Catalyzes the 2-thiolation of uridine at the wobble position (U34) of tRNA(Lys), tRNA(Glu) and tRNA(Gln), leading to the formation of s(2)U34, the first step of tRNA-mnm(5)s(2)U34 synthesis. Sulfur is provided by IscS, via a sulfur-relay system. Binds ATP and its substrate tRNAs. AH2 + ATP + S-sulfanyl-L-cysteinyl-[protein] + uridine(34) in tRNA = 2-thiouridine(34) in tRNA + A + AMP + diphosphate + H(+) + L-cysteinyl-[protein] Interacts with TusE. Belongs to the MnmA/TRMU family.