Bacterial Ohr and OsmC paralogues define two protein families with distinct functions and patterns of expression

Xanthomonas campestris Ohr (a protein involved in organic peroxide protecti on) and Escherichia coli OsmC (an osmotically inducible protein of unknown function) are related proteins. Database searches and phylogenetic analyses reveal that Ohr and OsmC homologues cluster into two related subfamilies o f proteins widely distributed in both Gram-negative and Grampositive bacter ia. To determine if these two subfamilies are functionally distinct, ohr an d osmC in Pseudomonas aeruginosa (a bacterium with one representative from each subfamily) were analysed. Only ohr mutants are hypersensitive to organ ic peroxide, and this phenotype can be restored by complementation with ohr but not osmC. In addition, expression of ohr ias highly induced only by or ganic peroxides, and not by other oxidants or stresses. In contrast, osmC w as induced by ethanol and osmotic stress. A similar pattern of regulation w as observed for Ohr and OsmC homologues in the Gram-positive bacterium Dein ococcus radiodurans, though uninduced expression was much higher and induct ion lower in this species. These data clearly support the conclusion that O hr and OsmC define two functionally distinct subfamilies with distinct patt erns of regulation.