Evidence for two calcium transport systems in the photosynthetic bacterium Chromatium vinosum

Abstract

Light-dependent Ca 2+ efflux via the Ca 2+/H + antiport in the photosynthetic purple sulfur bacterium Chromatium vinosum was inhibited by three phenothiazines: chlorpromazine; trifluoperazine and phenothiazine. The inhibitors had no effect on Ca 2+ uptake by C. vinosum in the dark nor any effect on the light-dependent efflux of either Na + or Tl + catalyzed, respectively, by the C. vinosum Na +/H + or K +/H + antiports. Ruthenium red and LaCl 3, neither of which inhibited light-dependent Ca 2+ efflux in C. vinosum, markedly inhibited Ca 2+ uptake in the dark by C. vinosum cells. Ruthenium red had no effect on the uptake of either Na +or the K + analog T1 + by C. vinosum cells in the dark. These results have been interpreted in terms of two separate Ca 2+ transport systems in C. vinosum: (i) a phenothiazine-sensitive and ruthenium red, La 3+-insensitive Ca 2+/H + antiport responsible for Ca 2+ efflux in the light; and (ii) a ruthenium red and La 3+-sensitive but phenothiazine-insensitive Ca 2+ uptake system.