Crystal structure of tRNA^His guanylyltransferase from Saccharomyces cerevisiae

tRNA maturation involves several steps, including processing, splicing, CCA addition, and posttranscriptional modifications. tRNA^His guanylyltransferase (Thg1) is the only enzyme known to catalyze templated nucleotide addition in the 3′–5′ direction, unlike other DNA and RNA polymerases. For a better understanding of its unique catalytic mechanism at the molecular level, we determined the crystal structure of GTP-bound Thg1 from Saccharomyces cerevisiae at the maximum resolution of 3.0 Å. The structure revealed the enzyme to have a tetrameric conformation that is well conserved among different species, and the GTP molecule was clearly bound at the active site, coordinating with two magnesium ions. In addition, two flexible protomers at the potential binding site (PBS) for tRNA^His were observed. We suggest that the PBS of the tetramer could also be one of the sites for interaction with partner proteins.