Site-directed mutagenesis of the CPa-1 protein of photosystem II: alteration of the basic residue pair (384,385)R to (384,385)G leads to a defect associated with the oxygen-evolving complex

The psbB gene encodes the intrinsic chlorophyll-a binding protein Cpa-1 (CP-47), a component of photosystem II in higher plants, algae, and cyanobacteria. Oligonucleotide-directed mutagenesis was used to introduce mutations into a segment of the psbB gene encoding the large extrinsic loop region of CPa-1 in the cyanobacterium Synechocystis sp. PCC 6803. Altered psbB genes were introduced into a mutant recipient strain (DEL-1) of Synechocystis in which the genomic psbB gene had been partially deleted. Initial target sites for mutagenesis were absolutely conserved basic residue pairs occurring within the large extrinsic loop. One mutation, RR384385GG, produced a strain with impaired photosystem II activity. This strain exhibited growth characteristics comparable to controls. However, at saturating light intensities this mutant strain evolved oxygen at only 50% of the rate of the control strains. Quantum yield measurements at low light intensities indicated that the mutant had 30% fewer fully functional photosystem II centers than do control strains of Synechocystis. Immunological analysis of a number of photosystem II protein components indicated that the mutant accumulates normal quantities of photosystem II proteins and that the ratio of photosystem II to photosystem I proteins is comparable to that found in control strains. Upon exposure to high light intensities the mutant cells exhibited a markedly increased susceptibility to photoinactivation. However, Tris-treated thylakoid membranes from both the mutant and wild-type exhibited comparable rates of photoinactivation. Thylakoid membranes isolated from RR384385GG exhibited only 15% of the H2O to 2,6-dichlorophenolindophenol electron transport rate observed in wild-type strains. The 1,5-diphenylcarbazide to 2,6-dichlorophenolindophenol electron transport rates of Tris-treated thylakoids from the mutant, however, were comparable to control rates. These results suggest that alteration of this basic residue pair leads to a defect associated with the oxygen-evolving complex of photosystem II.