DnaB and DciA: Mechanisms of Helicase Loading and Translocation on ssDNA

Replicative helicases are assembled on chromosomes by helicase loaders before initiation of DNA replication. Here, we investigate mechanisms used by the bacterial Vibrio cholerae ( Vc ) DnaB replicative helicase and the DciA helicase loader. Structural analysis of the ATPγS form of the Vc DnaB-ssDNA complex reveals a configuration distinct from that seen with GDP•AlF ₄ . With ATPγS, the amino-terminal (NTD) tier, previously found as an open spiral in the GDP•AlF ₄ complex, adopts a closed planar arrangement. Further, the DnaB subunit at the top of the carboxy-terminal spiral (CTD) tier is displaced by ∼25 Å between the two forms. We suggest that remodeling the NTD layer between closed planar and open spiral configurations and migration of two distinct CTDs to the top of the DnaB spiral, repeated three times, mediates hand-over-hand translocation. Biochemical analysis suggests that Vc DciA leverages its Lasso domain to contact DnaB near its Docking-Linker-Helix interface. Up to three copies of Vc DciA bind to Vc DnaB and suppress its ATPase activity during loading onto physiological DNA substrates. Our data suggest that DciA loads DnaB onto DNA using the ring-opening mechanism.