COMPLETE ASSIMILATION OF CYSTEINE BY A NEWLY ISOLATED NONSULFUR PURPLE BACTERIUM RESEMBLING RHODOVULUM-SULFIDOPHILUM (RHODOBACTER-SULFIDOPHILUS)

A rod-shaped, motile, phototrophic bacterium, strain SiCys, was enrich ed and isolated from a marine microbial mat, with cysteine as sole sub strate. During phototrophic anaerobic growth with cysteine, sulfide wa s produced as an intermediate, which was subsequently oxidized to sulf ate. The molar growth yield with cysteine was 103 g mol(-1), in accord ance with complete assimilation of electrons from the carbon and the s ulfur moiety into cell material. Growth yields with alanine and serine were proportionally lower. Thiosulfate, sulfide, hydrogen, and severa l organic compounds were used as electron donors in the light, whereas cystine, sulfite, or elemental sulfur did not support phototrophic an aerobic growth. Aerobic growth in the dark was possible with fructose as substrate. Cultures of strain SiCys were yellowish-brown in color a nd contained bacteriochlorophyll a, spheroidene, spheroidenone, and OH -spheroidene as major photosynthetic pigments. Taking the morphology, photosynthetic pigments, aerobic growth in the dark, and utilization o f sulfide for phototrophic growth into account, strain SiCys was assig ned to the genus Rhodovulum (formerly Rhodobacter) and tentatively cla ssified as a strain of R. sulfidophilum. In cell-free extracts in the presence of pyridoxal phosphate, cysteine was converted to pyruvate an d sulfide, which is characteristic for cysteine desulfhydrase activity (L-cystathionine gamma-lyase, EC 4.4.1.1).