Streptococcus mutans has a gene encoding nitrite reductase and generates nitric oxide in the oral cavity

To clarify the mechanism and pathological significance of NO generation by S. mutans, we cloned a nitric reductase gene from S. mutans and characterized the enzyme. Genomic study revealed that the cloned gene had 5-points mutations with frame-shift within the first 60-bps as compared with the reported database. The recombinant enzyme reduced nitrite to NO with a specific activity of 2.5 mu mol/min/mg protein. Nitrite reductase activity and NO generation by the nirJ(-/-) mutant were below detectable levels, suggesting that the enzyme is responsible for the generation of NO by S. mutans. The growth of the mutant strain, but not of wild strain, was strongly suppressed by the presence of nitrite. These observations suggest that the elimination of nitrite and/or generation of NO are critically important for the survival of S. mutans in the oral cavity.