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:11255023, PubMed:20354154). 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 kcat is 0.0056 sec(-1) with tRNA(Tyr) as substrate and 0.00586 sec(-1) with guanine as substrate. Heterodimer of a catalytic subunit QTRT1 and an accessory subunit QTRT2. Weakly associates with mitochondria, possibly via QTRT2. Belongs to the queuine tRNA-ribosyltransferase family.