In its thiocarboxylated form (CysO-COSH), is the sulfur donor in the CysM-dependent cysteine biosynthetic pathway. Amino-acid biosynthesis; L-cysteine biosynthesis. Thiocarboxylated by MoeZ. A covalent CysO-cysteine adduct is formed by the action of CysM. The structure of the adduct seems to be first a S-(glycyl)-cysteinate which leads to a N-(glycyl)-cysteinate after a S->N acyl shift (By similarity). Belongs to the sulfur carrier protein CysO family.