Properties of cell wall peptidoglycan synthesized by amino acid deprived relA mutants of Escherichia coli

Cell wall peptidoglycan synthesis in Escherichia coli is under stringent control. During amino acid deprivation, peptidoglycan synthesis is inhibited in re1A+ bacteria but not in re1A mutants. The relaxed synthesis of peptidoglycan by amino acid deprived re1A bacteria was inhibited by several beta-lactam antibiotics at concentrations which inhibited cell elongation in growing cultures suggesting that the transpeptidase activity of penicillin-binding protein (PBP-1B) was involved in this process. Structural studies on the peptidoglycan also indicated the involvement of transpeptidation in relaxed peptidoglycan synthesis. The peptidoglycan synthesized during amino acid deprivation was cross-linked to the existing cell wall peptidoglycan, and the degree of cross-linkage was the same as that of peptidoglycan synthesized by growing control cells. The relaxed synthesis of peptidoglycan was also inhibited by moenomycin, an inhibitor of the in vitro transglycosylase activities of PBPs, but the interpretation of this result depends on whether the transglycosylases are the sole targets of moenomycin in vivo. Most of the peptidoglycan lipoprotein synthesized by histidine-deprived re1A+ bacteria was in the free form as previously reported, possibly because of the restriction in peptidoglycan synthesis. In support of this proposal, most of the lipoprotein synthesized during histidine deprivation of re1A mutants was found to be covalently linked to peptidoglycan. Nevertheless, the peptidoglycan synthesized by amino acid deprived re1A bacteria was apparently deficient in bound lipoprotein as compared with peptidoglycan synthesized by normal growing control bacteria suggesting that the rate of lipoprotein synthesis during amino acid deprivation may be limiting.