Replication of the bacterial chromosome is initiated by binding the DnaA protein to oriC. Various factors control the ability of DnaA to bind and unwind DNA. Among them, Escherichia coli DiaA and Helicobacter pylori HobA have been characterized recently. They were found to interact with domain I of DnaA and stimulate DnaA binding to oriC. We examined HobA and DiaA functional homology and showed that, despite a high degree of structural similarity, they are not interchangeable because they are unable to interact with heterologous DnaA proteins. We revealed particular structural differences impeding formation of heterologous complexes and, consistently, we restored DiaA-enhanced oriC binding by the hybrid Ec(I)-Hp(II-IV)DnaA protein; i.e. H. pylori DnaA in which domain I was exchanged with that of E. coli. This proved that DiaA and HobA are functional homologs and upon binding to DnaA they exert a similar effect on orisome formation. Interestingly, we showed for the first time that the dynamics of DiaA- and HobA-stimulated orisome assembly are different. HobA enhances and accelerates HpDnaA binding to oriC, whereas DiaA increases but decelerates EcDnaA binding with oriC. We postulate that the different dynamics of orisome formation reflect the distinct strategies adopted by E. coli and H. pylori to regulate the frequency of the replication of their chromosomes. DiaA/HobA homolog have been identified in many proteobacteria and therefore might constitute a common, though species-specific, factor modulating bacterial orisome assembly.
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