Caerulomycin and Collismycin Antibiotics Share a trans Flavin-Dependent Assembly Line for 2,2-Bipyridine Formation and Sulfur Fate Differentiation

Linear nonribosomal peptide synthetases (NRPSs) and polyketide synthases (PKSs) template the modular biosynthesis of numerous nonribosomal peptides, polyketides and their hybrids though assembly line chemistry. This chemistry can be complex and highly varied, and thus challenges the understanding in the diverse polymerization processes of amino acid and carboxylate monomers programmed by various NRPSs and PKSs in nature. Here, we report that caerulomycin and collismycin peptide-polyketide hybrid antibiotics share an unusual assembly line that involves NRPS activity to recruit a flavoprotein acting in trans and catalyze C-C bond formation and heterocyclization during 2,2-bipyridine formation. Simultaneously, this assembly line provides dethiolated and thiolated 2,2-bipyridine intermediates through differential treatment of the sulfhydryl group arising from L-cysteine incorporation. Subsequent L-leucine extension, which does not contribute any atoms to either caerulomycins or collismycins, plays a key role in sulfur fate determination by selectively advancing one of the two 2,2-bipyridine intermediates down a path to the final products with or without sulfur decoration. These findings further the appreciation of assembly line chemistry and will facilitate the development of related molecules using synthetic biology approaches.