Abstract
Previously, we have found that an insertional inactivation of aaoSo, a gene encoding L-amino acid oxidase (LAAO), causes marked repression of the growth of Streptococcus oligofermentans. Here, we found that aaoSo 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 aaoSo and mutT was lethal. Quantitative PCR showed that the transcript abundance of mutT was reduced for 13-fold in the aaoSo insertional mutant, indicating that gene polarity derived from the inactivation of aaoSo 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 H2O2 (min·mg protein)–1]; and MutT showed the pyrophosphohydrolase activity, which removed mutagens such as 8-oxo-dGTP. Like paraquat, aaoSo mutations increased the expression of SOD, and addition of aminoacetone (final concentration, 5 mM) decreased the mutant’s growth by 11%, indicating that the aaoSo mutants are under ROS stress. HPLC did reveal elevated levels of cytoplasmic aminoacetone in both the deletion and insertional gene mutants of aaoSo. Electron spin resonance spectroscopy showed increased hydroxyl radicals in both types of aaoSo mutant. This demonstrated that inactivation of aaoSo 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 aaoSo-mutT operon in antioxidant defense in S. oligofermentans.
Highlights
Oxidative stress is encountered universally by all organisms living in an oxygen environment
This suggests that other inhibitory mechanisms might be present in addition to the H2O2 generated by L-amino acid oxidase (LAAO)
As sodA mRNA levels increase 3-fold in wild-type S. oligofermentans treated with the redox-cycling drug paraquat, which induces production of cellular endogenous superoxides, we propose that the accumulation of superoxide anion increases the expression of Mn-superoxide dismutase (SOD) in DaaoSo-mutT and DaaoSo mutants
Summary
Oxidative stress is encountered universally by all organisms living in an oxygen environment. Due to the higher energy that results from the presence of unpaired valence-shell electrons, ROS can cause severe damage to various biological macromolecules, especially DNA. This leads to mutagenesis, tumorigenesis, and aging [1,2]. Streptococcus oligofermentans is isolated from non-caries dental plaques in humans It uses multiple pathways for the generation of H2O2 from oxygen [9]. S. oligofermentans is one of those with the highest levels of tolerance to H2O2 [13] This makes it a suitable model organism for the study of the mechanisms by which bacteria protect themselves against oxidative stress
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
