Catalyzes cross-linking of the peptidoglycan cell wall (PubMed:3009484). Responsible for the determination of the rod shape of the cell (PubMed:1103132). Is probably required for lateral peptidoglycan synthesis and maintenance of the correct diameter during lateral and centripetal growth (PubMed:12519203). Preferential cleavage: (Ac)2-L-Lys-D-Ala-|-D-Ala. Also transpeptidation of peptidyl-alanyl moieties that are N-acyl substituents of D-alanine. Inhibited by mecillinam and benzylpenicillin. Cell wall biogenesis; peptidoglycan biosynthesis. Localizes preferentially in the lateral wall and at mid-cell in comparison with the old cell pole. Localization at mid-cell is dependent on active FtsI. Temperature-sensitive mutants grow as normal rods at 30 degrees Celsius but grow and divide as cocci during prolonged culturing at 42 degrees Celsius. Belongs to the transpeptidase family. MrdA subfamily.