Restoration by Fatty Acids of Active Transport in a Lactose Transport Mutant of Escherichia coli

A mutant of Escherichia coli ML 308-22 has lost its normal capacity to accumulate lactose analogues such as thiogalactosides without a reduction in the number and activity of the membrane carriers. The defect is only in the lactose transport system since transport of other sugars occurs at normal rates. Since the mutant was able to phosphorylate α-methylglucoside, the phosphotransferase system is believed to be fully operative. The addition of nucleotides, exogenous substrates, and surface active agents did not restore active galactoside transport in the mutant. However, the addition of extracts obtained from the wild type, ML 308, by osmotic shock or sonication enabled the mutant to accumulate up to 80% of the amount of galactoside accumulated by ML 308. The stimulating factor(s) in the shock fluid and cell-free extract was stable at 100° for 10 min and after proteinase treatment. A total lipid extract from ML 308 was partially stimulatory. When lipid classes were examined, phospholipids had little effect but free fatty acids were stimulatory. The stimulation of transport by fatty acids increased with increasing carbon-chain length to C18 and saturated fatty acids were found to be more stimulatory than unsaturated fatty acids. Addition of stearic acid at an optimal concentration of 0.2 mM caused an immediate increase in both the rate and the extent of [14C]thiomethylgalactoside ([14C]TMG) uptake in ML 308-22. Exit of [14C]TMG from the cell was slower in the presence of stearic acid. Growth of ML 308-22 in stearic acid medium had little effect on subsequent transport activity.