Transport of sugars and amino acids in bacteria. V. Further studies in the galactose transport system in Escherichia coli.

Abstract

Studies were made on the transport activities of three mutants of Escherichia coli K12 for galactose, glucose and fructose, and their activities to bind and phosphorylate sugars, thought to be intimately related with transport mechanisms. The mutants used were W3092 (galactokinase negative), S7 (galactokinase and galactose permease negative) and W2243A (glucose permease negative). It was found that W3092 lacked Enzyme II of a phosphoenol pyruvate dependent phosphotransferase system, which is specific for galactose, and that galactose, a gratuitous sugar for the organism, was actively taken up by the cells in the form of the free sugar. The cells were also shown to have glucokinase, a phosphotransferase system specific for glucose, galactose binding protein and low phosphorylating activity for fructose. Significant amounts of the glucose and fructose taken up by the cells were phosphorylated. The above activities were tested in parallel in S7 cells. Results indicated that, qualitatively, S7 cells had all the same activities as those of W3092 cells except that they had no galactose binding protein. From these results it was concluded that galactose transport into W3092 cells is mediated by a system involving the galactose binding protein, which results in free galactose as the initial product. On the contrary, glucose transport in W3092 cells seems to be catalyzed preferentially by the phosphotransferase system. Various activities related to sugar transport in W2243A cells were also examined. The organism lacked Enzyme II's of the phosphotransferase system, that are specific for galactose and glucose. It showed high galactose and glucose uptake activities and low fructose uptake activity. The galactose-(glucose-)binding protein was demonstrated in fluid obtained by subjecting the cells to osmotic shock but attempts to detect fructose binding protein failed. It was therefore concluded that the organism transports galactose and glucose via the galactose binding protein system and that the initial steps of metabolism of the sugars involve galactokinase or glucokinase. Galactokinase of W2243A was induced by galactose and was repressed by glucose. A model for the active transports of certain sugars and many amino acids, which are thought to be mediated by a binding protein system, was evaluated on the basis of the results and the unique significance of this active transport system in physiological regulation was discussed.