Pseudomonas aeruginosa is an opportunistic Gram-negative bacterial pathogen. A primary contributor to its ability to resist beta-lactam antibiotics is the expression, following detection of the beta-lactam, of the AmpC beta-lactamase. As AmpC expression is directly linked to the recycling of the peptidoglycan of the bacterial cell wall, an important question is the identity of the signaling molecule(s) in this relationship. One mechanism used by clinical strains to elevate AmpC expression is loss of function of penicillin-binding protein 4 (PBP4). As the mechanism of the beta-lactams is PBP inactivation, this result implies that the loss of the catalytic function of PBP4 ultimately leads to induction of antibiotic resistance. PBP4 is a bifunctional enzyme having both dd-carboxypeptidase and endopeptidase activities. Substrates for both the dd-carboxypeptidase and the 4,3-endopeptidase activities were prepared by multistep synthesis, and their turnover competence with respect to PBP4 was evaluated. The endopeptidase activity is specific to hydrolysis of 4,3-cross-linked peptidoglycan. PBP4 catalyzes both reactions equally well. When P. aeruginosa is grown in the presence of a strong inducer of AmpC, the quantities of both the stem pentapeptide (the substrate for the DD-carboxypeptidase activity) and the 4,3-cross-linked peptidoglycan (the substrate for the 4,3-endopeptidase activity) increase. In the presence of beta-lactam antibiotics these altered cell-wall segments enter into the muropeptide recycling pathway, the conduit connecting the sensing event in the periplasm and the unleashing of resistance mechanisms in the cytoplasm.