Catalyzes the ATP-dependent transfer of a sulfur to tRNA to produce 4-thiouridine in position 8 of tRNAs, which functions as a near-UV photosensor. Also catalyzes the transfer of sulfur to the sulfur carrier protein ThiS, forming ThiS-thiocarboxylate. This is a step in the synthesis of thiazole, in the thiamine biosynthesis pathway. The sulfur is donated as persulfide by IscS (By similarity). [ThiI sulfur-carrier protein]-S-sulfanyl-L-cysteine + a uridine in tRNA + ATP + H(+) + 2 reduced [2Fe-2S]-[ferredoxin] = [ThiI sulfur-carrier protein]-L-cysteine + a 4-thiouridine in tRNA + AMP + diphosphate + 2 oxidized [2Fe-2S]-[ferredoxin] [sulfur-carrier protein ThiS]-C-terminal Gly-Gly-AMP + AH2 + S-sulfanyl-L-cysteinyl-[cysteine desulfurase] = [sulfur-carrier protein ThiS]-C-terminal Gly-NH-CH2-C(O)SH + A + AMP + H(+) + L-cysteinyl-[cysteine desulfurase] Cofactor biosynthesis; thiamine diphosphate biosynthesis. Belongs to the ThiI family.