THE MURI GENE OF ESCHERICHIA-COLI IS AN ESSENTIAL GENE THAT ENCODES AGLUTAMATE RACEMASE ACTIVITY

The murI gene of Escherichia coli was recently identified on the basis of its ability to complement the only mutant requiring D-glutamic aci d for growth that had been described to date: strain WM335 of E. coli B/r (P. Doublet, J. van Heijenoort, and D. Mengin-Lecreulx, J. Bacteri ol. 174:5772-5779, 1992). We report experiments of insertional mutagen esis of the murI gene which demonstrate that this gene is essential fo r the biosynthesis of D-glutamic acid, one of the specific components of cell wall peptidoglycan. A special strategy was used for the constr uction of strains with a disrupted copy of murI, because of a limited capability of E. coli strains grown in rich medium to internalize D-gl utamic acid. The murI gene product was overproduced and identified as a glutamate racemase activity. UDP-N-acetylmuramoyl-L-alanine (UDP-Mur NAc-L-Ala), which is the nucleotide substrate of the D-glutamic-acid-a dding enzyme (the murD gene product) catalyzing the subsequent step in the pathway for peptidoglycan synthesis, appears to be an effector of the racemase activity.