Fig. S2.

Proposed pathway of carbide insertion. Carbide insertion begins with the transfer of a methyl group from one equivalent of SAM to the K cluster and the abstraction of a hydrogen atom from this methyl group by a 5-dA• radical that is generated upon homolytic cleavage of a second equivalent of SAM. The process continues with further deprotonation of the carbon-derived radical species concomitant with its incorporation into the L cluster as an interstitial carbide ion. Atoms of the clusters are colored as follows: Fe, orange; S, yellow; C, gray. The assembly proteins (Nif) housing these events are indicated.