Soluble nascent peptidoglycan in growing Escherichia coli cells.

Two homologous strains of Escherichia coli, one of which completely lacked the cell envelope Braun's lipoprotein, were compared with respect to their peptidoglycan synthesis and assembly. Both strains were auxotrophic for diaminopimelic acid and their uptake of radiolabeled diaminopimelic acid was comparable. Analysis of subcellular fractions obtained after mechanical disruption of the cells in a French pressure cell and sedimentation of the cell envelopes showed the existence of a soluble, chromatographically immobile macromolecular peptidoglycan. This labeled peptidoglycan contained a reduced degree of peptide side chain cross-linkages (19 mol % of labeled residues as compared to that present in the insoluble cell sacculus, 27 mol %). In addition, approximately 20% of its peptide side chains terminated in pentapeptide structures versus 1 to 4% in the sacculus. Furthermore, the soluble peptidoglycan of the parent strain also contained covalently bound lipoprotein (4.6%). Extraction of the cell envelope fraction with detergents afforded an additional amount of soluble peptidoglycan. This material was quite similar, in its degree of cross-linkage and amount of covalently bound lipoprotein, to the peptidoglycan present in the detergent-insoluble sacculus. These results indicate that peptidoglycan strands which are, in part, covalently linked to lipoprotein are late stage synthesis intermediates which subsequently become covalently attached to the preexisting sacculus.