Genetic engineering of membrane lipid

Methods for isolating mutant strains of E. coli defective in fatty acid biosynthesis (fab) or in phospholipid synthesis (pls) were developed and some of the mutants characterized with respect to their biochemical defects and their location on the E. coli chromosome. FabA and fabB mutants were employed extensively to define the requirement for cis unsaturated fatty acids in E. coli. Cis unsaturated fatty acids were replaced by certain branched chain fatty acids and at higher growth temperatures in part by intermediate chain length saturated fatty acids without serious loss in membrane function. These findings support the view that the physical properties of the acyl groups are of paramount importance. Utilizing an unsaturated fatty acid auxotroph and various exogenously supplied fatty acid analogues to modify membrane phospholipid structure, UDPgalactose lipopolysaccharide galactosyltransferase is found to be most active under conditions which are known to favor mobility of membrane fatty acyl groups. The differences in activity with the different membrane preparations can be attributed in some instances to differences in the temperature of the thermotropic lipid phase transitions, but reflect in other cases differences in the physical environment of the membrane which persist above the phase transitions.