Biotin synthase catalyses the last step of the biosynthesis of biotin in mi
croorganisms and plants. The active protein isolated from Bacillus sphaeric
us and Escherichia coli contains an iron-sulphur (FeS) cluster. The native
enzymes mere depleted of their iron and inorganic sulphide and the resultin
g apoenzymes mere chemically reconstituted with FeCl3 and Na-2[S-34] to giv
e labelled ((FeS)-S-34) enzymes. These enzymes were functional and when ass
ayed in vitro produced labelled biotin containing about 65% of S-34. These
data strongly support the hypothesis that the sulphur of biotin is derived
from the (FeS) centre of the enzyme. (C) 1998 Federation of European Bioche
mical Societies.