Restoration of the optimal redox state for the photosynthetic electron transfer system by auxiliary oxidants in an aerobic photosynthetic bacterium, Erythrobacter sp. OCh 114

In an aerobic photosynthetic bacterium, Erythrobacter sp. OCh 114, photosynthetic electron transfer (photooxidation of cytochromes) and light-driven proton release did not occur under anaerobic conditions. 'Auxiliary oxidants', such as nitrate, nitrite, trimethylamine N-oxide (TMAO) and chlorate, restored the optimal redox state for the photosynthetic electron transfer system, resulting in the photooxidation of c-type cytochrome and the release of protons from intact cells on illumination. These auxiliary oxidants oxidized c-type cytochrome(s) under anaerobic conditions in the dark, suggesting the presence of reductases for these auxiliary oxidants. The redox level of the electron transfer chain shifted towards oxidation in the presence of the auxiliary oxidants and was poised to allow photosynthetic electron transfer. KCN inhibited the photooxidation of cytochromes and the light-driven proton release in the presence of these auxiliary oxidants, except for TMAO. KCN also inhibited the auxiliary oxidant-induced oxidation of cytochromes. This suggests that KCN inhibits the reductases for these oxidants. Antimycin and myxothiazol enhanced the auxiliary oxidant-induced oxidation of cytochrome(s), suggesting the involvement of the cytochrome b-c₁ complex in nitrate, chlorate and nitrite reduction.