Mutants of Escherichia coli defective in membrane phospholipid synthesis. Effect of cessation of net phospholipid synthesis on cytoplasmic and outer membranes.

The effect of cessation of net phospholipid synthesis on the cytoplasmic and outer membranes of Escherichia coli was investigated in a mutant strain defective in the first enzyme of phospholipid synthesis, the sn-glycerol-3-phosphate (glycerol-P) acyltransferase. The glycerol-P (glycerol) auxotropic phenotype of this strain resulted from an altered membranous glycerol-P acyltransferase activity with an apparent Km for glycerol-P 10 times higher than that of the parental activity. When net phospholipid synthesis was halted during glycerol deprivation, both soluble and cell envelope protein synthesis continued. Fractionation of the membranes derived from glycerol-supplemented and glycerol-deprived cultures by isopycnic banding in sucrose gradients revealed that both the cytoplasmic and outer membranes of the deprived culture banded at higher buoyant densities. The protein/phospholipid ratio of both the cytoplasmic and outer membranes increased approximately 60% during the period of glycerol deprivation. The distribution of two cytoplasmic membrane activities, NADH oxidase and 1-acylglycerol-P acyltransferase, and an outer membrane activity, phospholipase A1, showed that the total membranes derived from glycerol-deprived cultures were separated cleanly into cytoplasmic and outer membrane fractions. Both cytoplasmic and outer membrane proteins were synthesized and integrated into their respective membranous structures when net phospholipid synthesis was halted. Hence, the biosynthesis of membrane phospholipid and membrane protein are not tightly coupled. Further, these data suggest that cellular control mechanisms exist which maintain the protein content of both membranous structures below the point where they are saturated with protein.