Oxygen-dependent electron flow influences photosystem II function and psbAgene expression in the cyanobacterium Synechococcus sp PCC 7942

During acclimated growth in Synechococcus sp, PCC 7942 a substantial propor tion of the electrons extracted from mater by photosystem II ultimately flo w back to oxygen, This flow increases rapidly under high light, which allow s Synechococcus to maintain photosystem II centers largely open, even under excessive excitation, The electron flow to oxygen with increasing light ac counts for the progressive discrepancy between the light response curve of measured oxygen evolution, and the light response curve of photosystem II a ctivity estimated from fluorescence measures. In cells under anoxia this fl exible electron sink is lost and photosystem II centers suffer partial clos ure at the growth light intensity, with closure becoming more severe under excess light. As predicted from earlier work this PSII closure results in r apid loss of psbAI message, encoding the D1:1 protein of PSII, and inductio n of psbAII/AIII encoding the alternate D1:2 protein. The changes in the mR NA pool are not, however, reflected at the protein level, and D1:1 remains in the thylakoid membranes. There is no accumulation of D1:2, despite some continued synthesis of other proteins. PSII closure, therefore, results in repression of psbAI and induction psbAII/AIII expression, but D1:1/D1:2 exc hange is blocked by anoxia, downstream from transcription. D1:1 protein and PSII activity are quite stable under anoxia and moderate illumination, Nev ertheless, upon recovery under oxygenic conditions, the existing D1:1 is lo st from the membranes, resulting in a transient drop in PSII activity. This suggests that under normal conditions the cells use oxygen to facilitate p reemptive turnover of D1 proteins.