Heterogeneity of newly inserted and preexisting murein in the sacculus of Escherichia coli.

In vivo studies of murein biosynthesis show that newly synthesized murein unexpectedly differs in its chemical composition from preexisting murein. New murein is loosely crosslinked with the preexisting sacculus; in a maturation process involving further transpeptidation, the final stage of crosslinkage in murein is achieved. Newly inserted murein initially carries pentapeptide subunits, which are the donor of the secondary transpeptidation reaction. In mutants with defective penicillin-binding protein 4 the secondary transpeptidation step is abolished. Uncoupling of the secondary transpeptidation reaction from crosslink formation during the initial insertion of new murein was also found in a mutant with a defect in lipoprotein biosynthesis. We conclude that the initial transpeptidation of murein and crosslink formation during the maturation of newly inserted murein are catalyzed by two different enzyme systems.