DELETION OF THE PEST-LIKE REGION OF PHOTOSYSTEM-2 MODIFIES THE Q(B)-BINDING POCKET BUT DOES NOT PREVENT RAPID TURNOVER OF D1

The rapid turn-over of the D1 polypeptide of the photosystem two compl ex has been suggested to be due to the presence of a ''PEST''-like seq uence located between putative transmembrane helices IV and V of D1 (G reenberg, B. M., Gaba, V., Mattoo, A. K. and Edelman, M. (1987) EMBO J . 6, 2865-2869). We have tested this hypothesis by constructing a dele tion mutant (Delta 226-233) of the cyanobacterium Synechocystis sp. PC C 6803 in which residues 226-233 of the D1 polypeptide, containing the PEST-like sequence, have been removed. The resulting mutant, Delta PE ST, is able to grow photoautotrophically and give light-saturated rate s of oxygen at wild type levels. However electron transfer on the acce ptor side of the complex is perturbed. Analysis of cells by thermolumi nescence and by monitoring the decay in quantum yield of variable fluo rescence 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 mu tant. Turnover of the D1 polypeptide as examined by pulse-chase experi ments using [S-35]methionine was enhanced in the Delta PEST mutant com pared to strain TC31 which is the wild type control. We conclude that the PEST sequence is not absolutely required for turnover of the D1 po lypeptide in vivo although deletion of residues 226-233 does have an e ffect on the redox equilibrium between Q(A) and Q(B).