TURNOVER OF THE D1 PROTEIN AND OF PHOTOSYSTEM-II IN A SYNECHOCYSTIS-6803 MUTANT LACKING TYR(Z)

The Photosystem II reaction center is rapidly inactivated by light, pa rticularly at higher light intensity. One of the possible factors caus ing this phenomenon is the oxidized primary donor, P680(+), which may be harmful to Photosystem II because of its highly oxidizing nature. H owever, no direct evidence specificially implicating P680(+) in photoi nhibition has been obtained yet. To investigate whether P680(+) is har mful to Photosystem II, turnover of the D1 protein and of the Photosys tem II reaction center complex were measured in vivo in a mutant of th e cyanobacterium Synechocystis sp. PCC 6803, in which the physiologica l donor to P680(+), Tyr(Z), was genetically deleted. In this mutant, D 1 degradation in the light is an order of magnitude faster than in wil d type. The most straightforward explanation of this phenomenon is tha t accumulation of P680(+) leads to an increased rate of turnover of th e Photosystem II reaction center complex, which is compatible with the hypothesis of destructive oxidation by P680(+) that is damaging to th e Photosystem II complex.