Catalyzes the radical-mediated insertion of two sulfur atoms into the C-6 and C-8 positions of the octanoyl moiety bound to the lipoyl domains of lipoate-dependent enzymes, thereby converting the octanoylated domains into lipoylated derivatives (PubMed:9808738, PubMed:24872381) (By similarity). Together with LIP2 is essential for mitochondrial protein lipoylation during seed development. Required for the lipoylation of mitochondrial pyruvate dehydrogenase component E2 proteins in leaves and roots (PubMed:24872381). [[Fe-S] cluster scaffold protein carrying a second [4Fe-4S](2+) cluster] + 4 H(+) + N(6)-octanoyl-L-lysyl-[protein] + 2 oxidized [2Fe-2S]-[ferredoxin] + 2 S-adenosyl-L-methionine = (R)-N(6)-dihydrolipoyl-L-lysyl-[protein] + 2 5'-deoxyadenosine + [[Fe-S] cluster scaffold protein] + 4 Fe(3+) + 2 hydrogen sulfide + 2 L-methionine + 2 reduced [2Fe-2S]-[ferredoxin] Binds 2 [4Fe-4S] clusters per subunit. One cluster is coordinated with 3 cysteines and an exchangeable S-adenosyl-L-methionine. Protein modification; protein lipoylation via endogenous pathway; protein N(6)-(lipoyl)lysine from octanoyl-[acyl-carrier-protein]: step 2/2. Expressed in leaves and flowers, but not in roots (PubMed:9808738). Expressed in roots, rosette leaves, cauline leaves, stems, flowers and siliques (PubMed:24872381). Embryonic lethality. Belongs to the radical SAM superfamily. Lipoyl synthase family.