| Abstract: | We have isolated mutants of Escherichia coli K-12 CS101B that have lost the ability to utilize gamma-aminobutyrate as a source of nitrogen. One class of mutants, which were not affected in the utilization of other nitrogen sources (proline, arginine, glycine), included many isolates with lesions in gamma-aminobutyrate transport or in its transamination and one mutant completely devoid of succinic semialdehyde dehydrogenase activity and exhibiting low gamma-aminobutyrate transport and transamination. gamma-Aminobutyrate-utilizing revertants of the latter recovered full transport and transamination capacities but remained dehydrogenaseless. Another class of mutants showed pleiotropic defects in nitrogen metabolism. One such mutant was lacking glutamate synthase activity. The genes specifying the synthesis of gamma-aminobutyrate permease, gabP, gamma-aminobutyrate transaminase, gabT, and succinic semialdehyde dehydrogenase, gabD, and the control gene, gabC, that coordinately regulates their expression all form a cluster on the E. coli chromosome, linked to the srl and recA loci (at 57.5 min). The mutations with pleiotropic effects on the metabolism of nitrogenous compounds are not linked to the gab cluster. |
