The radical S-adenosylmethionine (S-AdoMet) superfamily contains thousands of proteins that catalyze highly diverse conversions, most of which are poorly understood, owing to a lack of information regarding chemical products and radical-dependent transformations. We here report that NosL, involved in forming the indole side ring of the thiopeptide nosiheptide (NOS), is a radical S-AdoMet 3-methyl-2-indolic acid (MIA) synthase. NosL catalyzed an unprecedented carbon chain reconstitution of L-tryptophan to give MIA, showing removal of the Ca-N unit and shift of the carboxylate to the indole ring. Dissection of the enzymatic process upon the identification of products and a putative glycyl intermediate uncovered a radical-mediated, unusual fragmentation-recombination reaction. This finding unveiled a key step in radical S-AdoMet enzyme–catalyzed structural rearrangements during complex biotransformations. Additionally, NosL tolerated fluorinated L-tryptophan as the substrate, allowing for production of a regiospecifically halogenated thiopeptide that has not been found among the more than 80 members of the naturally occurring thiopeptide family.

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