The molecular mechanisms by which deep-sea bacteria such as Photobacterium profundum SS9 grow optimally at high pressure and cold temperatures are poorly understood. In this thesis, four previously identified P. profundum SS9R mini-transposon mutants were investigated. The high pressure-sensitive FL31 mutant has an insertion in a putative rctB gene. V. cholerae RctB protein is essential for replication initiation at the origin of chromosome II, oriCII. In this thesis, the potential link between pressure sensitivity, RctB and oriCII was investigated using a plasmid-based system in E. coli. The P. profundum oriCII was identified and the incompatibility region (incII) was shown to inhibit oriCII activity. The putative rctB gene was essential for oriCII function and when truncated, as in FL31, it conferred insensitivity to incII inhibition, indicating a link between downregulation of oriCII activity and survival at increased pressure. These findings provide the first characterisation of the replication origin of chromosome II in an organism other than V. cholerae and point to a critical control mechanism for chromosome duplication in response to changes in a key environmental factor, pressure. The FL24, FL25 and FL26 mutants display a cold-sensitive phenotype only on agar. In this study FL25 and FL26 were found to lack smooth LPS and displayed increased swimming motility compared to the parent, which could indicate alterations in their polar flagellum function. Since the polar flagellum can act as a surface sensor that induces changes in gene expression, the cold-sensitive colony growth phenotypes of the S-LPS mutants could stem from defects in this surface sensing mechanism. The observed lack of polar flagellum in FL24 rendered the mutant non-motile but was not directly responsible for its cold-sensitive phenotype. Hypothetically, the cold sensitivity of FL24 could stem from the transposon insertion in gene encoding the FliS chaperone and subsequent intracellular accumulation of flagellin.