CodY Regulates Thiol Peroxidase Expression as Part of the Pneumococcal Defense Mechanism against HO Stress.

is a facultative anaerobic pathogen. Although it maintains fermentative metabolism, during aerobic growth pneumococci produce high levels of HO, which can have adverse effects on cell viability and DNA, and influence pneumococcal interaction with its host. The pneumococcus is unusual in its dealing with toxic reactive oxygen species (ROS) in that it neither has catalase nor the global regulators of peroxide stress resistance. Previously, we identified pneumococcal thiol peroxidase (TpxD) as the key enzyme for enzymatic removal of HO, and showed that TpxD synthesis is up-regulated upon exposure to HO. This study aimed to reveal the mechanism controlling TpxD expression under HO stress. We hypothesize that HO activates a transcription factor which in turn up-regulates expression. Microarray analysis revealed a pneumococcal global transcriptional response to HO. Mutation of abolished HO-mediated response to high HO levels, signifying the need for an active TpxD under oxidative stress conditions. Bioinformatic tools, applied to search for a transcription factor modulating expression, pointed toward CodY as a potential candidate. Indeed, a putative 15-bp consensus CodY binding site was found in the proximal region of coding sequence. Binding of CodY to this site was confirmed by EMSA, and genetic engineering techniques demonstrated that this site is essential for TpxD up-regulation under HO stress. Furthermore, expression was reduced in a Δ mutant. These data indicate that CodY is an activator of expression, triggering its up-regulation under HO stress. In addition we show that HO specifically oxidizes the 2 CodY cysteines. This oxidation may trigger a conformational change in CodY, resulting in enhanced binding to DNA. A schematic model illustrating the contribution of TpxD and CodY to pneumococcal global transcriptional response to HO is proposed.