Dissociable Rpb4-Rpb7 subassembly of RNA polymerase II binds to single-strand nucleic acid and mediates a post-recruitment step in transcription initiation

The Rpb4 and Rpb7 subunits of yeast RNA polymerase II form a heterodimeric complex essential for promoter-directed transcription initiation in a recon stituted system. Results of template competition experiments indicate that the Rpb4-Rpb7 complex is not required for stable recruitment of polymerase to active preinitiation complexes, suggesting that Rpb4-Rpb7 mediates an es sential step subsequent to promoter binding. Sequence and structure-based a lignments revealed a possible OB-fold single-strand nucleic acid-binding mo tif in Rpb7, Purified Rpb4-Rpb7 complex exhibited both single-strand DNA- a nd RNA-binding activities, and a small deletion in the putative OB-fold nuc leic acid-binding surface of Rpb7 abolished binding activity without affect ing the stability of the Rpb4-Rpb7 complex or its ability to associate with polymerase, The same mutation destroyed the transcription activity of the Rpb4-Rpb7 complex. A separate deletion elsewhere in the OB-fold motif of Rp b7 also blocked transcription but did not affect nucleic acid binding, sugg esting that the OB-fold of Rpb7 mediates both DNA-protein and protein-prote in interactions required for productive initiation.