InterPro domain: IPR025845

Thg1 was originally characterised as synthesising the guanine nucleotide at the -1 position of the histidinyl tRNA (HtRNA). Thg1 has also been shown to have polymerase activity, which has been proposed to be the ancestral activity of this enzyme [ 1 , 2 ].

Thg1 polymerases contain an additional region of conservation C-terminal to the core palm domain that comprise of 5 helices and two strands [ 3 ]. This region has several well-conserved charged residues including a basic residue found towards the end of the first helix of this unit might contribute to the Thg1-specific active site [ 4 ]. This C-terminal module of Thg1 is predicted to form a helical bundle that functions equivalently to the fingers of the other nucleic acid polymerases, probably in interacting with the template HtRNA [ 4 ].

1. Template-dependent 3'-5' nucleotide addition is a shared feature of tRNAHis guanylyltransferase enzymes from multiple domains of life. Proc. Natl. Acad. Sci. U.S.A. 107, 674-9
2. tRNAHis guanylyltransferase catalyzes a 3'-5' polymerization reaction that is distinct from G-1 addition. Proc. Natl. Acad. Sci. U.S.A. 103, 8640-5
3. Presence of a classical RRM-fold palm domain in Thg1-type 3'- 5'nucleic acid polymerases and the origin of the GGDEF and CRISPR polymerase domains. Biol. Direct 5, 43