Structural basis of RNA polymerase III transcription initiation

RNA polymerase (Pol) III transcribes essential non-coding RNAs, including the entire pool of transfer RNAs, the 5S ribosomal RNA and the U6 spliceosomal RNA, and is often deregulated in cancer cells. The initiation of gene transcription by Pol III requires the activity of the transcription factor TFIIIB to form a transcriptionally active Pol III preinitiation complex (PIC). Here we present electron microscopy reconstructions of Pol III PICs at 3.4–4.0 Å and a reconstruction of unbound apo-Pol III at 3.1 Å. TFIIIB fully encircles the DNA and restructures Pol III. In particular, binding of the TFIIIB subunit Bdp1 rearranges the Pol III-specific subunits C37 and C34, thereby promoting DNA opening. The unwound DNA directly contacts both sides of the Pol III cleft. Topologically, the Pol III PIC resembles the Pol II PIC, whereas the Pol I PIC is more divergent. The structures presented unravel the molecular mechanisms underlying the first steps of Pol III transcription and also the general conserved mechanisms of gene transcription initiation. Detailed structures of yeast RNA polymerase III and its initiation complex shed light on how the transcription of essential non-coding RNAs begins and allow comparisons with other RNA polymerases. Structures of RNA polymerase III pre-initiation complex RNA polymerase III (Pol III) catalyses the transcription of short RNAs, including transfer RNAs and the 5S ribosomal RNA, which are essential for protein synthesis during cell growth. Pol III is predominantly regulated at the level of transcription initiation, and dysregulated Pol III activity is linked to diseases including cancer. Two independent studies in this issue, by Alessandro Vannini and colleagues and Christoph Müller and colleagues, describe electron cryo-microscopy structures of the yeast Pol III preinitiation complex, comprising the full 17-subunit Pol III and the three TFIIIB subunits (TBP, Brf1 and Bdp1) bound to promoter DNA in various functional states. The structures reveal the detailed mechanisms that underlie how Pol III is recruited to its target promoters and how promoter DNA is opened to form a stable transcription bubble, and also allow a comparison with the structures of Pol I and Pol II preinitiation complexes.