Comparison of lactose uptake in resting and energized Escherichia coli cells: high rates of respiration inactivate the lac carrier.

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RESUMO

The transport of lactose by Escherichia coli cells was radically different in the absence and in the presence of an exogenous energy source: in the former case, the time course of lactose accumulation was monotonous; in the latter case, lactose accumulation reached a maximum and then decreased to a final steady-state level lower than that observed in the absence of an energy source. We show that this "overshoot" is the result of a decrease in the influx rate and of an increase in the rate constant of efflux as lactose accumulates. These phenomena were irreversible. The extent of the overshoot was dependent upon the experimental conditions: it was maximal at alkaline pH, for low external potassium concentrations, and for relatively high external lactose concentrations (around or above the KT of uptake). The addition of an energy source to resting E. coli cells resulted in an increase in both the electrochemical gradient of protons and in the rate of respiration. We demonstrate that the overshoot is the result of the latter and unrelated to the former. We observed an irreversible decrease in the membrane potential as lactose accumulated in the presence of an exogenous energy source. We discuss the whole of our data in terms of an irreversible inactivation of the lactose carrier as a result of a possible interaction with the respiratory chain.

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