Polar Head Group Composition and Lipid Phase Transition 3209

The cls mutation conferring a defect in cardiolipin synthesis (Pluschke, G., Hirota, Y., and Overath, P. (1978) d BioC. Chem 253, 5048-5055) has been introduced into an Escherichia coli strain defective in unsaturated fatty acid synthesis in order to study the effect of changes in polar head group composition on the ordered c, fluid phase transition f the membrane phospholipids. The defect in cardiolipin formation is compensated by an increase in phosphatidylglycerol content, resulting in a decrease of the midpoint of the phase transition by 6°C. Starvation of the CIS mutant strain for the unsaturated fatty acid supplement leads to the incorporation of saturated acyl chains of reduced average length into the phospholipids, and growth is inhibited although the membrane remains in a fluid state. A revertant of this strain is described which retains the parental fabB-, f&-, and cls markers and grows in the absence of an unsaturated fatty acid supplement. A CES‘ derivative of the revertant can multiply in a restricted temperature range (35-43°C). It contains only saturated phospholipid acyl chains of an anornalously short average length of 14 carbon atoms but has the same polar head group composition as wild type E. coli. The results demonstrate that, under defined conditions, saturated acyl chains of reduced length are functionally equivalent to unsaturated chains.