Abstract
(1) Rhodobacter capsulatus was grown phototrophically at saturing light intensities under anaerobic conditions in a chemostat with malate as a limiting, anabolic substrate. (2) Below critical dilution, the optical density of the culture at 650 nm was independent of specific growth rate. There was a weak dependence of the levels of bacteriochlorophyll, photosynthetic reaction centre and b- and c-type cytochrome on specific growth rate. (3) Two values of specific growth rate, 0.075 h −1 (low) and 0.150 h −1 (high), were selected for detailed investigation. The proportionality constant relating electrochromic absorbance changes to membrane potential (Δψ) was not influenced by the specific growth rate. (4) The dependence of the dissipative membrane ionic current ( J dis) across the cytoplasmic membrane on its driving force, Δψ, was determined in cells harvested from cultures at high and low growth rate. In both cases the dependence was diodic. J dis in cells from low growth-rate cultures was greater than J dis in cells from high growth-rate cultures for the same values of Δψ above the threshold region although the maximum observed J dis was lower. These results indicate: (a) at given values of Δψ the membrane conductance was larger; (b) the maximum protonmotive activity was smaller in the low growth-rate cells. (5) Treatment of the harvested cells with the F 0-inhibitor venturicidin at concentrations sufficient to inhibit completely light-induced ATP synthesis resulted in increased values of Δψ during illumination. The threshold region in the J dis Δψ curves was shifted to greater values of Δψ with venturicidin and maximum values of J dis were not significantly reduced. The inhibitor had qualitatively similar effects on cells harvested from both the low and the high growth-rate cultures such that the differences in membrane conductance properties between the two sets of cells were still evident when the ATP synthase was blocked. (6) The maximum protonmotive activity was also smaller in chromatophores isolated from low growth-rate cultures than in chromatophores from high growth rate cultures (compare 4(b)) but the membrane conductance dependence on Δψ was similar in the two sets of chromatophores. Venturicidin treatment led to increased values of light-induced Δψ and to reduced maximum values of J dis. (7) It is suggested that in addition to the main consumer of the protonmotive force in the bacterial membranes, the ATP synthase, there is another major current-carrying pathway, a gated ionophore, whose threshold for conduction is slightly higher than that of the ATP synthase. In bacterial cells which are limited in their supply of anabolic substrate there is an adaptation which results in a proportionately greater ionic current through the ionophore and hence an increased futile cycling of ions across the cytoplasmic membrane. Soluble components responsible for the ionophoric activity are lost during the preparation of membranes from the bacteria.
Published Version
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