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Cyclopropanation of unactivated olefinic bonds via addition of a reactive one-carbon species is well developed in synthetic chemistry, whereas natural cyclopropane biosynthesis employing this strategy is very limited. Here, we identify a two-component cyclopropanase system, composed of a HemN-like radical  S -adenosyl- l -methionine (SAM) enzyme C10P and a methyltransferase C10Q, catalyzes chemically challenging cyclopropanation in the antitumor antibiotic CC-1065 biosynthesis. C10P uses its [4Fe-4S] cluster for reductive cleavage of the first SAM to yield a highly reactive 5′-deoxyadenosyl radical, which abstracts a hydrogen from the second SAM to produce a SAM methylene radical that adds to an  sp   2  -hybridized carbon of substrate to form a SAM-substrate adduct. C10Q converts this adduct to CC-1065 via an intramolecular S N 2 cyclization mechanism with elimination of  S -adenosylhomocysteine. This cyclopropanation strategy not only expands the enzymatic reactions catalyzed by the radical SAM enzymes and methyltransferases, but also sheds light on previously unnoticed aspects of the versatile SAM-based biochemistry.

A radical S-adenosyl-L-methionine enzyme and a methyltransferase catalyze cyclopropane formation in natural product biosynthesis