Identification and characterization of SnrA, an inducible oxygen-insensitive nitroreductase in Salmonella enterica serovar Typhimurium TA1535.

The biological activity of many nitrosubstituted compounds, many of which are produced commercially or have been identified as environmental contaminants, is dependent on metabolic activation catalyzed by nitroreductases. In the current study, we have cloned a nitroreductase gene, Salmonella typhimurium nitroreductase A (snrA), from S. enterica serovar Typhimurium strain TA1535, and characterized the purified gene product. SnrA is 240 amino acids in length and shares 87% sequence identity to the Escherichia coli homolog, E. coli nitroreductase A (NfsA). SnrA is the major nitroreductase in S. enterica serovar Typhimurium strain TA1535 and catalyzes nitroreduction through a ping-pong bi-bi mechanism in a NADPH and flavine mononucleotide (FMN) dependent manner. SnrA exhibits extremely low levels of FMN reductase activity but the nitroreductase activity of SnrA is competitively inhibited by exogenously added FMN. Treatment of TA1535 with paraquat resulted in induction of nitroreductase activity, suggesting that SnrA is a member of the S. enterica serovar Typhimurium SoxRS regulon associated with cellular defense against oxidative damage. Examination of the microbial genomes databases shows that SnrA homologs are widely distributed in the microbial world, being present in isolates of both Archea and Eubacteria. Southern hybridization and PCR failed to detect the snrA gene in the closely related S. enterica serovar Typhimurium strain TA1538. S. enterica serovar Typhimurium strains TA1535 and TA1538 and their derivatives are commonly used in mutagenicity testing. Differences in metabolic capacity between these two strains may have implications for the interpretation of mutagenicity data.