FIGURE 7.

Schematic and molecular illustration of the PGN structures. A, in S. aureus, the PGN subunit is composed of d-isoglutamine and l-lysine and is cross-linked via a pentaglycine bridge (43). It comprises C-terminally either d-Ala-d-Ala or d-Ala-pentaglycine. B, B. subtilis PGN from vegetative cells (53, 55), however, contains d-isoglutamic acid and amidated meso-DAP and lacks a d-alanine at the meso-DAP α-carboxyl group, and it is directly cross-linked with d-alanine of the next subunit. In addition, B. subtilis PGN has 1,6-anhydro-MurNAc, shown in gray, at its terminus instead of a reducing MurNAc.