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