THE PERIPLASMIC, GROUP-III CATALASE OF VIBRIO-FISCHERI IS REQUIRED FOR NORMAL SYMBIOTIC COMPETENCE AND IS INDUCED BOTH BY OXIDATIVE STRESS AND BY APPROACH TO STATIONARY-PHASE

The catalase gene, katA, of the sepiolid squid symbiont Vibrio fischer i has been cloned and sequenced. The predicted amino acid sequence of KatA has a high degree of similarity to the recently defined group III catalases, including those found in Haemophilus influenzae, Bacteroid es fragilis, and Proteus mirabilis, Upstream of the predicted start co don of katA is a sequence that closely matches the consensus sequence for promoters regulated in Escherichia coli by the alternative sigma f actor encoded by rpoS, Further, the level of expression of the cloned katA gene in an E. coli rpoS mutant is much lower than in wild-type E. coli. Catalase activity is induced three-to fourfold both as growing V. fischeri cells approach stationary phase and upon the addition of a small amount of hydrogen peroxide during logarithmic growth. The cata lase activity was localized in the periplasm of wild-type V. fischeri cells, where its role could be to detoxify hydrogen peroxide coming fr om the external environment. No significant catalase activity could be detected in a katA null mutant strain, demonstrating that KatA is the predominately expressed catalase in V. fischeri and indicating that V . fischeri carries only a single catalase gene. The catalase mutant wa s defective in its ability to competitively colonize the light organs of juvenile squids in coinoculation experiments with the parent strain , suggesting that the catalase enzyme plays an important role in the s ymbiosis between V. fischeri and its squid host.