| Abstract: | A specific gamma-aminobutyrate (GABA) transport system in Escherichia coli K-12 cells with a K(m) of 12 muM and a V(max) of 278 nmol/ml of intracellular water per min is described. Membrane vesicles contained d-lactate-dependent activity of the system. Mutants defective in GABA transport were isolated; they lost the ability to utilize GABA as a nitrogen source, although the activities of glutamate-succinylsemialdehyde transaminase (GSST) (EC 2.6.1.19) and succinylsemialdehyde dehydrogenase (SSDH) (EC 1.2.1.16), the enzymes that catalyze GABA utilization, remained as high as in the parental CS101B strain. The ability to utilize l-ornithine, l-arginine, putrescine, l-proline, and glycine as a nitrogen source was preserved in the mutants. The genetic lesions resulting in the loss of GABA transport, gabP5 and gabP9, mapped in the gab gene cluster in close linkage to gabT and gabD, the structural genes of GSST and SSDH, and to gabC, a gene controlling the utilization of GABA, arginine, putrescine, and ornithine. The synthesis of the GABA transport carrier is subject to dual physiological control by (i) catabolite repression and (ii) nitrogen availability. Experiments with glutamine synthetase (EC 6.3.1.2)-negative and with glutamine synthetase-constitutive strains strongly indicate that this enzyme is the effector in the regulation of GABA carrier synthesis by route (ii). |
