The oxyR mutant also failed to grow in Na + -depleted medium, implying that this condition mimics oxidative stress (Fig. 5). Approximately half the genes up regulated in both ATCC 33277 parent and rprY mutant strains under Na + stress were reported to be associated with the OxyR regulon [18]. It was demonstrated that expression of several of these genes was maximal during normal anaerobic growth and were downregulated in an oxyR mutant grown under the same conditions, consistent with OxyR was a positive regulator of these genes [18]. Our **data** show that under Na + stress expression of these genes was higher in the mutant consistent with RprY being a repressor. A possible scenario is that under anaerobic conditions OxyR constitutively activates oxidative stress genes and RprY, acting as a repressor, fine-tunes the expression of a subset of these to maintain a balance that prevents a superoxidative stress response. Sodium depletion disrupts the balance triggering induction of RprY in order to restore homeostasis and eventual growth as in the parent strain. However, in the absence of RprY a superoxidative stress response ensues from which the mutant cannot recover and grow.