Catalytic subunit of the queuine tRNA-ribosyltransferase (TGT) that catalyzes the base-exchange of a guanine (G) residue with queuine (Q) at position 34 (anticodon wobble position) in tRNAs with GU(N) anticodons (tRNA-Asp, -Asn, -His and -Tyr), resulting in the hypermodified nucleoside queuosine (7-(((4,5-cis-dihydroxy-2-cyclopenten-1-yl)amino)methyl)-7-deazaguanosine) (PubMed:19414587, PubMed:29862811). Catalysis occurs through a double-displacement mechanism. The nucleophile active site attacks the C1' of nucleotide 34 to detach the guanine base from the RNA, forming a covalent enzyme-RNA intermediate. The proton acceptor active site deprotonates the incoming queuine, allowing a nucleophilic attack on the C1' of the ribose to form the product (By similarity). guanosine(34) in tRNA + queuine = guanine + queuosine(34) in tRNA Heterodimer of a catalytic subunit QTRT1 and an accessory subunit QTRT2. Weakly associates with mitochondria, possibly via QTRT2. Expressed in brain, heart, kidney, liver, ling, skeletal muscle, spleen and testis. Belongs to the queuine tRNA-ribosyltransferase family. Truncated N-terminus.