Human Dmc1 protein binds DNA as an octameric ring.

The bacterial RecA protein has been the most intensively studied enzyme in homologous genetic recombination. The core of RecA is structurally homologous to that of the F1-ATPase and helicases. Like the F1-ATPase and ring helicases, RecA forms a hexameric ring. The human Dmc1 (hDmc1) protein, a meiosis-specific recombinase, is homologous to RecA. We show that hDmc1 forms octameric rings. Unlike RecA and Rad51, however, hDmc1 protein does not form helical filaments. The hDmc1 ring binds DNA in the central channel, as do the ring helicases, which is likely to represent the active form of the protein. These observations indicate that the conservation of the RecA-like ring structure extends from bacteria to humans, and that some RecA homologs may form both rings and filaments, whereas others may function only as rings.