Counter-transport mediated by the lactose permease of Escherichia coli

Abstract

When the two main energy yielding pathways, respiration and the membrane ATPase of Escherichia coli are poisoned, the lactose permease is unable to accomplish accumulative transport of thiogalactosides, but the efflux of preloaded substrate can be coupled to a transiently uphill transport of exogenous substrate. This transient uphill transport, called overshoot has been reexamined with the possibility of an obligate H+ cotransport in mind. Overshoot can be diminished but not suppressed by a proton-conducting uncoupler, carbonyl cyanide m chlorophenylhydrazone, (CCCP) and by a liposoluble cation, triphenyl-methyl phosphonium (TPMP+). The effect of other factors, such as temperature, amount of permease and pH were also explored. The overshoot was found to decrease with increasing pH, until at pH 8 it became negligible. This is in sharp contrast with the relatively flat pH dependence of uphill and downhill transport in unpoisoned cells. CCCP and TPMP+ had no inhibitory effect on the overshoot at pH 6 and below.