A Repurposed S-Adenosyl-l-Methionine Synthetase Mediates Cytidyl-Cyclitol Natural Product Assembly

Nucleoside natural products exhibit diverse chemical architectures and potent biological activities, yet the biosynthetic strategies that generate their structural diversity remain incompletely understood. Here, we elucidate the early stage biosynthetic pathway of the cytidyl-cyclitol natural product K-563 and its derivatives. The cyclitol component is generated by the myo-inositol-1-phosphate synthase (MIPS) family enzyme KesM and is subsequently coupled to the cytidine moiety by KesL using cytidine 5'-triphosphate (CTP), followed by further modifications catalyzed by the phosphatase KesI and the unique dehydrogenase complex KesJ/KesK. The X-ray crystal structure and mutagenesis analyses reveal that KesL adopts the canonical fold of S-adenosyl-l-methionine (SAM) synthetases, which catalyze the adenosylation of l-methionine with adenosine 5'-triphosphate (ATP) in primary metabolism, while the KesL active site is extensively remodeled to accept the cyclitol phosphate with CTP as an atypical substrate pair, thereby generating the cytidyl-cyclitol core structure. This work not only expands the chemical logic of nucleoside biosynthesis but also demonstrates how the SAM synthetase-like protein scaffold is repurposed to catalyze an unusual nucleoside transfer reaction for specialized secondary metabolite assembly.