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Characterization of SodC, a Periplasmic Superoxide Dismutase from Burkholderia cenocepacia

Infectious Diseases Research Group, Siebens-Drake Research Institute, Departments of Microbiology and Immunology,¹ Medicine, University of Western Ontario, London, Ontario N6A 5C1, Canada²

Received 27 September 2006/ Returned for modification 8 November 2006/ Accepted 13 February 2007

Burkholderia cenocepacia is a gram-negative, non-spore-forming bacillus and a member of the Burkholderia cepacia complex. B. cenocepacia can survive intracellularly in phagocytic cells and can produce at least one superoxide dismutase (SOD). The inability of O₂^– to cross the cytoplasmic membrane, coupled with the periplasmic location of Cu,ZnSODs, suggests that periplasmic SODs protect bacteria from superoxide that has an exogenous origin (for example, when cells are faced with reactive oxygen intermediates generated by host cells in response to infection). In this study, we identified the sodC gene encoding a Cu,ZnSOD in B. cenocepacia and demonstrated that a sodC null mutant was not sensitive to a H₂O₂, 3-morpholinosydnonimine, or paraquat challenge but was killed by exogenous superoxide generated by the xanthine/xanthine oxidase method. The sodC mutant also exhibited a growth defect in liquid medium compared to the parental strain, which could be complemented in trans. The mutant was killed more rapidly than the parental strain was killed in murine macrophage-like cell line RAW 264.7, but killing was eliminated when macrophages were treated with an NADPH oxidase inhibitor. We also confirmed that SodC is periplasmic and identified the metal cofactor. B. cenocepacia SodC was resistant to inhibition by H₂O₂ and was unusually resistant to KCN for a Cu,ZnSOD. Together, these observations establish that B. cenocepacia produces a periplasmic Cu,ZnSOD that protects this bacterium from exogenously generated O₂^– and contributes to intracellular survival of this bacterium in macrophages.

Published ahead of print on 26 February 2007.

Editor: J. N. Weiser

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