Repression of photosynthesis gene expression by formation of a disulfide bond in CrtJ

Many species of purple photosynthetic bacteria repress synthesis of their photosystem in the presence of molecular oxygen. The bacterium Rhodobacter capsulatus mediates this process by repressing expression of bacteriochlorophyll, carotenoid, and light-harvesting genes via the aerobic repressor, CrtJ. In this study, we demonstrate that CrtJ forms an intramolecular disulfide bond in vitro and in vivo when exposed to oxygen. Mutational and sulfhydryl-specific chemical modification studies indicate that formation of a disulfide bond is critical for CrtJ binding to its target promoters. Analysis of the redox states of aerobically and anaerobically grown cells indicates that they have similar redox states of approximately -200 mV, thereby demonstrating that a change in midpoint potential is not responsible for disulfide bond formation. In vivo and in vitro analyses indicate that disulfide bond formation in CrtJ is insensitive to the addition of hydrogen peroxide but is sensitive to molecular oxygen. These results suggest that disulfide bond formation in CrtJ may differ from the mechanism of disulfide bond formation used by OxyR.