Lactose Permeation Via the Arabinose Transport System in Escherichia coli K-12

Anne Messer
1974 Journal of Bacteriology  
This paper describes the isolation and characterization of a mutant of Escherichia coli that transports lactose and its analog thiomethylgalactoside via the arabinose permeation system. Unlike transport via the lactose permease, this transport is not inhibited by thiodigalactoside, but was inhibited by arabinose, xylose, and fucose. The site of the mutation was in the arabinose C gene and confers constitutivity on the entire arabinose operon. Furthermore, this transport was found in 24
more » ... ound in 24 independently isolated arabinose-constitutive strains, and in strains which had been induced with arabinose and then starved to remove all traces of it. It was therefore concluded that lactose and thiomethylgalactoside are low-affinity substrates of at least one component of the normal arabinose permeation system. The mechanism by which bacterial permeases interact with their substrates in the course of substrate transport remains poorly understood. It has been proposed in the case of the most thoroughly studied system, the lactose permease (1, 3, 4, 7) , that the binding site for an inhibitor substrate analog, thiodigalactoside (TDG), is distinct from the site for substrate binding. In the present study, as a test of this proposal, a mutant was sought in which lactose transport is not subject to TDG inhibition. Such mutants were sought among revertants from a y-strain. In the mutant studied, the genetic alteration was not as expected in the lactose permease nor were the properties consistent with any of the galactoside transport systems described by Rotman et al. (14) . On the basis of genetic and other experiments, it is concluded that lactose transport in this mutant in the presence of TDG is mediated by a totally distinct transport system, that for the pentose arabinose. The arabinose operon (Fig. 1) has been examined mainly as an example of a positive genetic control system, both in Escherichia coli B/r (5) and in E. coli K-12 (15). Brown and Hogg (2) report that E. coli B/r has two active transport systems for arabinose, both under the control of araC. They have identified two genes, one associated with low-affinity uptake and the other with both high-affinity uptake and the arabinose binding protein. Schleif (15) has demonstrated two kinetically distinguishable arabi-I Present address:
doi:10.1128/jb.120.1.266-272.1974 fatcat:mb5tryq6pffa7nuq7hc3vyol6m