ThiFSGH and ThiI are required for the biosynthesis of the thiazole moiety of thiamin in Escherichia coli. The overproduction, purification, and characterization of ThiFS and the identification of two of the early steps in the biosynthesis of the thiazole moiety of thiamin are described here. ThiS isolated from E. coli thiI+ is post-translationally modified by converting the carboxylic acid group of the carboxyl-terminal glycine into a thiocarboxylate. The thiI gene plays an essential role in the formation of the thiocarboxylate because ThiS isolated from a thiI- strain does not contain this modification. ThiF catalyzes the adenylation by ATP of the carboxyl-terminal glycine of ThiS. This reaction is likely to be involved in the activation of ThiS for sulfur transfer from cysteine or from a cysteine-derived sulfur donor.