Structural insight into Helicobacter pylori DNA replication initiation.

While increasing knowledge is accumulating about the molecular mechanisms allowing the human pathogen Helicobacter pylori to survive and to subvert host defenses, much less is known about fundamental aspects of its biology, including DNA replication. We have studied the initiation step of chromosome replication of H. pylori and particularly the interaction between the initiator protein DnaA and its recently identified regulator HobA. This work has recently culminated in the determination of the crystal structure of the domains I and II of DnaA (DnaA(I-II)) in complex with HobA. By combining the structure with a variety of biochemical experiments we show that a tetramer of HobA can accommodate up to four DnaA molecules organized in a particular conformation within the complex. Mutations of the HobA interface that impaired the binding with DnaA were designed and proved to be lethal once introduced into H. pylori. These features suggest that HobA provides a molecular scaffold onto which regular oligomers of DnaA can assemble. The HobA-promoted oligomerization of DnaA could have a determinant role in the formation of the open complex. We propose a speculative model of HobA-dependent DnaA oligomerization leading to DNA unwinding. More generally, the parallel we draw with Escherichia coli DnaA and DiaA (HobA-like E. coli protein) will direct new studies that will contribute to the understanding of bacterial DNA replication.