INACTIVATION OF THE WATER-OXIDIZING ENZYME IN MANGANESE-STABILIZING PROTEIN-FREE MUTANT-CELLS OF THE CYANOBACTERIA SYNECHOCOCCUS PCC7942 AND SYNECHOCYSTIS PCC6803 DURING DARK INCUBATION AND CONDITIONS LEADING TO PHOTOACTIVATION

The previously constructed MSP (manganese stabilizing protein-psbO gene product)-free mutant of Synechococcus PCC7942 (Bockholt R, Masepohl B and Pistorius E K(1991) FEES Lett 294: 59-63) and a newly constructed MSP-free mutant of Synechocystis PCC6803 were investigated with respect to the inactivation of the water-oxidizing enzyme during dark incubation. O-2 evolution in the MSP-free mutant cells, when measured with a sequence of short saturating light flashes, was practically zero after an extended dark adaptation, while O-2 evolution in the corresponding wild type cells remained nearly constant. It could be shown that this inactivation could be reversed by photoactivation. With isolated thylakoid membranes from the MSP-free mutant of PCC7942, it could be demonstrated that photoactivation required illumination in the presence of Mn2+ and Ca2+, while Cl- addition was not required under our experimental conditions. Moreover, an extended analysis of the kinetic properties of the water-oxidizing enzyme (kinetics of the S-3-->(S-4)-->S-0 transition, S-state distribution, deactivation kinetics) in wild type and mutant calls of Synechococcus PCC7942 and Synechocystis PCC6803 was performed, and the events possibly leading to the reversible inactivation of the water-oxidizing enzyme in the mutant cells are discussed. We could also show that the water-oxidizing enzyme in the MSP-free mutant cells is more sensitive to inhibition by added NH4Cl- suggesting that NH3 might be a physiological inhibitor of the water oxidizing enzyme in the absence of MSP.