INTERACTION OF THE PHOTOSYNTHETIC AND RESPIRATORY ELECTRON-TRANSPORT CHAINS PRODUCING SLOW O-2 SIGNALS UNDER FLASHING LIGHT IN SYNECHOCYSTIS SP PCC-6803

We investigated the slow signal of apparent O-2 release under brief li ght flashes by using mutants of Synechocystis sp. PCC 6803 which lacke d CP43 and D1. The slow signal was present at higher amplitudes in the mutants. It was inhibited by starving the mutants of glucose (>90%), by 10 mM NaN3 (85%) and by boiling samples for 2 min (100%). In the mu tants and in the wild-type, the slow signal was 95% inhibited by the c ombination of DBMIB (2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone) and HQNO (2-n-heptyl-4-hydroxyquinoline-N-oxide). In the wild type, th e addition of DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea) or CCCP ( carbonylcyanide m-chlorophenylhydrazone) completely inhibited photosyn thetic O-2 evolution, yet failed to inhibit the slow signal. We explai n the kinetics of the wild-type signal as a positive deflection due to the inhibition of respiration by PS I activity, and a negative deflec tion due to the stimulation of respiration by electrons originating fr om PS II. We found no evidence of a 'meta-stable S-3' in Synechocystis sp. PCC 6803 that could contribute to the slow signal of apparent O-2 release. We present a calculation which involves only averaging, divi sion and subtraction, that can remove the contribution of the slow sig nal from the true photosynthetic O-2 Signal and provide up to a 10-fol d improved accuracy of the S-state models.