Cation-induced,inhibitor-resistant photosystem II reactions in cyanobacterial membranes

Ferricyanide-supported oxygen evolution in sonic vesicles from the cyanobacterium $\text{Spirulina}\text{platensis}$ is only partially sensitive to inhibition by 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), and addition of cations to inhibited membranes stimulates the rate of oxygen evolution. The order of cation effectiveness (M³⁺ > M²⁺ > M⁺) suggests that this stimulation is due at least in part to surface charge screening effects which permit freer access of anionic ferricyanide to the vesicle membrane surface; La³⁺, Ca²⁺, and K⁺ are most effective in this regard. Ferricyanide photoreduction is completely sensitive to 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), and neither mono- nor divalent cations affect this inhibition. Addition of La³⁺, on the other hand, causes a nearly complete restoration of ferricyanide-supported oxygen evolution. We conclude that the membrane surfaces of these vesicles are uniquely different from those o higher plants; sites of ferricyanide reduction associated with the interphotosystem chain are surface localized, and the primary acceptor region of photosystem II is susceptible to a trivalent cation-specific reaction in which ferricyanide may directly oxidize the primary acceptor.