Is the sulfur donor in the synthesis of the thiazole phosphate moiety of thiamine phosphate. Cofactor biosynthesis; thiamine diphosphate biosynthesis. C-terminal thiocarboxylation occurs in 2 steps, it is first acyl-adenylated (-COAMP) by TtuC, then thiocarboxylated (-COSH) by the cysteine desulfurases IscS or SufS. Belongs to the sulfur carrier protein ThiS family.