The zinc ribbon domains of the general transcription factors TFIIB and Brf: conserved functional surfaces but different roles in transcription initiation

The function of the conserved zinc-binding domains in the related Pol II- a nd Pol III-specific factors TFIIB and Brf was investigated. Three-dimension al structure modeling and mutagenesis studies indicated that for both facto rs, the functional surface of the zinc ribbon fold consists of a small cons erved patch of residues located on one face of the domain comprised mainly of the second and third antiparallel beta strands. Previous studies have sh own that the TFIIB zinc ribbon is essential for recruitment of Pol II into the preinitiation complex. In contrast, Pol III recruitment assays and in v itro transcription demonstrate that the disruption of the Brf zinc ribbon d oes not lead to a defect in Pol III recruitment but, rather, a defect in op en complex formation. Therefore, the same conserved surface of the zinc rib bon domain has been adapted to serve distinct roles in the Pol II and Pol I II transcription machinery.