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Galactose transport in Escherichia coli. II. Characteristics of the exit process.

B. Horecker,J. Thomas,J. Monod

Published 1960 in Journal of Biological Chemistry

ABSTRACT

Paper I of this series was concerned with the uptake of galactose in a mutant strain of Escherichia coli, ML 32,400. This organism is incapable of growth on galactose because of a complete lack of galactokinase, but it will accumulate this sugar to levels which greatly exceed those found in the external medium. The transport system has a high affinity for galactose, is highly specific, and is sensitive to energy uncoupling agents such as dinitrophenol(1). Ordinarily, galactose utilization in E. coli is an inducible process (2) ; in the parent strain ML 30, for example, galactokinase is present only in induced cells (1). Yet the transport of galactose in the mutant strain ML 32,400, derived from ML 30, is not dependent upon previous induction. On the contrary, the capacity of the constitutive transport system is depressed in a striking way when growth occurs in the presence of galactose and certain compounds related to galactose (1) although these compounds are not utilized. Thus, cells grown on glucose or lactose show little activity toward galactose, and the level of galactose accumulation is also depressed by growth in the presence of certain galactosidase inducers, such as isopropyl thiogalactoside and methyl thioga1actoside.l Growth of the organism in the presence of traces of galactose, which are not utilized, yields cells which have a much smaller capacity to accumulate galactose, compared with that of cells which have been cultivated on the same carbon source in the absence of galactose. The reduced capacity for galactose can be shown to be caused by an increase in the rate of loss of galactose from the cells, rather than by a decrease in the rate of galactose uptake. These results are not compatible with exit of sugar by simple diffusion and lend strong support to the “transporter” hypothesis proposed by Kepes (3) to explain galactoside accumulation. Kepes has found the exit of thiodigalactosides from E. coli cells to be inhibited by such agents as p-chloromercuribenzoate and accelerated by other galactosides such as a-methyl thiogalactoside. These observations could not be explained in terms of passive diffusion and led Kepes to suggest that it is necessary for the substrate to combine with a hypothetical “transporter” for both entry and exit. The present results obtained with the galactose transport system are consistent with this view.

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