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Efflux and the Steady State in -Methylglucoside Transport in Escherichia coli

Department of Microbiology, School of Medicine, University of Virginia, Charlottesville, Virginia 22903

ABSTRACT

Efflux and the steady state in a group translocation system, the -methylglucoside (MG) transport system, were investigated. The maximum intracellular level of -methylglucoside is a function of a steady state. There is no inhibition of MG influx as the intracellular pool of MG, and -methylglucoside-6-phosphate (MGP) rises. This steady state has three components: MG influx, action of an MGP phosphatase, and MG efflux. The phosphatase is the rate-limiting step (half-time = 5.0 min); thus, the true efflux rate (half-time = 2.0 min) cannot be simply measured from the kinetics of MG loss from the cell. Under our steady-state conditions the percentage of intracellular radioactivity present as MGP was 71%. Under conditions of zero influx, after an efflux of 12 min the percentage present as MGP fell to 55%. However, when fluoride was present during the efflux period, the percentage of the sugar as MGP increased to about 85%. Fluoride greatly inhibits both influx and phosphatase activity (half-time = 50 min). The efflux of MG from the cell is apparently also fluoride-sensitive but to a lesser extent (half-time = 4.1 min). These data are summarized in a model describing the three components of the steady-state and effect of fluoride.

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