Molecular basis for RNA polymerase-dependent transcription complex recycling by the helicase-like motor protein HelD.

In bacteria, transcription complexes stalled on DNA represent a major source of roadblocks for the DNA replication machinery that must be removed in order to prevent damaging collisions. Gram-positive bacteria contain a transcription factor HelD that is able to remove and recycle stalled complexes, but it was not known how it performed this function. Here, using single particle cryo-electron microscopy, we have determined the structures of Bacillus subtilis RNA polymerase (RNAP) elongation and HelD complexes, enabling analysis of the conformational changes that occur in RNAP driven by HelD interaction. HelD has a 2-armed structure which penetrates deep into the primary and secondary channels of RNA polymerase. One arm removes nucleic acids from the active site, and the other induces a large conformational change in the primary channel leading to removal and recycling of the stalled polymerase, representing a novel mechanism for recycling transcription complexes in bacteria.