Site-directed mutagenesis of the basic residues 321K to 321G in the CP 47 protein of photosystem II alters the chloride requirement for growth and oxygen-evolving activity in Synechocystis 6803
Abstract
CP 47, a component of photosystem II (PSII) in higher plants, algae and cyanobacteria, is encoded by the psbB gene. Site-specific mutagenesis 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. Alteration of a lysine residue occurring at position 321 to glycine produced a strain with allured PSII activity. This strain grew at wild-type rates in complete BG- 11 media (480 μM 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 intensities indicated that the mutant had 66% of the fully functional PSII centers contained in the control strain. The mutant proved to be extremely sensitive to photoinactivation at high light intensities, exhibiting a 3-fold increase in the rate of photoinactivation. When this mutant was grown in media depleted of chloride (30 μM chloride), it lost the ability to grow photoautotrophically while the control strain exhibited a normal rate of growth. The effect of chloride depletion on the growth rate of the mutant was reversed by the addition of 480 μM bromide to the chloride-depleted BG-11 media. In the presence of glucose, the mutant and control strains grew at comparable rates in either chloride-containing or chloride-depleted media. Oxygen evolution rates for the mutant were further depressed (28% of control rates) under chloride-limiting conditions. Addition of bromide restored these rates to those observed under chloride-sufficient conditions. Measurements of the variable fluorescence yield indicated that the mutant assembled fewer functional centers in the absence of chloride. These results indicate that the mutation K321G in CP 47 affects PSII stability and/or assembly tinder conditions where chloride is limiting.