Figure 1: Function encoded in structure.
Hoffmann et al.2 resolved three structures of the enzyme RNA polymerase (Pol) III, which transcribes short RNAs. Pol III contains specialized subunits — the C82–C34–C31 heterotrimer and the C53–C37 heterodimer — that enable it to terminate and reinitiate transcription. The authors find that the heterotrimer packs onto the enzyme's clamp-head domain, and extends helices that help to hold the DNA awaiting transcription in a tight grip in the structure's cleft. The heterodimer connects to the lobe and jaw domains on the lower side of the cleft. By contrast, DNA in the active centre (where transcription occurs) is held loosely. During termination, the untranscribed DNA strand, which contains terminator signal sequences, makes contact with C37. C37 might transmit termination signals to the lower side of the cleft, releasing the downstream DNA. The authors propose that the loose grip in the active centre then enables the DNA to easily separate from the enzyme. For simplicity, not all Pol III subunits are shown.
