Cytochrome<i>c</i>‐549 ‐ an endogenous cofactor of cyclic photophosphorylation in the cyanobacterium<i>Anacystis nidulans</i>?

Abstract

The cyanobacterium Anacystis nidulans synthesizes fairly large amounts of low potential cytochrome c ‐549 ( E ′ o = −260 mV) during late logarithmic and stationary growth phases. We show here that isolated cytochrome c ‐549 catalyzed high rates of cyclic photophosphorylation by membrane preparations of A. nidulans when added in about 50% reduced state; fully oxidized or reduced cytochrome c ‐549 was far less efficient. A similarly decisive role of proper redox poising was documented with ferredoxin as the cofactor of cyclic photophosphorylation. Inhibitor studies indicated that plastoquinone and cytochrome f are involved in cyclic electron flow catalyzed by cytochrome c ‐549 and ferredoxin.