OXYGEN REGULATION OF NADH OXIDASE EXPRESSION IN STREPTOCOCCI

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We have previously revealed that two distinct NADH oxidases corresponding to H_2O_2-forming oxidase (Nox-1) and H_2O-forming oxidase (Nox-2) were induced in an oxygen tolerant Streptococcus mutans by exposure to oxygen. Furthermore, sequence searches indicated that the Nox-1 protein is homologue of Salmonella typhimurium AhpF, the flavoprotein component of alkyl hydroperoxide reductase AhpR involved in defense against oxidative stress, and suggested the presence of ahpC homologue upstream of nox-1 gene with S.typhimurium AhpC, the non-flavopretein component of AhpR.In this study, we determined the complete sequence of the S.mutans ahpC comprised of 558 bp encoding AhpC of 186 amino acid residues, confirmed the identity of these proteins as an AhpR system in S.mutans, demonstrated that together the Nox-1 and AhpC catalyzes the NADH-dependent reduction of organic hydroperoxides or H_2O_2 to their respective alcohols and/or H_2O, as well as the four-electron reduction of 0_2 by Nox-2, and proposed that Nox-2 from S.mutans contains a cysteinyl redox center ; the Cys44 of Nox-2 protein plays a key role in the overall four-electron reduction of O_2 to H_2O, by demonstrating that replacement of Cys44 with Ser provides for an altered O_2 reduction stoichiometry in which H_2O_2, not 2H_2O is the product. Furthermore, we confirmed that these proteins of Nox-1, Nox-2, and AhpC are induced by oxygen, and presented the first evidence that Nox-2 plays an important role in aerobic energy metabolism through the regeneration of NAD, but Nox-1 contributes negligibly, by analyses of knockout mutants of Nox-1, Nox-2, and/or AhpC.We also demonstrated a clear function for Nox-1 as part of an AhpR system in vivo in combination with AhpC.