Catalyzes the reversible oxidation-reduction of the R-enantiomer of free methionine sulfoxide to methionine. Does not act on S-enantiomer of free methionine sulfoxide or R-enantiomer of dabsylated methionine sulfoxide. Involved in protection against oxidative stress. [thioredoxin]-disulfide + H2O + L-methionine = [thioredoxin]-dithiol + L-methionine (R)-S-oxide Single deletion of fRMsr has complete growth inhibition on methionine-R-sulfoxide medium and fRMsr and MXR1 double deletion completely blocks the growth on both methionine-R-sulfoxide and methionine-S-sulfoxide medium. FRMsr and MXR2 double deletion has no effect on growth on methionine-S-sulfoxide medium. Single mutant or any of the double mutants show no growth defects in methionine medium, even the fRMsr, MXR1 and MXR2 triple deletion mutant is viable and grows similarly to wild-type. Single deletion of fRMsr has an increased sensitivity to oxidative stress and a decreased life span of 18% compared to wild-type. FRMsr and MXR1, as well as fRMsr and MXR2 double mutants, and fRMsr, MXR1 and MXR2 triple mutant show 20% reduction in life span compared with wild-type cells. Present with 2610 molecules/cell in log phase SD medium. Belongs to the free Met sulfoxide reductase family.