Removal of stable tyrosine radical D+ affects the structure or redox properties of tyrosine Z in manganese-depleted photosystem II particles from Synechocystis 6803.

Abstract

Photosystem II contains two redox-active tyrosines, D and Z. To understand the function of the dark stable tyrosine radical, D+, we have characterized two site-directed mutations at the D tyrosine residue in the transformable cyanobacterium, Synechocystis sp. PCC 6803, through the use of purified photosystem II particles (Noren, G. H., Boerner, R. J., and Barry, B. A. (1991) Biochemistry 30, 3943-3950). In manganese-depleted mutant particles, a light-induced EPR signal is observed. This signal contains a stable component, due to a chlorophyll radical, and an unstable component. The lineshape of the unstable, oxidized component, which we call M+, is obtained by subtraction; it has a lineshape different from tyrosine Z+/D+ and a g value of 2.004. Up to one M+ spin per reaction center can be photooxidized. The characteristic light-induced EPR signal ascribed to Z+ is not detected; under the same conditions, Z+ is detected in control preparations. The M+ radical lineshape is similar to the light-induced photosystem II radical identified in a site-directed mutant in the D1 polypeptide (YF161D1) (Noren, G. H., and Barry, B. A. (1992) Biochemistry 31, 3335-3342). Optical measurements on manganese-depleted photosystem II particles from control and D2 mutant preparations show that charge recombination kinetics between Q-A and an oxidized redox-active component are similar, to within a factor of two, in all three preparations. We conclude that lack of the stable tyrosine D+ alters the structure or redox properties of tyrosine Z in manganese-depleted preparations.