Involved in the post-transcriptional modification of the uridine at the wobble position (U34) of tRNA(Lys), tRNA(Glu) and tRNA(Gln). Catalyzes the conversion of 2-thiouridine (S2U-RNA) to 2-selenouridine (Se2U-RNA). Acts in a two-step process involving geranylation of 2-thiouridine (S2U) to S-geranyl-2-thiouridine (geS2U) and subsequent selenation of the latter derivative to 2-selenouridine (Se2U) in the tRNA chain. (2E)-geranyl diphosphate + 5-methylaminomethyl-2-thiouridine(34) in tRNA + H(+) + H2O + selenophosphate = (2E)-thiogeraniol + 5-methylaminomethyl-2-selenouridine(34) in tRNA + diphosphate + phosphate (2E)-geranyl diphosphate + 5-methylaminomethyl-2-thiouridine(34) in tRNA = 5-methylaminomethyl-S-(2E)-geranyl-thiouridine(34) in tRNA + diphosphate 5-methylaminomethyl-S-(2E)-geranyl-thiouridine(34) in tRNA + H(+) + selenophosphate = (2E)-thiogeraniol + 5-methylaminomethyl-2-(Se-phospho)selenouridine(34) in tRNA 5-methylaminomethyl-2-(Se-phospho)selenouridine(34) in tRNA + H2O = 5-methylaminomethyl-2-selenouridine(34) in tRNA + phosphate Monomer. Belongs to the SelU family.