Catalyzes the conversion of dethiobiotin (DTB) to biotin by the insertion of a sulfur atom into dethiobiotin via a radical-based mechanism. (4R,5S)-dethiobiotin + [sulfur carrier]-SH + 2 reduced [2Fe-2S]-[ferredoxin] + 2 S-adenosyl-L-methionine = 2 5'-deoxyadenosine + [sulfur carrier]-H + biotin + 2 L-methionine + 2 oxidized [2Fe-2S]-[ferredoxin] Binds 1 [4Fe-4S] cluster. The cluster is coordinated with 3 cysteines and an exchangeable S-adenosyl-L-methionine. Binds 1 [2Fe-2S] cluster. The cluster is coordinated with 3 cysteines and 1 arginine. Cofactor biosynthesis; biotin biosynthesis; biotin from 7,8-diaminononanoate: step 2/2. Homodimer. Belongs to the radical SAM superfamily. Biotin synthase family.