[en] By using the glutamate-amidated tetrapeptide l-alanyl-d-isoglutaminyl-(l)-meso-diamino-pimelyl-(l)-d-alanine as a probe, there appears to exist in the membranes of Escherichia coli K12 strain 44 a dd-carboxypoptidase-transpeptidase system which does not recognize this peptide and a dd-carboxypoptidase-transpeptidase system which recognizes it. The dd-carboxypeptidase-endopeptidase system is essentially hydrolytic. It catalyzes the hydrolysis of UDP-N-acetyl-muramyl-pentapeptide into UDP-N-acetylmuramyl-tetrapeptide and the hydrolysis of the wall peptidoglycan peptide dimer into monomers. These activities are not inhibited by the glutamate-amidated tetrapeptide. The system may consist either of two enzyme proteins having predominantly carboxypeptidase activity and endopeptidase activity, respectively, or of one enzyme protein of which the functioning would depend upon the environmental conditions. The dd-carboxypeptidase-transpeptidase system (a) catalyzes concomitant hydrolysis (carboxypeptidase activity) and transfer (natural model transpeptidase activity) reactions with the pentapeptide l-alanyl-γ-d-glutamyl-(l)-meso-diaminopimelyl-(l)-d-alanyl-d-alanine. The transfer reaction leads to the synthesis of a dimer that is identical to the one which occurs in the E. coli wall peptidoglycan; (b) utilizes the glutamate-amidated tetrapeptide as an acceptor. Simultaneous exposure of the pentapeptide and the glutamate-amidated tetrapeptide to the enzyme system leads to the formation of an hybrid monoamidated peptide dimer and causes a decreased hydrolysis of the pentapeptide; (c) by virtue of its own carboxypeptidase activity, it appears to exert some endopeptidase activity. Both carboxypeptidase and endopeptidase activities of this system are inhibited by the glutamate-amidated tetrapeptide, but this represents only a small fraction of the total hydrolytic activity of the membrane Brij-36T extract. (d) The system catalyzes an unnatural model transpeptidation reaction in which glycine replaces d-alanine at the C-terminal position of the nucleotide UDP-N-acetylmuramyl-pentapeptide. This system may also consist either of two enzyme proteins having predominantly natural model transpeptidase activity and unnatural model transpeptidase activity, respectively, or of one enzyme protein of which the functioning would depend upon the environmental conditions. Whatever the exact situation, the E. colidd-carboxypeptidase-transpeptidase system is in many respects, similar to the dd-carboxy-peptidase-transpeptidase single polypeptide enzymes isolated from Streptomyces strains R39 and R61.