tRNA(His) guanylyltransferase (, histidine tRNA guanylyltransferase, Thg1p, Thg1) is an enzyme with systematic name p-tRNA(His):GTP guanylyltransferase (ATP-hydrolysing). This enzyme catalyses the following chemical reaction
The enzyme requires a divalent cation for activity.
Thg1 enzymes are the only nucleic acid polymerases known to be capable of catalyzing nucleotide addition in the 3â²âÂÂ5â² direction.