FUNCTIONAL-CHARACTERIZATION OF MUTANT STRAINS OF THE CYANOBACTERIUM SYNECHOCYSTIS SP PCC-6803 LACKING SHORT DOMAINS WITHIN THE LARGE, LUMEN-EXPOSED LOOP OF THE CHLOROPHYLL-PROTEIN CP47 IN PHOTOSYSTEM-II

Several autotrophic mutant strains of Synechocystis sp. PCC 6803 carrying short deletions or a single-site mutation within the large, lumen-exposed loop (loop E) of the chlorophyll a-binding photosytem II core protein, CP47, are analyzed for their functional properties by measuring the flash-induced pattern of thermoluminescence, oxygen yield, anti fluorescence quantum yield. A physiological and biochemical characterization of these mutant strains has been given in two previous reports [Eaton-Rye, J. J., and Vermaas, W. F. J. (1991) Plant Mel. Biol. 17, 1165-1177; Haag, E., Eaten-Rye, J. J., Renger, G., and Vermaas, S. F. J. (1993) Biochemistry 32, 4444-4454]. The results of the present study show that deletion of charged and conserved amino acids in a region roughly located between residues 370 and 390 decreases the binding affinity of the extrinsic PS II-O protein to photosystem II. Marked differences with PSII-O deletion mutants are observed with respect to Ca2+ requirement and the flash-induced pattern of oxygen evolution. Under conditions where a sufficient light activation is provided, the psbB mutants assayed in this study reveal normal S-state parameters and lifetimes. The results bear two basic implications: (i) the manganese involved in water oxidation can still be bound ina functionally normal or only slightly distorted manner, and (ii) the binding of the extrinsic PS II-O protein to photosystem II is impaired in mutants carrying a deletion in the domain between residues 370 and 390, but the presence of the PS II-O protein is still of functional relevance for the PS II complex, e.g., for maintenance of a high-affinity binding site for Ca2+ and/or involvement during the process of photoactivation.