An RNA polymerase II holoenzyme responsive to activators.

RNA polymerase II requires multiple general transcription factors to initiate site-specific transcription. These proteins can assemble in an ordered fashion onto promoter DNA in vitro, and such ordered assembly may occur in vivo (Fig. 1a). Some general transcription factors can interact with RNA polymerase II in the absence of DNA, however, suggesting that RNA polymerase II may also assemble into a multi-component complex containing a subset of initiation factors before binding to promoter DNA (Fig. 1b). Here we present evidence from the yeast Saccharomyces cerevisiae for such an RNA polymerase II holoenzyme, a multi-subunit complex containing roughly equimolar amounts of RNA polymerase II, a subset of general transcription factors, and SRB regulatory proteins. Transcription by this holoenzyme is stimulated by the activator protein GAL4-VP16, a feature not observed with purified RNA polymerase II and general transcription factors alone. We propose that the holoenzyme is a form of RNA polymerase II readily recruited to promoters in vivo.