Insights into the initiation of chromosome II replication of the pressure-loving deep-sea bacterium Photobacterium profundum SS9.

How DNA metabolism is adapted to survival of organisms such as the bacterium Photobacterium profundum SS9 at high pressure is unknown. Previously, a high pressure-sensitive P. profundum SS9 transposon mutant (FL31) was identified, with an insertion in a putative rctB gene. The Vibrio cholerae RctB protein is essential for replication initiation at the origin of chromosome II, oriCII. Using a plasmid-based system in E. coli we have identified the replication origin of chromosome II from P. profundum SS9 and have shown that the putative rctB gene, disrupted in FL31, is essential for oriCII function. Moreover, we found that a region corresponding to the V. cholerae oriCII incompatibility region (incII) exerts an inhibitory effect on P. profundum oriCII. The truncated rctB gene in FL31 confers insensitivity to incII inhibition, indicating that the C-terminus of RctB is important for the negative regulation of replication. The RctB proteins of V. cholerae and P. profundum are partially interchangeable, but full functionality is achieved only with the cognate origin. Our findings provide the first characterization of the replication origin of chromosome II in a deep-sea bacterium.