activating sequence. In addition, TFIIS recruitment to Gal4-binding sites is dependent on Gal4, SAGA, and Mediator but not on RNA polymerase II (Pol II). The association of TFIIS is also necessary for the optimal recruitment of TATA-binding protein and Pol II to the GAL1 promoter. These results provide strong evidence that TFIIS plays an important role in the initiation of transcription at GAL1 in addition to its well-characterized roles in transcription elongation.