Investigate Natural Product Indolmycin and the Synthetically\nImproved Analogue Toward Antimycobacterial Agents

Indolmycin\n(IND) is a microbial natural product that selectively\ninhibits bacterial tryptophanyl-tRNA synthetase (TrpRS). The tryptophan\nbiosynthesis pathway was recently shown to be an important target\nfor developing new antibacterial agents against Mycobacterium\ntuberculosis (Mtb). We investigated the antibacterial\nactivity of IND against several mycobacterial model strains. A TrpRS\nbiochemical assay was developed to analyze a library of synthetic\nIND analogues. The 4″-methylated IND compound, Y-13, showed\nimproved anti-Mtb activity with a minimum inhibitory concentration\n(MIC) of 1.88 μM (∼0.5 μg/mL). The MIC increased\nsignificantly when overexpression of TrpRS was induced in the genetically\nengineered surrogate M. bovis BCG.\nThe cocrystal structure of Mtb TrpRS complexed with IND and ATP has\nrevealed that the amino acid pocket is in a state between the open\nform of apo protein and the closed complex with the reaction intermediate.\nIn whole-cell-based experiments, we studied the combination effect\nof Y-13 paired with different antibacterial agents. We evaluated the\nkilling kinetics, the frequency of resistance to INDs, and the mode\nof resistance of IND-resistant mycobacteria by genome sequencing.\nThe synergistic interaction of Y-13 with the TrpE allosteric inhibitor,\nindole propionic acid, suggests that prospective IND analogues could\nshut down tryptophan biosynthesis and protein biosynthesis in pathogens,\nleading to a new class of antibiotics. Finally, we discuss a strategy\nto expand the genome mining of antibiotic-producing microbes specifically\nfor antimycobacterial development.