FIG. 1.

Peptidoglycan fragments and modifications. (A) Unmodified MurNAc-GlcNAc disaccharide residues are depicted with stem peptide structures, the cross-linking bridge (CL), and a disaccharide fragment from the adjacent glycan strand. The 1,6-anhydro-MurNAc residues are generated following cleavage by lytic transglycosylases, and this is depicted on one of the MurNAc residues. The structure depicted represents a fragment from a Gram-negative organism which has meso-DAP in the third residue of the stem peptide, while most Gram-positive organisms have an l-Lys at this position. (B) Nod1 and Nod2 ligands, shown with boxes around the minimal motifs sensed by these proteins, depicted on an unmodified, Gram-negative MurNAc residue. The three modifications to the glycan backbone discussed in the text are shown in the lower portion of the panel.