Thiamine pyrophosphate 1 is an essential cofactor in all living systems(1). Its biosynthesis involves the separate syntheses of the pyrimidine 2 and thiazole 3 precursors, which are then coupled(2). Two biosynthetic routes to the thiamine thiazole have been identified. In prokaryotes, five enzymes act on three substrates to produce the thiazole via a complex oxidative condensation reaction, the mechanistic details of which are now well established(2-6). In contrast, only one gene product is involved in thiazole biosynthesis in eukaryotes (THI4p in Saccharomyces cerevisiae)(7). Here we report the preparation of fully active recombinant wild-type THI4p, the identification of an iron-dependent sulphide transfer reaction from a conserved cysteine residue of the protein to a reaction intermediate and the demonstration that THI4p is a suicide enzyme undergoing only a single turnover.