A novel two-gene requirement for the octanoyltransfer reaction of Bacillus subtilis lipoic acid biosynthesis

The Bacillus subtilis genome encodes three apparent lipoyl ligase homologues: yhfJ, yqhM and ywfL, which we have renamed lplJ, lipM and lipL respectively. We show that LplJ encodes the sole lipoyl ligase of this bacterium. Physiological and biochemical characterization of a ΔlipM strain showed that LipM is absolutely required for the endogenous lipoylation of all lipoate-dependent proteins, confirming its role as the B. subtilis octanoyltransferase. However, we also report that in contrast to Escherichia coli, B. subtilis requires a third protein for lipoic acid assembly, LipL. B. subtilis ΔlipL strains are unable to synthesize lipoic acid despite the presence of LipM and the sulphur insertion enzyme, LipA, which should suffice for lipoic acid biosynthesis based on the E. coli model. LipM is only required for the endogenous lipoylation pathway, whereas LipL also plays a role in lipoic acid scavenging. Expression of E. coli lipB allows growth of B. subtilisΔlipL or ΔlipM strains in the absence of supplements. In contrast, growth of an E. coliΔlipB strain can be complemented with lipM, but not lipL. These data together with those of the companion article provide evidence that LipM and LipL catalyse sequential reactions in a novel pathway for lipoic acid biosynthesis.