Generates 3->3 cross-links in peptidoglycan, catalyzing the cleavage of the mDap(3)-D-Ala(4) bond of a tetrapeptide donor stem and the formation of a bond between the carbonyl of mDap(3) of the donor stem and the side chain of mDap(3) of the acceptor stem. Is specific for donor substrates containing a stem tetrapeptide since it cannot use pentapeptide stems (By similarity). Is irreversibly inactivated by the beta-lactams carbapenems via the formation of a covalent adduct resulting from acylation of the catalytic Cys. Cell wall biogenesis; peptidoglycan biosynthesis. Monomer. Lack of this gene alone does not alter in vitro and in vivo growth rate or colony and cell shape and morphology. But loss of both LdtMt1 and LdtMt2 severely alters cellular shape, intracellular morphology, physiology and virulence: the length of mutant cells are shorter than wild-type, they have deep surface depressions and bulges, they possess large unstained vacuole-like structures, the thickness of the peptidoglycan layer is smaller, the protein localization is altered, and in vitro and in vivo growth and virulence are severely attenuated. Moreover, double-mutant cells are more sensitive to vancomycin and amoxicillin-clavulanate. Truncated N-terminus.