Evidence for C-Methylation Proceeding via a Cyclopropane Intermediate during Tabtoxin Biosynthesis

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.