An Iron-Binding Protein, Dpr, fromStreptococcus mutansPrevents Iron-Dependent Hydroxyl Radical Formation In Vitro

Abstract

Thedprgene is an antioxidant gene which was isolated from theStreptococcus mutanschromosome by its ability to complement an alkyl hydroperoxide reductase-deficient mutant ofEscherichia coli, and it was proven to play an indispensable role in oxygen tolerance inS. mutans. Here, we purified the 20-kDadprgene product, Dpr, from a crude extract ofS. mutansas an iron-binding protein and found that Dpr formed a spherical oligomer about 9 nm in diameter. Molecular weight determinations of Dpr in solution by analytical ultracentrifugation and light-scattering analyses gave values of 223,000 to 292,000, consistent with a subunit composition of 11.5 to 15 subunits per molecule. The purified Dpr contained iron and zinc atoms and had an ability to incorporate up to 480 iron and 11.2 zinc atoms per molecule. UnlikeE. coliDps and two other members of the Dps family, Dpr was unable to bind DNA. One hundred nanomolar Dpr prevented by more than 90% the formation of hydroxyl radical generated by 10 μM iron(II) salt in vitro. The data shown in this study indicate that Dpr may act as a ferritin-like iron-binding protein inS. mutansand may allow this catalase- and heme-peroxidase-deficient bacterium to grow under air by limiting the iron-catalyzed Fenton reaction.