Cooperative Clamp-Mediated Promoter Recognition by Poxviral RNAP and Its TBP/TFIIB-Like Partner

The recruitment of RNA polymerase (RNAP) to gene promoters is a critical step in gene expression. For RNAP II, this process is initiated by the TATA-box binding protein (TBP) and transcription factor IIB (TFIIB), with homologs of these TBP/TFIIB pairs found in all known multi-subunit RNAP systems. Here, we describe a previously unknown mode of promoter recognition by the poxviral intermediate transcription factor 3 (VITF-3). This heterodimeric factor comprises an atypical TBP/TFIIB pair forming a stable ring structure inert towards DNA in the absence of viral RNAP (vRNAP). Promoter recognition instead requires concerted VITF-3 and vRNAP binding, as revealed by cryo-EM analysis of the intermediate pre-initiation complex (iPIC). During iPIC formation, vRNAP facilitates ring opening and loading of VITF-3 onto the promoter, anchoring the polymerase at the transcription start site. Our findings propose vRNAP as a clamp loader for VITF-3 and reveal a thus far unknown transcription initiation mechanism.