Biosynthesis of the thiazole moiety of thiamin in Escherichia coli: Identification of an acyldisulfide-linked protein-protein conjugate that is functionally analogous to the ubiquitin/E1 complex

A covalently linked protein-protein conjugate between ThiF and ThiS thiocar boxylate was found in a partially purified coexpressed ThiF/ThiS protein mi xture by using Fourier transform mass spectrometry. The Cys-184 of ThiF and the C terminus of ThiS thiocarboxylate were identified to be involved in t he formation of this complex by using both mutagenesis and chemical modific ation methods. A complementation study of Escherichia coli thiF(-) using th iF(C184S) suggests that this conjugate is an essential intermediate involve d in the biosynthesis of the thiazole moiety of thiamin. This ThiF/ThiS con jugate is the first characterized example of a unique acyldisulfide interme diate in a biosynthetic system, This protein conjugate is also an example o f an ubiquitin-E1 like protein-protein conjugate in prokaryotes and support s a strong evolutionary link between thiamin biosynthesis and the ubiquitin conjugating system.