REGULATION OF ATP SYNTHESIS IN CHLOROPLASTS
After chloroplasts have reached steady state in the light, the onset of darkness leads to a condition in which the proton gradient rapidly dissipates, while the ATP/ADP+Pi ratio remains high. Under these conditions, FₒF₁ initially catalyzes ATPase-dependent proton pumping back to the lumen, the reverse of synthase activity. This wasteful consumption of the energy captured in light is rapidly halted by formation of a disulfide bond in the γ subunit that stops rotation and ATP hydrolysis (6). Upon illumination, the first reducing equivalents from PSI that reduce ferredoxin are diverted from FNR to thioredoxin reductase that catalyzes the reduction of thioredoxin. Thioredoxin in turn reduces the disulfide of the γ subunit to activate FₒF₁.