The mechanism of thioamide formation by the YcfA-YcfC system in 6-thioguanine biosynthesis

6-thioguanine (6-TG) is a therapeutic medication for childhood acute lymphoblastic leukemia (ALL) and a potent antimicrobial agent. Its biosynthesis relies on the YcfA-YcfC system, yet the formation of its critical thioamide moiety remains incompletely understood. Here, we provide a detailed biochemical and structural characterization of YcfA, including apo and substrate-bound crystal structures, which reveal that substrate adenylation and L-cysteine addition are key initial steps in the reaction cascade. Cryo-electron microscopy (cryo-EM) and functional analyses highlight YcfA's assembly into a two-layered heptameric structure, essential for the enzymatic function. GTP serves a dual role as a substrate and oligomerization enhancer. Additionally, pyridoxal 5'-phosphate (PLP), a cofactor for YcfC, the partner enzyme in this system, promotes YcfA oligomerization but inhibits its activity by obstructing GTP binding. Biochemical and structural evidence confirms that YcfC acts as a C‒S lyase, which is essential for thioamide formation in the presence of PLP. Exploiting substrate flexibility, we synthesized a seleno analog with antimicrobial properties. Multi-omics analyses of the biosynthetic precursor underscore its potential as an antibiotic. Collectively, our findings unravel the distinct architecture and functionality of the YcfA-YcfC system, offering an evolutionary perspective on noncanonical thioamide biosynthesis and a foundation for synthetic biology applications in drug development.