Abstract
The rapid turn-over of the D1 polypeptide of the photosystem two complex has been suggested to be due to the presence of a "PEST"-like sequence located between putative transmembrane helices IV and V of D1 (Greenberg, B. M., Gaba, V., Mattoo, A. K. and Edelman, M. (1987) EMBO J. 6, 2865-2869). We have tested this hypothesis by constructing a deletion mutant (delta 226-233) of the cyanobacterium Synechocystis sp. PCC 6803 in which residues 226-233 of the D1 polypeptide, containing the PEST-like sequence, have been removed. The resulting mutant, delta PEST, is able to grow photoautotrophically and give light-saturated rates of oxygen at wild type levels. However electron transfer on the acceptor side of the complex is perturbed. Analysis of cells by thermoluminescence and by monitoring the decay in quantum yield of variable fluorescence following saturating flash excitation indicates that Q-B, but not Q-A, is destabilized in this mutant. Electron transfer on the donor side of photosystem two remains largely unchanged in the mutant. Turnover of the D1 polypeptide as examined by pulse-chase experiments using [35S]methionine was enhanced in the delta PEST mutant compared to strain TC31 which is the wild type control. We conclude that the PEST sequence is not absolutely required for turnover of the D1 polypeptide in vivo although deletion of residues 226-233 does have an effect on the redox equilibrium between QA and QB.
Highlights
The rapid turn-over of the DI polypeptide of the photosystem two complex has been suggested to be due to the presence of a "PEST"-like sequence located between putative transmembrane helices IV and V of DI
We conclude that the PEST sequence is not absolutely required for turnover of the DI polypeptide in vivo deletion of residues 226-233 does have an effect on the redox equilibrium between QA and QB'
Because Glu 235 found in tobacco D1 is replaced by a tyrosine residue in Synechocystis, the PEST-like region is located between residues Glu 226 and Ser232
Summary
The rapid turn-over of the DI polypeptide of the photosystem two complex has been suggested to be due to the presence of a "PEST"-like sequence located between putative transmembrane helices IV and V of DI Turnover of the DI polypeptide as examined by pulse-chase experiments using [35Sl m ethionine was enhanced in the .:i.PEST mutant compared to strain TC31 which is the wild type control. Of the Dl polypeptide is its high rate of turnover in vivo compared to other PSII components (Edelman and Reisfeld, 1978; Wettern et al, 1983). It is believed that this turnover represents a repair cycle to replace Dl that has been damaged or in the process of being irreparably damaged (Ohad et al, 1984) The nature of this photoinhibitory damage is uncertain, but in vivo one possible mechanism may be the action of singlet oxygen generated by aberrant chlorophyll triplet states within PSII (Telfer et al, 1994; reviewed by Barber and Andersson, 1992). The signaling mechanisms involved in the turnover of Dl, as well as the enzymology of Dl degradation, are poorly understood
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