Role of the dpr Product in Oxygen Tolerance in Streptococcus mutans

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Abstract
We have previously identified and characterized the alkyl hydroperoxide reductase of Streptococcus mutans , which consists of two components, Nox-1 and AhpC. Deletion of both nox-1 and ahpC had no effect on the sensitivity of S. mutans to cumene hydroperoxide or H 2 O 2 , implying that the existence of another antioxidant system(s) independent of the Nox-1–AhpC system compensates for the deficiency. Here, a new antioxidant gene ( dpr for Dps-like peroxide resistance gene) was isolated from the S. mutans chromosome by its ability to complement an ahpCF deletion mutant of Escherichia coli with a tert -butyl hydroperoxide-hypersensitive phenotype. The dpr gene complemented the defect in peroxidase activity caused by the deletion of nox-1 and ahpC in S. mutans . Under aerobic conditions, the dpr disruption mutant carrying a spectinomycin resistance gene ( dpr ::Spc r mutant) grew as well as wild-type S. mutans in liquid medium. However, the dpr ::Spc r mutant could not form colonies on an agar plate under air. In addition, neither the dpr ::Spc r ahpC ::Em r :: nox-1 triple mutant nor the dpr ::Spc r sod ::Em r double mutant was able to grow aerobically in liquid medium. The 20-kDa dpr gene product Dpr is an iron-binding protein. Synthesis of Dpr was induced by exposure of S. mutans cells to air. We propose a mechanism by which Dpr confers aerotolerance on S. mutans.