Mutagenesis of the D-E loop of photosystem II reaction centre protein D1. Function and assembly of photosystem II.

The sequence connecting alpha-helices D and E of the D1 protein in photosystem II (PSII) is longer than that found in the corresponding loop of the L subunit in the rhodobacterial reaction centre. This sequence was mutated in order to determine its role in oxygenic photosynthesis. Site-specific mutants, including point mutations and deletions of different size, of the PEST-like region and the putative cleavage area in the D-E loop of the D protein were constructed in Synechocystis sp. PCC 6803. The effects of mutations on the functional and structural properties of PSII and turnover of the D1 protein were examined. Our results demonstrate that deletion of either the PEST-like sequence (deltaR225-F239) or the putative cleavage region (deltaG240-V249, deltaR225-V249) of the D1 protein resulted in severe perturbations on the function of the QB electron acceptor of PSII. However, PSII centres of the mutant with deleted PEST region remained functional enough to support autotrophic growth whereas deletions of the putative cleavage region prevented autotrophic growth. Although enhanced degradation rates of the mutant D1 proteins under low-light growth conditions demonstrate that neither the PEST-like sequence nor the putative cleavage region are required for D1 proteolysis, it became clear that the extension in the D-E loop of the D1 protein is essential for proper PSII assembly and photoautotrophic growth. Moreover, modifications of the D-E loop resulted in transcriptional activation of the psbA gene, indicating that neither light intensity, as such, nor the activity of the electron transfer chain are the only determinants in regulation of psbA gene transcription.