The functions of cytochrome c-550 and a 12 kDa protein in cyanobacterial oxygen evolution were studied with directed deletion mutants delta psbV and delta psbU of Synechocystis sp. PCC 6803, and the following results were obtained. (1) In contrast to the delta psbU mutant which is capable of autotrophic growth in the absence of Ca2+ or Cl- at a reduced rate, the delta psbV mutant lacking cytochrome c-550 could not grow at all without Ca2+ or Cl-. (2) The delta psbV mutant had a significantly reduced thermoluminescence emission intensity and flash oxygen yield, whereas the delta psbU mutant showed slight decreases in thermoluminescence intensity and flash oxygen yield, indicating corresponding decreases in the concentrations of O2-evolving centers in these mutants. (3) The delta psbV and delta psbU mutants exhibited elevated peak temperature for the thermoluminescence B- and Q-bands indicative of more stable S2 states. (4) The rise time of the O2 signal during the S3-[S4]-S0 transition was increased slightly in the delta psbV mutant but not in the delta psbU mutant. (5) The oxygen evolution was inactivated 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) Photoactivation of the oxygen-evolving complex after removal of the manganese cluster by hydroxylamine showed a higher quantum yield in the delta psbV mutant than in the delta psbU mutant or wild type. Taken together, these results indicated that cytochrome c-550 plays a substantial role 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 functional features of cytochrome c-550 and the 12 kDa protein were compared with those of the 23 and 17 kDa proteins in higher plant photosystem II and of the 33 kDa protein in both algal and plant photosystem II.