The Thg1 protein from Saccharomyces cerevisiae is responsible for adding a GMP residue to the 5' end of tRNA His [1]. The catalytic domain Thg1 contains a RRM (ferredoxin) fold palm domain, just like the viral RNA-dependent RNA polymerases, reverse transcriptases, family A and B DNA polymerases, adenylyl cyclases, diguanylate cyclases (GGDEF domain) and the predicted polymerase of the CRISPR system [2]. Thg1 possesses an active site with three acidic residues that chelate Mg++ cations [2]. Thg1 catalyzes polymerization similar to the 5'-3' polymerases [2][3].

1: tRNAHis maturation: An essential yeast protein catalyzes addition of a guanine nucleotide to the 5' end of tRNAHis. Gu W, Jackman JE, Lohan AJ, Gray MW, Phizicky EM; 17;0:0-0. PMID:14633974

2: 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. Anantharaman V, Iyer LM, Aravind L; Biol Direct. 2010;5:43. PMID:20591188

3: tRNA(His) guanylyltransferase (THG1), a unique 3'-5' nucleotidyl transferase, shares unexpected structural homology with canonical 5'-3' DNA polymerases. Hyde SJ, Eckenroth BE, Smith BA, Eberley WA, Heintz NH, Jackman JE, Doublie S; Proc Natl Acad Sci U S A. 2010;107:20305-20310. PMID:21059936