Oxidized OxyR Up-Regulates Expression to Suppress Plating Defects of and Catalase-Deficient Strains.

It is well established that in bacteria, such as , OxyR is a transcriptional regulator that mediates the response to HO by activating the OxyR regulon, which consists of many genes that play vital roles in oxidative stress resistance. In , OxyR regulates, however, in both reduced and oxidized states, the production of HO scavengers, including major catalase KatB and NADH peroxidase AhpCF. Here we showed that the mutant carried a plating defect manifested as division arresting, a phenotype that can be completely suppressed by an OxyR variant constitutively existing in oxidized form (OxyR). This effect of OxyR could not be solely attributed to the increment in KatB production, since the suppression was also observed in the absence of KatB. Although expression of peroxidase CcpA was greatly activated by OxyR, the contribution of the protein in alleviating plating defect was negligible. We eventually identified AhpCF as the critical factor, when produced at substantially elevated levels by OxyR, to protect the cell from HO attack. Our data indicate that AhpCF is a particularly important peroxidase in oxidative stress resistance in , not only playing a compensatory role for catalase, but also by itself providing sufficient protection from killing of HO generated abiotically.