Figure 1.

(A) PG cross-linking modes associated with Ltds and PBPs. X represents the third position amino acid (either m-DAP or L-lysine-based amino acids). (B) A synthetic mimic of the stem peptide modified with a fluorescent handle (green hexagon) is covalently incorporated within growing PG scaffold. First, a terminal D-Ala residue is removed by Ltd, leading to a covalent intermediate. Second, this acyl-donor is captured by the third position amino acid within existing PG thus leading to its cross-linking with PG and generating a measurable fluorescent signal.