MEMBRANE LIPID-COMPOSITION AND CELL-SIZE OF ACHOLEPLASMA-LAIDLAWII STRAIN-A ARE STRONGLY INFLUENCED BY LIPID ACYL-CHAIN LENGTH

The small, cell-wall-less prokaryote Acholeplasma laidlawii strain A-E F22 could grow with membrane lipids having an average acyl chain lengt h (C) over bar(n) varying over 14.5- almost 20 carbons by exogenous su pplementation with selected fatty acids. For 16 < (C) over bar(n) < 18 , the cells grew with lipids containing 100% (mol/100 mol) monounsatur ated acyl chains, whereas for (C) over bar(n) < 16 and (C) over bar(n) > 18, cell growth only occurred with gradually lower fractions of uns aturated chains. (C) over bar(n) was actively increased and decreased by chain elongation or de novo fatty acid synthesis upon incorporation of short-chain and long-chain fatty acids, respectively. The membrane lipid composition was strongly affected by the acyl chain length and unsaturation, and the metabolic responses are readily explained as a r egulation mechanism based on the established phase equilibria of the i ndividual lipids in the A. laidlawii membrane. Monoglucosyldiacylglyce rol (Glc-acyl(2)Gro) was the dominating lipid with short chains but th e fraction of this lipid decreased with increasing (C) over bar(n), co rrelating with the decreasing lamellar to nonlamellar phase transition temperatures for this lipid. The fractions of diglucosyldiacylglycero l (Glc(2)-acyl(2)Gro) and phosphatidylglycerol (PtdGro), forming lamel lar phases only, increased with increasing (C) over bar(n) over the en tire chain-length interval. A weaker correlation was usually observed between the relative amount of a lipid and the extent of chain unsatur ation; however, the fractions of Glc(2)-acyl(2)Gro and PtdGro increase d clearly with an increasing degree of unsaturation. Moreover, the syn thesis of the nonbilayer-forming lipids acyl(2)Gro and monoacyl-Glc-ac yl(2)Gro was strongly stimulated by a high degree of chain saturation. Concomitantly, the phase equilibria of Glc-acyl(2)Gro an shifted towa rds lamellar phases at the growth temperature. The fraction of the thr ee potentially nonbilayer-forming lipids varied over 10-80% (mol/100 m ol) total lipids as a function of the acyl chain composition. The comb ined molar fractions of the three phospholipids increased strongly wit h chain unsaturation. However, the fraction of phosphate moieties in t he different lipids was constant over the entire chain-length interval . It is concluded that the regulation of the membrane lipid compositio n aims at maintaining similar phase equilibria and surface charge dens ities of the lipid bilayer. The size of A. laidlawii cells was changed in a systematic manner and correlated qualitatively with the packing properties of the lipids. Cell diameters were increased by an increase in acyl chain length and saturation, and was affected by additives su ch an n-dodecane and acyl(2)Gro.