Energy transduction in the photosynthetic membranes of the cyanobacterium (blue-green alga) P-lectonema boryanum

Abstract

Membrane preparations isolated from the photosynthetic lamellae of the cyanobacterium Plectonema boryanum generate upon illumination a transmembrane pH gradient of approximately 2 to 3 pH units (acid inside), as determined from the distribution of either fluorescent or radioactive amines (9 aminoacridine and [14C]methylamine, respectively). Using the distribution of permeant ions to measure the electrical potential across the membrane, it was found that the latter is practically nil under conditions in which the deltapH is formed and photophosphorylation takes place. In agreement with the above findings cyclic photophosphorylation in this membrane preparation is inhibited by agents shown to collapse the deltapH but not by agents which should collapse the electrical potential. It is deduced that the pattern of proton movement in the photosynthetic lamellae of intact Plectonema spheroplasts corresponds to that of the cell-free membrane system, as both preparations show similar light dependent accumulation of fluorescent amine. It is concluded that the pattern of energy transduction in Plectonema photosynthetic lamellae is similar to that of chloroplast thylakoid membranes and not to that of bacterial cytoplasmic membranes. The evolutionary implications of the findings are discussed and a model for the directionality of H+ movements in the whole cell is presented.