Involved in the post-transcriptional modification of the uridine at the wobble position (U34) of tRNA(Lys), tRNA(Glu) and tRNA(Gln) (PubMed:14594807, PubMed:22983156, PubMed:24971911, PubMed:29862510). Catalyzes the conversion of 2-thiouridine (S2U-RNA) to 2-selenouridine (Se2U-RNA) (PubMed:14594807, PubMed:24971911, PubMed:29862510). 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 (PubMed:24971911, PubMed:29862510). (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 kcat is 0.00014 min(-1) for the geranylation reaction. kcat is 0.53 min(-1) for the selenation reaction. Monomer. Disruption mutant is incapable of incorporating selenium into tRNA. Disruption does not affect 2 thiouridine generation (PubMed:14594807). Mutant cannot produce geranylated RNA (PubMed:22983156). Belongs to the SelU family.