Metabolite uptake by serineglycine auxotrophs of Escherichia coli.

Microbial growth has been shown, in many instances to be dependent upon the uptake of metabolites from the culture medium into the cell (1, 2). The formation of the essential “uptake systems” appears not only to be under genetic control but, in some cases, to be an inducible capacity of the organism (1,2). This communication is concerned with the uptake of metabolites by two amino acid auxotrophs of Escherichia coli K-12: strain S/G, which responds equally well to L-serine and to glycine, and strain S, which first was believed to respond only to L-serine (3, 4). Subsequent experiments showed strain S to be capable of growth on glycine after prolonged lag periods (5), and suggested that it has the characteristics of a “cryptic mutant” (1) (i.e. a mutant apparently unable to utilize an exogenous substrate due to a faulty “uptake mechanism” but possessing the enzymes necessary for the endogenous metabolism of the substrate). Strain S also was found to grow readily on a variety of glycinecontaining peptides. Its growth response to such peptides, unlike that to free glycine, was not inducible (5). However, the main difference between the utilization of free glycine and of a peptide such as glycylglycine under the usual conditions of growth tests appeared to be the relatively faster growth rate permitted by the dipeptide. A comparison of the amino acid and peptide metabolism of strains S and S/G led to the hypothesis that both auxotrophs can interconvert endogenous serine and glycine and that strain S, but not strain S/G, is deficient in the mechanism by which free glycine is taken up from the culture fluid (5). In an attempt to provide direct proof for this hypothesis, a study has been made of the ability of each strain to take up CY-labeled glycine and glycylglycine from the medium under a variety of environmental conditions.