ASSOCIATION OF AN ACTIVATOR WITH AN RNA-POLYMERASE-II HOLOENZYME

RNA polymerase II holoenzymes have been described that consist of RNA polymerase II, a subset of general transcription factors, and four SRB proteins. The SRB proteins, which were identified through a selection for genes involved in transcription initiation by RNA polymerase IT i n vivo, are a hallmark of the holoenzyme. We report here the isolation and characterization of additional SRB genes. We show that the produc ts of all nine SRB genes identified thus far are components of the RNA polymerase II holoenzyme and are associated with a holoenzyme subcomp lex termed the mediator of activation. The holoenzyme is capable of re sponding to a transcriptional activator, suggesting a model in which a ctivators function, in part, through direct interactions with the holo enzyme. Immunoprecipitation experiments with anti-SRB5 antibodies demo nstrate that the acidic activating domain of VP16 specifically binds t o the holoenzyme. Furthermore, the holoenzyme and the mediator subcomp lex bind to a VP16 affinity column. These results provide a more compl ete description of the RNA polymerase II holoenzyme and suggest that t his form of the transcription apparatus can be recruited to promoters via direct interactions with activators.