Galactoside-proton symport in a lacYUN mutant of Escherichia coli investigated by analysis of transport progress curves.

The kinetics of galactoside-proton symport catalysed by a wild-type strain and one carrying a mutation, previously reported to cause uncoupling of the symport reaction, have been examined. The mutation does not affect the stoichiometry during the initial period of uptake, when the internal concentration of galactoside is low, but it does result in much greater competition from the galactoside as it is accumulated. Simple methods for the analysis of the uptake progress curves have been developed and used to estimate the initial rate of uptake and affinity for internal galactoside. The maximum rate of uptake is decreased by a factor of 2 at most whereas the affinity for internal galactoside is increased up to 50-fold by the mutation. The pH-dependence of the galactoside efflux reaction is changed in a manner which suggests that the defect is in the interaction between proton-binding and galactoside-binding sites rather than in the structure of either site.