SOME OBSERVATIONS ON THE SYNTHESIS AND FUNCTION OF THE PHOTOSYNTHETIC APPARATUS IN RHODOSPIRILLUM RUBRUM

Abstract

The purified chromato-phores isolated from cells of R. rubrum grown anaerobically at high and low light intensities have markedly different chlorophyll contents, the differences being closely correlated with the differences in chlorophyll content of the cells from which they were prepared. It is therefore evident that light-induced changes of cellular chlorophyll content reflect mainly changes in the amount of chlorophyll that is incorporated in the photosynthetic apparatus, rather than changes in the amount of chromato-phore material in the cell. In chromatophores prepared from cells growing photosynthetically under steady state conditions, the. rate of photophosphorylation measured on the basis of chlorophyll content is inversely related to the amount of chlorophyll present in the chromatophores. Measured on the basis of protein content, however, the rate of phosphorylation is practically constant in chromatophores with very different chlorophyll contents. These facts indicate that the capacity for photophosphorylation is not limited by the amount of chlorophyll in the photosynthetic apparatus, but rather by some enzymatic component of the system. When initially depigmented cells of R. rubrum, produced under strictly aerobic conditions of growth, are subjected to oxygen limitation in the dark, a rapid synthesis of chlorophyll ensues the differential rate of chlorophyll synthesis is of the same order as that which occurs during anaerobic growth in the light. The synthesis of chlorophyll under semi-aerobic conditions in the dark is accompanied by the formation of photochemically active chromatophores. The rate of photophosphorylation by such chromatophores as measured on the basis of protein content increases with the chlorophyll content of the cells, eventually attaining a value considerably higher than that characteristic of chromatophores prepared from cells grown under photosynthetic conditions.