2.8.1.4	ATP + [ThiI sulfur-carrier protein]-S-sulfanyl-L-cysteine + uracil in tRNA + 2 reduced ferredoxin [iron-sulfur] cluster = AMP + diphosphate + 4-thiouracil in tRNA + [ThiI sulfur-carrier protein]-L-cysteine + 2 oxidized ferredoxin [iron-sulfur] cluster