Biotin Synthase: Purification, Characterization as a [2Fe-2S] Cluster Protein, and in Vitro Activity of the Escherichia coli bioB Gene Product

We report here the first purification of the protein encoded by the Escherichia coli bioB gene. One species of this protein runs on native gels with an electrophoretic mobility typical of a protein with m = 82 kDa, suggesting the protein is a dimer (gene sequence predicts m = 38.7 kDa). There are two iron- and two acid-labile sulfur atoms per protein monomer. Solutions containing the protein are red and have an absorbance spectrum characteristic of proteins with [2Fe-2S] clusters. In its oxidized native state, the protein is EPR-silent. Upon addition of dithionite, the protein's UV-visible absorbance spectrum is very slowly bleached, and an EPR active species is produced that displays a signal at g_avg = 1.95. All these results are consistent with this protein containing one [2Fe-2S] cluster per monomer. The EPR spin quantitation is only 5–10% of expected. Since this protein loses iron upon reduction with dithionite, the low-spin quantitation is probably due to cluster instability in the reduced state. Another species of the bioB gene products has also been purified which runs on native gels with an electrophoretic mobility typical of a protein with m = 104 kDa. This species appears to be a dimer with one [2Fe-2S] cluster per dimer. The 104-kDa protein can be converted to the 82-kDa protein upon incubation with Fe³⁺ and S²⁻. The bioB gene product we have isolated is active in the conversion of dethiobiotin to biotin in vitro in the presence of NADPH, AdoMet, Fe³⁺ or Fe²⁺, and additional unidentified factors from the crude extracts of E. coli. The K_m for dethiobiotin in this reaction has been found to be 2 °M.