Sequencing and expression of complementary DNA for the general transcription factor BTF3.

The initiation of transcription of eukaryotic genes involves the ordered assembly of a multiprotein complex on proximal promoter elements such as the TATA box. In addition to RNA polymerase II (otherwise RNA pol II, RNA polymerase B), four general transcription factors are required for initiation of transcription: BTF1 (also referred to as TFIID) which has recently been cloned from yeast, BTF2, BTF3 and STF. The first step in assembly of the initiation complex is the stable binding of BTF1 to the TATA box, which is facilitated by STF. Neither BTF2 nor BTF3 bind directly to the promoter proximal elements, but BTF3 can form a stable complex with RNA pol II. We recently purified BTF3, which is a protein of relative molecular mass 27,000, but further studies have been hampered by its low abundance in cells. On the basis of sequences from peptides of BTF3, we have now cloned two complementary DNAs, one for a protein (BTF3a) with all the characteristics of purified BTF3, and one for a shorter protein (BTF3b) lacking the first 44 residues of BTF3a and which is transcriptionally inactive, despite its ability to bind RNA pol II.