THIAMIN BIOSYNTHESIS IN ESCHERICHIA-COLI - IDENTIFICATION OF THIS THIOCARBOXYLATE AS THE IMMEDIATE SULFUR DONOR IN THE THIAZOLE FORMATION

ThiFSGH and ThiI are required for the biosynthesis of the thiazole moi ety of thiamin in Escherichia coli. The overproduction, purification, and characterization of ThiFS and the identification of two of the ear ly steps in the biosynthesis of the thiazole moiety of thiamin are des cribed here. ThiS isolated from E. coli thiI(+) is post-translationall y modified by converting the carboxylic acid group of the carboxyl-ter minal glycine into a thiocarboxylate. The thiI gene plays an essential role in the formation of the thiocarboxylate because ThiS isolated fr om 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 sulfu r transfer from cysteine or from a cysteine-derived sulfur donor.