FUNCTIONAL-CHARACTERIZATION OF SYNECHOCYSTIS SP. PCC-6803 DELTA-PSBU AND DELTA-PSBV MUTANTS REVEALS IMPORTANT ROLES OF CYTOCHROME C-550 IN CYANOBACTERIAL OXYGEN EVOLUTION

The functions of cytochrome c-550 and a 12 kDa protein in cyanobacteri al oxygen evolution were studied with directed deletion mutants Delta psbV and Delta psbU of Synechocystis sp. PCC 6803, and the following r esults were obtained. (1) In contrast to the Delta psbU mutant which i s capable of autotrophic growth in the absence of Ca2+ or Cl- at a red uced rate, the Delta psbV mutant lacking cytochrome c-550 could not gr ow at all without Ca2+ or Cl-. (2) The Delta psbV mutant had a signifi cantly reduced thermoluminescence emission intensity and flash oxygen yield, whereas the Delta psbU mutant showed slight decreases in thermo luminescence intensity and flash oxygen yield, indicating correspondin g decreases in the concentrations of O-2-evolving centers in these mut ants. (3) The Delta psbV and Delta psbU mutants exhibited elevated pea k temperature for the thermoluminescence B- and Q-bands indicative of more stable S-2 states. (4) The rise time of the O-2 signal during the S-3-[S-4]-SO transition was increased slightly in the Delta psbV muta nt but not in the Delta psbU mutant. (5) The oxygen evolution was inac tivated in the dark rapidly in the Delta psbV mutant with a half-time of 28 min, but this did not happen in the Delta psbU mutant. (6) Photo activation of the oxygen-evolving complex after removal of the mangane se cluster by hydroxylamine showed a higher quantum yield in the Delta psbV mutant than in the Delta psbU mutant or wild type. Taken togethe r, these results indicated that cytochrome c-550 plays a substantial r ole in maintaining the stability and function of the manganese cluster in algal photosystem II, whereas the 12 kDa protein plays primarily a regulatory role in maintaining normal S-state transitions. These func tional features of cytochrome c-550 and the 12 kDa protein were compar ed with those of the 23 and 17 kDa proteins in higher plant photosyste m II and of the 33 kDa protein in both algal and plant photosystem II.