RecD function is required for high-pressure growth of a deep-sea bacterium

A genomic library derived from the deep-sea bacterium Photobacterium profundum SS9 was conjugally delivered into a previously isolated pressure-sensitive SS9 mutant, designated EC1002 (E. Chi and D. H. Bartlett, J. Bacteriol. 175:7533-7540, 1993), and exconjugants were screened for the ability to grow at 280-atm hydrostatic pressure. Several clones were identified that had restored high-pressure growth. The complementing DNA was localized and in all cases found to possess strong homology to recD, a DNA recombination and repair gene. EC1002 was found to be deficient in plasmid stability, a phenotype also seen in Escherichia coli recD mutants. The defect in EC1002 was localized to a point mutation that created a stop codon within the recD gene. Two additional recD mutants were constructed by gene disruption and were both found to possess a pressure-sensitive growth phenotype, although the magnitude of the defect depended on the extent of 3' truncation of the recD coding sequence. Surprisingly, the introduction of the SS9 recD gene into an E. coli recD mutant had two dramatic effects. At high pressure, SS9 recD enabled growth in the E. coli mutant strain under conditions of plasmid antibiotic resistance selection and prevented cell filamentation. Both of these effects were recessive to wild-type E. coli recD. These results suggest that the SS9 recD gene plays an essential role in SS9 growth at high pressure and that it may be possible to identify additional aspects of RecD function through the characterization of this activity.