Conformational regulation of charge recombination reactions in a photosynthetic bacterial reaction center

In bright light the photosynthetic reaction center ( RC) of Rhodobacter sphaeroides stabilizes the P-870(+) center dot Q(A)(-) charge-separated state and thereby minimizes the potentially harmful effects of light saturation. Using X-ray diffraction we report a conformational change that occurs within the cytoplasmic domain of this RC in response to prolonged illumination with bright light. Our observations suggest a novel structural mechanism for the regulation of electron transfer reactions in photosynthesis.