Mutational and Physiological Enhancement of Photosynthetic Energy Conversion in Rhodopseudomonas capsulata

Abstract

Photosynthetic growth of Rhodopseudomonas capsulata, a nonsulfur purple bacterium, is severely inhibited by inorganic arsenate when this anion is present equimolar to orthophosphate in the culture medium.Studies on an arsenate-resistant mutant indicate that its resistance can be explained by a significantly increased photophosphorylation capacity of the energy-converting machinery, which can compensate for the decrease in adenosine triphosphate regeneration rate caused by esterification of arsenate in place of phosphate. Repeated subculture of the mutant in the presence of arsenate, under certain conditions, yields cell populations that have strikingly elevated contents of membrane-bound reactioncenter bacteriochlorophyll, cytochromes, and photophosphorylation coupling factor. Membranes from such cells show unusually high photophosphorylation activity, and can be expected to provide a useful experimental material for more refined analysis of the mechanism of the energy-conversion process.Results of previous studies (1, 2) on photosynthetic bacteria are consistent with the idea that the relative concentrations of the intracellular adenylate nucleotides define an important signal system in the complex regulatory networks that control both biosynthesis and the integration of synthetic activities with energy conversion. It may be expected that the rate of regeneration and the adenylate nucleotide profile in growing cells will be strongly affected by the presence of inorganic arsenate (Asi), which substitutes for orthophosphate (Pi) in numerous biochemical reactions, thereby leading to the formation of unstable esters. Asi, when present equimolar to Pi in the medium, is indeed very inhibitory for photosynthetic growth of the nonsulfur purple bacterium, Rhodopseudomonas capsulata (2). Mutants resistant to Asi can be readily isolated, and a study of one such organism (strain Z-1) suggested that the Asi-resistance in this particular strain is due to a significantly enhanced activity of the photophosphorylation system (2). This increased activity presumably enables the cells to compensate for the decrease in ATP regeneration rate occasioned by the Asi versus Pi competition in the energyconversion process. In this communication, we summarize supporting evidence that shows that the energy-converting membranes of Rps. capsulata Z-1 contain elevated amounts of reaction center bacteriochlorophyll (BChl), photophosphorylation coupling factor, and cytochromes. These modifications are associated with, and presumably responsible for, a very high photophosphorylation capacity of the energyconverting membranes of this organism. For routine transfers, inoculated cultures were incubated at 34°C and illuminated with banks of Lumiline lamps (Sylvania) at a light intensity of about 900 footcandles (fc); other details were as described by Sojka and Gest (1). The arsenate-resistant mutant, Rps. capsulata Z-1 (2), was similarly cultivated in a medium of the same composition, but supplemented with 10 mM sodium arsenate (Asi/Pi in th...

A microbiologist's odyssey: Bacterial viruses to photosynthetic bacteria

“…These included study of: phosphorylation 'coupling factors' (Melandri et al 1970;Lien and Gest 1973); cytoplasmic inorganic pyrophosphatases (Klemme and Gest 1971a, b); ATPase activity (Bose and Gest 1965); photophosphorylation in an arsenate-resistant mutant of R. capsulatus (Lien et al 1971(Lien et al , 1973; and the effects of redox poise on photophosphorylation activity (Bose and Gest 1963b). Of particular interest to me was the complex problem of how energy conversion is integrated with biosynthesis in growing cells.…”

Genetics of Rhodospirillaceae.

“…One such mutant, strain Z-1, exhibits enhanced rates of photophosphorylation. After prolonged culture in the presence of arsenate, cells have elevated contents of cytochromes, reaction center bacteriochlorophyll, and photophosphorylation coupling factor (129,281). This class of mutants should prove useful for analyzimg mechanisms of energy conservation in photosynthetic bacteria.…”