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 thought to play a role in adaptation to the nonreplicative state of M.tuberculosis. Is irreversibly inactivated by the beta-lactams carbapenems via the formation of a covalent adduct resulting from acylation of the catalytic Cys; ertapenem and imipenem are the most efficient drugs for in vitro LdtMt1 inactivation. Cephalosporins (cefotaxime, cephalothin, and ceftriaxone) also form covalent adducts with LdtMt1, although the acylation reaction was 7- to 1,000-fold slower and leads to elimination of one of the drug side chains. A high drug concentration (360 uM) of ceftriaxone is required for full inhibition of enzyme activity. Is not inhibited by ampicillin. Cell wall biogenesis; peptidoglycan biosynthesis. Monomer. Up-regulated 17-fold under nutrient starvation. Consists of two domains connected by a short loop, which form a bent shape. The N-terminal domain (residues 32-122) resembles a c-type immunoglobulin (Ig) domain, and the C-terminal contains the catalytic site that is located in a tiny tunnel. The peptidoglycan structure of stationary-phase M.tuberculosis is atypical since it contains a majority (80%) of 3->3 cross-links synthesized by L,D-transpeptidases that predominate over the 4->3 cross-links formed by the D,D-transpeptidase activity of classical penicillin-binding proteins (PubMed:18408028). In fact, 3-3 cross-linkages predominate throughout all growth phases and the ratio of 4-3/3-3 linkages does not vary significantly under any growth condition (PubMed:22906310).