THE GLUTAMATE RACEMASE ACTIVITY FROM ESCHERICHIA-COLI IS REGULATED BYPEPTIDOGLYCAN PRECURSOR UDP-N-ACETYLMURAMOYL-L-ALANINE

The murI gene product of Escherichia coli was recently identified as t he glutamate racemase activity which catalyzes the formation of D-glut amic acid, one of the essential components of bacterial cell-wall pept idoglycan [Doublet et al. (1993) J. Bacteriol. 175, 2970-2979]. We her e describe the purification to homogeneity and the kinetic properties of this enzyme. In vitro, the glutamate racemase activity shows an abs olute 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 af finity of the enzyme for this activator is particularly high (K-D = 4 mu M) and specific, since no other peptidoglycan precursor from UDP-Gl cNAc to UDP-MurNAc-pentapeptide is an effector. Minor chemical modific ations 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 r egulates the activity of the glutamate racemase in vivo. It adjusts th e formation of D-glutamic acid to the requirements of peptidoglycan sy nthesis and avoids an excessive racemization of the intracellular pool of L-glutamic acid.