Properties of the glutamate transport system in Escherichia coli

Abstract

The properties of the glutamate transport system in two glutamate-utilizing mutants of Escherichia coli K-12 were investigated. Growth in the presence of glutamate enhanced the capacity of the bacteria for glutamate uptake. Accumulation of glutamate was found to be an energy-linked highly temperature-dependent process. Nonlinear double reciprocal plots of uptake were obtained in the absence of an exogenous energy source and in the presence of glucose or glycerol. Addition of gamma-aminobutyrate, succinate, ketoglutarate, or aspartate accelerated glutamate uptake and brought about "normalization" of its kinetics. Straight-line kinetics of uptake was also observed when succinate served as the source of energy. Under these conditions, aspartate and alpha-ketoglutarate inhibited glutamate uptake in a noncompetitive fashion, whereas gamma-aminobutyrate activated the system. A number of other amino acids were found to act as "noncompetitive" inhibitors. d-glutamate and some derivatives of glutamate with an unsubstituted alpha-carboxylic and alpha-amino group inhibited l-glutamate uptake in a strictly competitive fashion. An allosteric permease model, consistent with all of these findings, is proposed.