Catalyzes cross-linking of the peptidoglycan cell wall at the division septum (By similarity). Binds penicillin (PubMed:9045804). Preferential cleavage: (Ac)2-L-Lys-D-Ala-|-D-Ala. Also transpeptidation of peptidyl-alanyl moieties that are N-acyl substituents of D-alanine. Cell wall biogenesis; peptidoglycan biosynthesis. Produced in the stationary phase of growth. Inactivation causes no changes in the cell morphology or growth rate of exponentially growing cells. Belongs to the transpeptidase family. FtsI subfamily.