Adaptation to Oxygen ROLE OF TERMINAL OXIDASES IN PHOTOSYNTHESIS INITIATION IN THE PURPLE PHOTOSYNTHETIC BACTERIUM, RUBRIVIVAX GELATINOSUS

The appearance of oxygen in the Earth's atmosphere via oxygenic photosynthesis required strict anaerobes and obligate phototrophs to cope with the presence of this toxic molecule. Here we show that in the anoxygenic phototroph Rubrivivax gelatinosus, the terminal oxidases (cbb(3), bd, and caa(3)) expand the range of ambient oxygen tensions under which the organism can initiate photosynthesis. Unlike the wild type, the cbb(3)(-)/bd(-) double mutant can start photosynthesis only in deoxygenated medium or when oxygen is removed, either by sparging cultures with nitrogen or by co-inoculation with strict aerobes bacteria. In oxygenated environments, this mutant survives nonphoto-synthetically until the O-2 tension is reduced. The cbb(3) and bd oxidases are therefore required not only for respiration but also for reduction of the environmental O-2 pressure prior to anaerobic photosynthesis. Suppressor mutations that restore respiration simultaneously restore photosynthesis in nondeoxygenated medium. Furthermore, induction of photosystem in the cbb(3)(-) mutant led to a highly unstable strain. These results demonstrate that photosynthetic metabolism in environments exposed to oxygen is critically dependent on the O-2 -detoxifying action of terminal oxidases.