Structural basis for activation and mechanism of the bacterial hexameric motors RadA and ComM in DNA branch migration during homologous recombination

Some DNA helicases play key roles in genome maintenance and plasticity through their branch migration activity in distinct pathways of homologous recombination. RadA is a highly conserved bacterial helicase involved in both DNA repair in all species and in natural transformation in Gram positive firmicutes. In others, ComM is the natural transformation-specific helicase. Both RadA and ComM form hexameric rings and use ATP hydrolysis as a source of energy. In this work we present the cryoEM structures of RadA and ComM interacting with DNA and ATP analogues. This is the first structure reported for ComM, and the first structure of RadA in its active state. In particular, this study unveiled a molecular switch for ATP hydrolysis and DNA binding coupling in RadA and the role of the Lon protease-like domain, shared by RadA and ComM. Together our results provide new insights on the branch migration mechanism at the molecular level in the context of homologous recombination. ### Competing Interest Statement The authors have declared no competing interest. RadA model information is available under accession PDB 8RXC and density map under accession EMDB 19573. dsDNA-bound ComM main map is available under EMDB 19574 and PDB 8RXD, while map relating to subpopulation 2 is under EMDB 19575. Coordinates of ComM Lon domain are available under the accession codes PDB: 8RXS and EMDB 19578. Coordinates for dsDNA-bound ComM trimer are available under accession code PDB: 8RXK and EMDB 19577. Density map for the dsDNA-bound ComM hexamer used for focused refinement are submitted under coordinates EMDB 19581. Coordinates and density map for ComM hexamer in absence of DNA are available under accession codes PDB: 8RXT and EMDB 19579. Density map for ComM dodecamers coordinating dsDNA area available under EMDB 19580 accession code.