Cytoplasmic membrane of Rhizobium meliloti bacteroids. I. Alterations in lipid composition, physical properties, and respiratory proteins

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Abstract
Differentiation of Rhizobium meliloti cells in alfalfa root nodules is characterized by alteration in the lipid composition and physical properties of the bacteroid cytoplasmic membrane. Nitrogen-fixing bacteroids isolated from nodules showed an altered phospholipid and fatty acid composition and lacked aliphatic alcohols normally associated with the bacterial cell wall. Bacteroid cytoplasmic membrane lipids were more fluid, but responded specifically to the presence of calcium ion with an increase in the ordered packing of fatty acid side chains, as determined from electron spin resonance spectra of spin probes intercalated in the membrane. This cation also specifically enhanced rates of respiration of the bacteroids and rates of reduction of the spin probe. Bacteroid cytoplasmic membranes differed from those of air-grown cells, both in the amount of cytochromes b and c per cell and in the nature of the terminal oxidase. Cytochrome aa3, was present in bacteroids at less than 1/10 the level found in air-grown cells. The observed differences in the structure and function of the cytoplasmic membrane of bacteroids may be associated with requirements of the bacterial symbiont for utilization of carbon substrates and provision of reducing power in support of the nitrogenase enzyme system which is expressed in the nodule environment.