Characterization of two cell-envelope fractions from chemotrophically grownRhodospirillum rubrum

Two cell-envelope fractions were isolated from chemotrophically grown cells ofRhodospirillum rubrum. On the basis of electron-microscopic investigations, chemical analysis, distribution of components involved in respiration, and polyacrylamide gel electrophoresis, the heavy fraction (ρ²⁰=1.246 g per cm³) was identified as cell-wall, and the light fraction (ρ²⁰=1.145 g per cm³) as cytoplasmic-membrane fragments. Electron micrographs showed cell-wall fragments as open structures while cytoplasmic-membrane preparations were composed of closed membrane vesicles. With respect to the main classes of chemical compounds, cell wall could be distinguished from cytoplasmic membranes by a rather low ratio of phospholipids per protein and a high ratio of carbohydrates per protein. The relative proportion of individual neutral sugars as well as phospholipids (except for lysophosphatidyl ethanolamine) revealed no significant differences between both envelope fractions. Fatty acid analysis demonstrated a higher proportion of saturated fatty acids in cell-wall than in cytoplasmic-membrane fractions. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the fractions showed distinct protein compositions. While in cell-wall preparations polypeptides of 43000 and 14000 daltons predominated, 56000- and 52000-dalton polypeptides were the main protein subunits of cytoplasmic membranes. Cross contaminations of both cell-envelope fractions were defined.