Strains lacking either of these two mediators

have been shown to be more sensitive to pro-oxidants such as hydrogen peroxide, menadione and methyl viologen or paraquat (7, 9), suggesting that oxyR and rpoS are essential for survival and growth under oxidative conditions. Similar results have been found in other bacterial species and the role of OxyR in the response to oxidative stress is well established. selleck antibody For example, oxyR mutants of Pseudomonas aeruginosa are hypersensitive to pro-oxidants including H2O2 and paraquat (16) while E. coli with deletions of oxyR are hypersensitive to hydrogen peroxide and have increased rates of spontaneous mutation during aerobic growth (17). Similarly, oxyR mutants of Brucella abortus, Erwinia carotovora and Xanthomonas campestris, all show increased sensitivity to pro-oxidants (17–20). Negative regulation of oxyR by RpoS has been reported in E. coli (21). In particular the degree of β-galactosidase expression from a single-copy oxyR::lacZ fusion in a RpoS-defective strain has been shown to be higher than in its parental strain as the cells enter into, and remain in, the stationary phase growth (21). Additionally, increased expression of RpoS prevents the normal expression of oxyR (21). However, in contrast to this,

Schellhorn observed a significant reduction in oxyR expression in an E. coli rpoS::Tn10 mutant (22), a result supported by our own observations with B. pseudomallei in RG7422 molecular weight which Methocarbamol low amounts of CAT activity were observed in oxyR::CAT/rpoS−, which contains a chromosomal oxyR::CAT fusion and is null for rpoS. More significantly, isogenic replacement of RpoS in strain oxyR::CAT/rpoS−/RpoS restored oxyR::CAT expression to the extent seen in the parental strain (oxyR::CAT), suggesting that RpoS acts as a positive regulator of oxyR transcription in

B. pseudomallei. Three genes have been shown to be under transcriptional control of OxyR, namely dpsA (23), katG (24) and gorA (25). The expression pattern of katG during growth of B. pseudomallei has been previously examined using a chromosomal katG::CAT fusion as a reporter. CAT activity was observed to increase during early exponential growth, reaching a maximum value in the early stationary phase growth, after which it declined in the late stationary phase growth (6). Significantly, expression was greater in an oxyR mutant strain during all phases of growth, suggesting that katG expression is negatively regulated by OxyR during normal growth, although further studies showed that katG was positively regulated by OxyR during oxidative stress (6). The negative regulation of katG by oxyR was confirmed in this study, a greater degree of CAT expression being seen in katG::CAT as compared to katG::CAT/oxyR−.