The structure of a DnaA/HobA complex from Helicobacter pylori provides insight into regulation of DNA replication in bacteria.

Bacterial DNA replication requires DnaA, an AAA+ ATPase that initiates replication at a specific chromosome region, oriC, and is regulated by species-specific regulators that directly bind DnaA. HobA is a DnaA binding protein, recently identified as an essential regulator of DNA replication in Helicobacter pylori. We report the crystal structure of HobA in complex with domains I and II of DnaA (DnaA(I-II)) from H. pylori, the first structure of DnaA bound to one of its regulators. Biochemical characterization of the complex formed shows that a tetramer of HobA binds four DnaA(I-II) molecules, and that DnaA(I-II) is unable to oligomerize by itself. Mutagenesis and protein-protein interaction studies demonstrate that some of the residues located at the HobA-DnaA(I-II) interface in the structure are necessary for complex formation. Introduction of selected mutations into H. pylori shows that the disruption of the interaction between HobA and DnaA is lethal for the bacteria. Remarkably, the DnaA binding site of HobA is conserved in DiaA from Escherichia coli, suggesting that the structure of the HobA/DnaA complex represents a model for DnaA regulation in other Gram-negative bacteria. Our data, together with those from other studies, indicate that HobA could play a crucial scaffolding role during the initiation of replication in H. pylori by organizing the first step of DnaA oligomerization and attachment to oriC.