SITE-DIRECTED MUTAGENESIS OF THE BASIC RESIDUES K-321 TO (321)G IN THE CP-47 PROTEIN OF PHOTOSYSTEM-II ALTERS THE CHLORIDE REQUIREMENT FOR GROWTH AND OXYGEN-EVOLVING ACTIVITY IN SYNECHOCYSTIS-6803

CP 47, a component of photosystem II (PSII) in higher plants, algae an d cyanobacteria, is encoded by the psbB gene. Site-specific mutagenesi s has been used to alter a portion of the psbB gene encoding the large extrinsic loop E of CP 47 in the cyanobacterium Synechocystis 6803, A lteration of a lysine residue occurring at position 321 to glycine pro duced a strain with altered PSII activity. This strain grew at wild-ty pe rates in complete BG-11 media (480 mu M chloride). However, oxygen evolution rates for this mutant in complete media were only 60% of the observed wild-type rates. Quantum yield measurements at low light int ensities indicated that the mutant had 66% of the fully functional PSI I centers contained in the control strain. The mutant proved to be ext remely sensitive to photoinactivation at high light intensities, exhib iting a 3-fold increase in the rate of photoinactivation. When this mu tant was grown in media depleted of chloride (30 mu M chloride), it lo st the ability to grow photoautotrophically while the control strain e xhibited a normal rate of growth. The effect of chloride depletion on the growth rate of the mutant was reversed by the addition of 480 mu M bromide to the chloride-depleted BG-11 media. In the presence of gluc ose, the mutant and control strains grew at comparable rates in either chloride-containing or chloride-depleted media, Oxygen evolution rate s for the mutant were further depressed (28% of control rates) under c hloride-limiting conditions. Addition of bromide restored these rates to those observed under chloride-sufficient conditions. Measurements o f the variable fluorescence yield indicated that the mutant assembled fewer functional centers in the absence of chloride. These results ind icate that the mutation K321G in CP 47 affects PSII stability and/or a ssembly under conditions where chloride is limiting.