The thioquinolobactin siderophore from Pseudomonas fluorescens ATCC 17400 utilizes a variation of the sulfur transfer chemistry found in thiamine and molydobterin biosynthesis. A JAMM motif protein cleaves the C-terminal amino acid residues following a diglycine moiety on a small sulfur carrier protein, and the modified C terminus is activated and sulfurylated, forming a thiocarboxylate.