Retardation of electron donation to Photosystem I in aged cyanobacteria and its reversal by metal cations

Abstract

The dark reduction of photooxidized P-700 in the cyanobacterium Anacystis nidulans becomes slower as a consequence of aging. In cells, of which the envelope has been enzymatically permeabilized to electrolytes, the reduction of P-700 by endogenous electron donors is accelerated by KCl plus valinomycin, but not by KCl alone. The K +-valinomycin system is ineffective, however, in the case of aged but unpermeabilized cells. These results suggest that a consequence of aging in Anacystis is an increase in the negative electric potential at the inner thylakoid surface, which makes electron donation to P-700 by acidic donors (cytochrome c-553 and plastocyanin) less probable. This situation is reversed by the permeant K +-valinomycin system, which suppresses the inner surface potential and collapses the inside-outside potential difference, but not by K + alone, since the thylakoid membrane is impermeable to it.