tRNA-guanine transglycosylase (TGT) is involved in the post-transcriptional modification of tRNA.  TGT is involved in the replacement of guanine-34 by queuine precursor (preQ1) in the anticodon of tRNA.  In archaebacteria TGT (ATGT) catalyzes the introduction of archaeosin precursor (preQ0) into the D-loop of tRNA.