MULTIPLE SETS OF BASAL FACTORS INITIATE TRANSCRIPTION BY RNA-POLYMERASE-II

The minimal requirements for transcription initiation from supercoiled templates were determined for the two major forms of TATA-binding fac tors found in cell extracts, the 300-kDa B-TFIID and the 1000-kDa D-TF IID complexes. As had been observed for the TATA-binding protein (TBP) subunit (Parvin and Sharp, 1993), transcription from the IgH promoter minimally requires TFIID activity plus TFIIB and RNA polymerase II. T his minimal reaction is only active on negatively supercoiled template DNA. In contrast, the supercoiled templates encoding the adenovirus m ajor late promoter (MLP), or several other promoters, require the addi tion of TFIIF to the minimal reaction. Further addition of TFIIE and T FIIH boosts the level of transcription from these latter promoters but is not required. In contrast to the complete reaction on linear templ ate, transcription from supercoiled IgH or MLP templates does not requ ire the hydrolysis of the beta-gamma bond of ATP. Fourteen different c ore promoters were compared in complete and minimal basal transcriptio n reactions reconstituted with one of the three TATA activities: TBP, B-TFIID, and D-TFIID. Of these 14 promoters, only the IgH was active i n the absence of TFIIF, and the other promoters demonstrated different levels of transcription depending on which basal factors were present in reaction. It is proposed that a significant level of basal transcr iption only requires a minimal set of factors, and stimulation by upst ream activators may in part be mediated by the inclusion of additional basal factors into the initiation reaction.