Transport Systems for l-Methionine in Escherichia coli

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The transport of l-methionine into cells of Escherichia coli is described. The transported methionine undergoes a fairly rapid conversion to other cellular metabolites, primarily polyamines, but within 2 min, at least 65% of the intracellular label remains as methionine. The uptake process, which is temperature dependent, allows the accumulation of methionine against a concentration gradient. The dependence of the initial rate of uptake on the extracellular substrate concentration indicates the presence of multiple transport systems, whose kinetic behavior can be approximated by two systems, one with KT = 7.5 × 10−8 M and VT = 200 pmol per μliter of cell water per min, and the other with KT = 40 × 10−6 M and VT = 1,550 pmol per μliter per min. Both systems are highly specific for l-methionine. Methionine derivatives substituted on the amino or carboxyl group were somewhat effective as inhibitors of l-methionine uptake, whereas d-methionine, ethionine, or other amino acids were poorly inhibitory, if at all. The uptake process is dependent on metabolic energy, but apparently this energy can be derived either from glycolysis or from oxidative phosphorylation. Efflux of methionine was demonstrable, and both the influx and efflux process were susceptible to inhibition by N-ethylmaleimide. The intracellular pool of l-methionine was estimated to be 0.1 to 0.3 mM. The transport in two mutant strains defective in methionine uptake (metD and metP) showed that the high-affinity transport system was lost, whereas the low-affinity system remained more or less intact.

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