MUTAGENESIS OF THE D-E LOOP OF PHOTOSYSTEM-II REACTION-CENTER PROTEIND1 - FUNCTION AND ASSEMBLY OF PHOTOSYSTEM-II

The sequence connecting alpha-helices D and E of the D1 protein in pho tosystem 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 di fferent size, of the PEST-like region and the putative cleavage area i n the D-E loop of the DI protein were constructed in Synechocystis sp. PCC 6803. The effects of mutations on the functional and structural p roperties of PSII and turnover of the DI protein were examined. Our re sults demonstrate that deletion of either the PEST-like sequence (Delt a R225-F239) or the putative cleavage region (Delta G240-V249, Delta R 225-V249) of the D1 protein resulted in severe perturbations on the fu nction of the Q(B) electron acceptor of PSII. However, PSII centres of the mutant with deleted PEST region remained functional enough to sup port autotrophic growth whereas deletions of the putative cleavage reg ion prevented autotrophic growth. Although enhanced degradation rates of the mutant D1 proteins under low-light growth conditions demonstrat e that neither the PEST-like sequence nor the putative cleavage region are required for D1 proteolysis, it became clear that the extension i n 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 th at neither light intensity, as such, nor the activity of the electron transfer chain are the only determinants in regulation of psbA gene tr anscription.