Stereospecificity of Tripeptide Utilization in a Methionine Auxotroph of Escherichia coli K-12

Abstract

The stereospecificity of peptide utilization in Escherichia coli K-12 4212, a methionine auxotroph, was investigated using diastereomers of trimethionine and trimethionine methyl ester. Of the eight stereoisomers examined, only l -Met- l -Met- l -Met, l -Met- l -Met- d -Met, and d -Met- l -Met- l -Met and the corresponding methyl esters serve as growth substrates. Triornithine-resistant mutants of strain 4212 were isolated which failed to transport d -Met- l -Met- l -Met. These results provide evidence that an oligopeptide containing a d residue at its amine terminus can enter E. coli by the oligopeptide transport system.