Kinetic Analysis of 3′–5′ Nucleotide Addition Catalyzed by Eukaryotic tRNA^His Guanylyltransferase

The tRNA^His guanylyltransferase (Thg1) catalyzes the incorporation of a single guanosine residue at the −1 position (G_–1) of tRNA^His, using an unusual 3′–5′ nucleotidyl transfer reaction. Thg1 and Thg1 orthologs known as Thg1-like proteins (TLPs), which catalyze tRNA repair and editing, are the only known enzymes that add nucleotides in the 3′–5′ direction. Thg1 enzymes share no identifiable sequence similarity with any other known enzyme family that could be used to suggest the mechanism for catalysis of the unusual 3′–5′ addition reaction. The high-resolution crystal structure of human Thg1 revealed remarkable structural similarity between canonical DNA/RNA polymerases and eukaryotic Thg1; nevertheless, questions regarding the molecular mechanism of 3′–5′ nucleotide addition remain. Here, we use transient kinetics to measure the pseudo-first-order forward rate constants for the three steps of the G_–1 addition reaction catalyzed by yeast Thg1: adenylylation of the 5′ end of the tRNA (<i>k</i>_aden), nucleotidyl transfer (<i>k</i>_ntrans), and removal of pyrophosphate from the G_–1-containing tRNA (<i>k</i>_ppase). This kinetic framework, in conjunction with the crystal structure of nucleotide-bound Thg1, suggests a likely role for two-metal ion chemistry in all three chemical steps of the G_–1 addition reaction. Furthermore, we have identified additional residues (K44 and N161) involved in adenylylation and three positively charged residues (R27, K96, and R133) that participate primarily in the nucleotidyl transfer step of the reaction. These data provide a foundation for understanding the mechanism of 3′–5′ nucleotide addition in tRNA^His maturation