Involved in FAD and FMN biosynthesis. Catalyzes the 2-thiolation of uridine at the wobble position (U34) of tRNA, leading to the formation of s(2)U34. ATP + riboflavin = ADP + FMN + H(+) ATP + FMN + H(+) = diphosphate + FAD AH2 + ATP + S-sulfanyl-L-cysteinyl-[protein] + uridine(34) in tRNA = 2-thiouridine(34) in tRNA + A + AMP + diphosphate + H(+) + L-cysteinyl-[protein] Cofactor biosynthesis; FAD biosynthesis; FAD from FMN: step 1/1. Cofactor biosynthesis; FMN biosynthesis; FMN from riboflavin (ATP route): step 1/1. In the N-terminal section; belongs to the RibF family. In the C-terminal section; belongs to the MnmA/TRMU family.