DnaA proteins of Escherichia coli and Bacillus subtilis: coordinate actions with single-stranded DNA-binding protein and interspecies inhibition during open complex formation at the replication origins.

DnaA-mediated unwinding of the AT-rich region in the replication origins of Escherichia coli and Bacillus subtilis was analysed in vitro with and without single-stranded DNA-binding protein (SSB). In the presence of SSB, the unwound region was larger by a defined number of base pairs. Although the overall structure of the origins is very different, the size and structure of the unwound region were similar. The unwinding reaction at oriC of one organism was inhibited by DnaA protein of the other bacterium. Similarly, hybrid DnaA proteins with swapped DNA-binding domains were inactive and inhibitory to 'open complex' formation at both origins. We suggest that the inhibition is due to inactive mixed complexes.