Plays a central role in 2-thiolation of mcm(5)S(2)U at tRNA wobble positions of cytosolic tRNA(Lys), tRNA(Glu) and tRNA(Gln). Also essential during biosynthesis of the molybdenum cofactor. Acts by mediating the C-terminal thiocarboxylation of sulfur carriers urm1 and mocs2a. Its N-terminus first activates urm1 and mocs2a as acyl-adenylates (-COAMP), then the persulfide sulfur on the catalytic cysteine is transferred to urm1 and mocs2a to form thiocarboxylation (-COSH) of their C-terminus. The reaction probably involves hydrogen sulfide that is generated from the persulfide intermediate and that acts as nucleophile towards urm1 and mocs2a. Subsequently, a transient disulfide bond is formed. Does not use thiosulfate as sulfur donor; nfs1 probably acting as a sulfur donor for thiocarboxylation reactions (By similarity). [molybdopterin-synthase sulfur-carrier protein]-C-terminal Gly-Gly + ATP + H(+) = [molybdopterin-synthase sulfur-carrier protein]-C-terminal Gly-Gly-AMP + diphosphate [molybdopterin-synthase sulfur-carrier protein]-C-terminal Gly-Gly-AMP + AH2 + S-sulfanyl-L-cysteinyl-[cysteine desulfurase] = [molybdopterin-synthase sulfur-carrier protein]-C-terminal Gly-NH-CH2-C(O)SH + A + AMP + H(+) + L-cysteinyl-[cysteine desulfurase] Binds 1 zinc ion per subunit. tRNA modification; 5-methoxycarbonylmethyl-2-thiouridine-tRNA biosynthesis. In the N-terminal section; belongs to the HesA/MoeB/ThiF family. UBA4 subfamily.