Involvement of the NADH oxidase-encoding noxA gene in oxidative stress responses in Corynebacterium glutamicum

Abstract

Corynebacterium glutamicum ORF NCgl0328, designated noxA, encodes an NADH oxidase enzyme. The noxA gene, which was preferentially expressed in the log growth phase, was found to be under the control of the whcA, whcB, and whcE genes, which play regulatory roles in cells under oxidative stress. While noxA transcription was minimal in whcE-deleted mutant cells (Delta whcE) during growth, its transcription was maximal even in the stationary phase in Delta whcA cells. The transcription levels of noxA in Delta whcB and whcB-overexpressing cells were comparable to the levels only in the log growth phase in Delta whcA and whcA-overexpressing cells, respectively. Direct binding of purified WhcA to the promoter region of noxA was observed in vitro. The DNA-protein interaction was only possible in the presence of the reducing agent dithiothreitol. A noxA-deleted mutant strain and a strain overexpressing the noxA gene (P-180-noxA) were established, and these strains were found to exhibit defective cell growth. The Delta noxA and P-180-noxA strains were sensitive to the redox-cycling oxidant menadione, suggesting a role of noxA in redox balancing. Accordingly, the purified NoxA enzyme exhibited NADH-oxidizing activity. Taken together, these data show that noxA plays a role in oxidative stress responses and also that the gene is under direct control of the WhcA protein, which was shown to be a regulatory DNA-binding protein. Furthermore, the involvement and roles of the whcA, whcB, and whcE genes in regulating the expression of noxA were demonstrated.