THE DNA-BINDING DOMAIN OF THE VACCINIA VIRUS EARLY TRANSCRIPTION FACTOR SMALL-SUBUNIT IS AN EXTENDED HELICASE-LIKE MOTIF

The vaccinia virus early transcription factor (VETF) is an ATP-depende nt activator of the early class of viral genes, VETF is a heterodimeri c protein that binds an initiator-like element surrounding the start s ite of transcription. Previous studies indicated that the small subuni t of VETF contacts the promoter DNA, We have taken a mutational approa ch to determine sequences in the VETF small subunit that are important for DNA binding, Two types of sequences were targeted for mutation: o nes resembling motifs that are conserved in the nucleic acid helicase family and positively charged residues in predicted alpha-helices, Mut ations affecting transcription activation were clustered in two region s, One mutation that impaired DNA binding is located near the N-termin us within the putative ATP-binding pocket that comprises helicase doma in I. DNA binding was also severely reduced by mutations in a sequence resembling helicase domain VI and two putative alpha-helices that fla nk this domain in the C-terminal third of the polypeptide, These resul ts indicate that the DNA binding domain in the small subunit of VETF i s not isolated within a separable domain as is the case with most tran scription factors, but rather, spans most of the length of the 637 res idue polypeptide. A model for VETF structure is suggested in which the active site for ATP hydrolysis is integrated within an extended DNA-b inding domain such that the structure and function of each domain infl uences that of the other.