Protein on the cell surface of the moderately halophilic phototrophic bacterium Rhodospirillum salexigens

Abstract

A cell surface protein (Mr 68,000) of the moderately but obligately halophilic phototrophic bacterium Rhodospirillum salexigens was identified by two independent methods: first, by labeling the cell surface with radioactive iodine and lactoperoxidase, and second, by washing cells in 30% sucrose to remove proteins attached to the cell surface by ionic bonds. The identified protein very likely represents the outermost layer of the cell envelope of R. salexigens as observed by electron microscopy. The protein was isolated. Its isoelectric point was determined to be 4.4; the excess of acidic over basic amino acids was found to be 18.3 mol %; and its average hydrophobicity was 2.26 kJ per residue. Although extremely halophilic bacteria and their cell walls have been the focus of extensive work (for a review, see reference 10), moderately halophilic bacteria have not been studied in such detail yet. The latter are found in all