New method for study of peptide transport in bacteria.

The transport system for glycylmethionine in Escherichia coli B and Salmonella typhimurium LT2 was examined by a new approach which may be applied to other types of exogenous materials. Physiological auxotrophs were prepared by growing wild strains in a methionine-containing medium to repress the methionine biosynthetic enzymes. Immediate protein synthesis was shown to take place in such physiological auxotrophs only in the presence of either exogenous methionine or a methionine peptide, e.g., glycylmethionine. Protein synthesis was dependent on glycylmethionine taken up by the cell and was indicated by assaying for the inducible enzyme lysine decarboxylase at 5- to 15-min intervals. Uptake was studied by using low concentrations of glycylmethionine, therefore making uptake by permease the limiting step in incorporation of methionine into protein, and by addition of competitor peptides to media containing saturating concentrations of glycylmethionine. Lysine decarboxylase activity in S. typhimurium LT2 was about 80 times that present in E. coli B. Glycylmethionine transport had a K(m) of the order of 1 muM in S. typhimurium. Structural specificities observed for peptide transport by other workers were confirmed for E. coli B. Competitive inhibition of glycylmethionine uptake by dipeptides was observed in E. coli.