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Previously, we have found that an insertional inactivation of  aao So  , a gene encoding L-amino acid oxidase (LAAO), causes marked repression of the growth of  Streptococcus oligofermentans . Here, we found that  aao So   and  mutT , a homolog of pyrophosphohydrolase gene of  Escherichia coli , constituted an operon. Deletion of either gene did not impair the growth of  S. oligofermentans , but double deletion of both  aao So   and  mutT  was lethal. Quantitative PCR showed that the transcript abundance of  mutT  was reduced for 13-fold in the  aao So   insertional mutant, indicating that gene polarity derived from the inactivation of  aao So   attenuated the expression of  mutT . Enzymatic assays were conducted to determine the biochemical functions of LAAO and MutT of  S. oligofermentans . The results indicated that LAAO functioned as an aminoacetone oxidase [47.75 nmol H 2 O 2  (min·mg protein) –1 ]; and MutT showed the pyrophosphohydrolase activity, which removed mutagens such as 8-oxo-dGTP. Like paraquat,  aao So   mutations increased the expression of SOD, and addition of aminoacetone (final concentration, 5 mM) decreased the mutant’s growth by 11%, indicating that the  aao So   mutants are under ROS stress. HPLC did reveal elevated levels of cytoplasmic aminoacetone in both the deletion and insertional gene mutants of  aao So  . Electron spin resonance spectroscopy showed increased hydroxyl radicals in both types of  aao So   mutant. This demonstrated that inactivation of  aao So   caused the elevation of the prooxidant aminoacetone, resulting the cellular ROS stress. Our study indicates that the presence of both LAAO and MutT can prevent endogenous metabolites-generated ROS and mutagens. In this way, we were able to determine the role of the  aao So -mutT  operon in antioxidant defense in  S. oligofermentans .

Role of Operon aaoSo-mutT in Antioxidant Defense in Streptococcus oligofermentans