The transglycosylase complex QTRT1:QTRTD1 exchanges guanine for queuine at nucleotide 34 of tRNA(Tyr) (Chen et al. 2010, Chen et al. 2011). The QTRT1 subunit is responsible for the transglycosylase activity. Eukaryotes are unable to synthesize queuine and must obtain it from dietary sources or symbiotic gut flora. As inferred from the mouse homologs, QTRT1:QTRTD1 associates with the outer mitochondrial membrane (Boland et al. 2009). The homologous enzyme in Escherichia coli is tgt.