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Mutation of Phe-363 in the photosystem II protein CP47 impairs photoautotrophic growth, alters the chloride requirement, and prevents photosynthesis in the absence of either PSII-O or PSII-V in Synechocystis sp. PCC 6803.
The deletion of the amino acids between Gly-351 and Thr-365 within the large, lumen-exposed, hydrophilic region (loop E) of the photosystem II (PSII) chlorophyll a-binding protein CP47 produced a strain of Synechocystis sp. PCC 6803 that failed to assemble stable PSII centers [Eaton-Rye, J. J., and Vermaas, W. F. J. (1991) Plant Mol. Biol. 17, 1165-1177]. The importance of two conserved Phe residues at positions 362 and 363 within this deletion has been investigated. The F363R strain had impaired photoautotrophic growth and an enhanced sensitivity to photoinactivation, demonstrating that Phe is required at position 363 for normal PSII function. In contrast, photoautotrophic growth in strains N361K and F362R was unaffected. Uniquely, among the mutant strains tested, F363R was unable to grow under chloride-limiting conditions, and this effect was reversed by replacing chloride with bromide. The removal of the manganese-stabilizing protein (PSII-O), the 12 kDa extrinsic protein (PSII-U), and cytochrome c-550 (PSII-V) was investigated in each mutant in vivo. In N361K and F362R, removal of PSII-V produced a more deleterious effect than the removal of PSII-O, but even so, all strains remained photoautotrophic. In contrast, the absence of PSII-V and PSII-O in F363R produced obligate photoheterotrophic strains. The removal of PSII-U increased the susceptibility of PSII to heat inactivation and further decreased the stability of PSII in F363R, demonstrating that PSII-U can contribute to the stabilization of mutations that have been introduced into CP47. The order of importance of the selective removal of the extrinsic proteins in strains carrying mutations in loop E of CP47 was found to be as follows: DeltaPSII-V >/= DeltaPSII-O > DeltaPSII-U.
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