Plays a central role in 2-thiolation of mcm(5)S(2)U at tRNA wobble positions of tRNA(Lys), tRNA(Glu) and tRNA(Gln). Directly binds tRNAs and probably acts by catalyzing adenylation of tRNAs, an intermediate required for 2-thiolation. It is unclear whether it acts as a sulfurtransferase that transfers sulfur from thiocarboxylated URM1 onto the uridine of tRNAs at wobble position. Prior mcm(5) tRNA modification by the elongator complex is required for 2-thiolation. May also be involved in protein urmylation. tRNA modification; 5-methoxycarbonylmethyl-2-thiouridine-tRNA biosynthesis. Interacts with NCS2 and URM1. May act by forming a heterodimer with NCS2. Component of a large molecular weight complex of more than 250 kDa. Belongs to the TtcA family. CTU1/NCS6/ATPBD3 subfamily.