The murI gene product of Escherichia coli was recently identified as the glutamate racemase activity which catalyzes the formation of D-glutamic acid, one of the essential components of bacterial cell-wall peptidoglycan [Doublet et al. (1993) J. Bacteriol. 175, 2970-2979]. We here describe the purification to homogeneity and the kinetic properties of this enzyme. In vitro, the glutamate racemase activity shows an absolute requirement for UDP-N-acetylmuramoyl-L-alanine (UDP-MurNAc-L-Ala), the substrate of the D-glutamic acid-adding enzyme which catalyzes the subsequent step in the pathway for peptidoglycan synthesis. The affinity of the enzyme for this activator is particularly high (KD = 4 microM) and specific, since no other peptidoglycan precursor from UDP-GlcNAc to UDP-MurNAc-pentapeptide is an effector. Minor chemical modifications of the UDP-MurNAc-L-Ala molecule, such as the reduction of the uracyl moiety, suppress its activating effect. This specific in vitro requirement most likely represents the physiological mechanism which regulates the activity of the glutamate racemase in vivo. It adjusts the formation of D-glutamic acid to the requirements of peptidoglycan synthesis and avoids an excessive racemization of the intracellular pool of L-glutamic acid.