BACTERIAL PHOTOPHOSPHORYLATION: REGULATION BY REDOX BALANCE

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Abstract
An optimal oxidation-reduction potential is required for maximal coupling of light-activated electron transport to the phosphorylation processes. Either the photochemical apparatus, in vivo, can be maintained within a restricted redox range, or the rate of photo phosphorylation is normally subject to metabolic regulation by changes in the temporal concentrations of certain physiological reductants and oxidants. Extensive shift of the redox potential, with consequent inhibition of phosphorylation, can be induced in intact cells of Rhodospirillum rubrum by addition of low concentrations of certain redox dyes; under these conditions, the cells form a "dark" fermentation of endogenous reserves even though they are under continuous illumination. This striking metabolic effect, which is also caused by antimycin A, indicates that photophosphorylation activity inhibits fermentation. "Over-reduction" (or "over-oxidation") inhibits phosphorylation, due to severe displacement of the redox potential of one or more components of the electron transfer chain. Photophosphorylation by isolated particles is activated by adjusting the redox potential with mild reducing systems (hydrogenase action or addition of ascorbate alone). The inhibition caused by "over-reduction" with external electron donors can be effectively reversed by supplementation with certain electron acceptors. Restoration of photophosphorylation can occur in the absence of net electron transfer between the exogenous donor and acceptor systems. Addition of the electron acceptor primarily adjust the redox potential to an electrochemical range which is optimal for the operation of cyclic photophosphorylation. If dyes are present under favorable redox conditions, the phosphorylation becomes resistant to antimycin-A.