Changes in the composition of Escherichia coli murein as it ages during exponential growth

Escherichia coli murein was specifically labeled with [14C]diaminopimelic acid in the mutant strains W7 (dap lysA) and BUG6. Pulse-labeled heat-denatured E. coli cells were digested with 2 mg of egg-white lysozyme per ml to degrade the murein completely and free any lipoprotein-bound muropeptide trimers, dimers, and monomers. Pulse-chase experiments showed that the relative percentage of trimers and dimers found in the newly synthesized murein increased somewhat with time at the expense of monomers. The increase in cross-links indicated that the radioactive monomers served as acceptors in multisite transpeptidations occurring after the labeling period. The content of nonreducing monomers (C7 and C8) remained unaltered, indicating that the oligosaccharide chain length did not change with time. A gradual conversion of the reducing disaccharide tetrapeptide monomer to its tripeptide analog occurred during chasing. Braun lipoprotein was linked to about 2% of the murein subunits within 30 s of the incorporation of subunits into insoluble murein, and after one-half a generation of chase, lipoprotein-associated muropeptides had approached the maximum (16% of the total murein subunits). The distribution of muropeptides was similar in lipoprotein-linked and lipoprotein-free murein, showing that the enzyme that links Braun lipoprotein to murein does not discriminate between monomers, dimers, and trimers. No evidence for a chasable, soluble polymer of murein was found in our experiments. Hence, our data support the idea that new murein is incorporated directly into the sacculus without first existing as a soluble intermediate.