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. Is essential for virulence in a mouse model of acute infection. 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. Is expressed at a level higher than other LDT paralogs at all phases of growth. Inactivation of this gene leads to altered colony morphology and growth, attenuation of persistence and increased susceptibility to amoxicillin-clavulanate both in vitro and in the mouse model during the chronic phase of infection. Loss of both ldtMt1 and ldtMt2 results in phenotypes that are unique and/or severe compared to the single mutants lacking only ldtMt1 or ldtMt2. The double gene deletion 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.