Natural product discovery in the genomic era requires validation and accurate assignment of enzyme function to identify novel biosynthetic pathways and molecules. We establish the biosynthetic logic for a rare C-methylation event at the C5-position of a lysine backbone during tabtoxin biosynthesis. The C-methylation is introduced via an unexpected cyclopropane ring formation and hydrolysis reaction sequence catalyzed by a novel SAM-dependent cyclopropane synthase (TblA) and a dual function cyclopropane hydrolase/N-acyltransferase (TabB) acting on a cyclic enamine substrate derived from l-2,3,4,5-tetrahydro-dipicolinic acid (THDPA). A new metabolic branch of lysine biosynthesis is established that bypasses the acylation dependent pathways and provides direct access to meso-diaminopimelate via reversible TabD-catalyzed transamination on THDPA, the branching metabolite for tabtoxin and lysine biosynthesis. Our findings fill a knowledge gap by explaining the mechanistic basis and the order of enzyme action during incorporation of a branching C1-unit that is required for construction of monobactams in the tabtoxin family of natural products which have promising antimicrobial and herbicidal activity.
Li et al. (2026) studied this question.