The transcription elongation factor TFIIS is a component of RNA polymerase II preinitiation complexes

In this article, we provide direct evidence that the evolutionarily conserved transcription elongation factor THIS functions during preinitiation complex assembly. First, we identified THIS in a mass spectrometric screen of RNA polymerase II (PoI II) preinitiation complexes (PICs). Second, we show that the association of THIS with a promoter depends on functional PIC components including Mediator and the SAGA complex. Third, we demonstrate that THIS is required for efficient formation of active PICs. Using truncation mutants of THIS, we find that the PoI II-binding domain is the minimal domain necessary to stimulate PIC assembly. However, efficient formation of active PICs requires both the PoI II-binding domain and the poorly understood N-terminal domain. Importantly, Domain III, which is required for the elongation function of THIS, is dispensable during PIC assembly. The results demonstrate that TFIIS is a PIC component that is required for efficient formation and/or stability of the complex.