This gene encodes the catalytic subunit of tRNA-guanine transglycosylase. tRNA-guanine transglycosylase is a heterodimeric enzyme complex that plays a critical role in tRNA modification by synthesizing the 7-deazaguanosine queuosine, which is found in tRNAs that code for asparagine, aspartic acid, histidine and tyrosine. A pseudogene of this gene is located on the long arm of chromosome X.