tRNA His guanylyltransferase catalyzes a 3′-5′ polymerization reaction that is distinct from G −1 addition

Yeast tRNAHis guanylyltransferase, Thg1, is an essential protein that adds a single guanine to the 5′ end (G−1 ) of tRNAHis . This G−1 residue is required for aminoacylation of tRNAHis by histidyl-tRNA synthetase, bothin vitro andin vivo. The guanine nucleotide addition reaction catalyzed by Thg1 extends the polynucleotide chain in the reverse (3′-5′) direction of other known polymerases, albeit by one nucleotide. Here, we show that alteration of the 3′ end of the Thg1 substrate tRNAHis unleashes an unexpected reverse polymerase activity of wild-type Thg1, resulting in the 3′-5′ addition of multiple nucleotides to the tRNA, with efficiency comparable to the G−1 addition reaction. The addition of G−1 forms a mismatched G·A base pair at the 5′ end of tRNAHis , and, with monophosphorylated tRNA substrates, it is absolutely specific for tRNAHis . By contrast, reverse polymerization forms multiple G·C or C·G base pairs, and, with preactivated tRNA species, it can initiate at positions other than −1 and is not specific for tRNAHis . Thus, wild-type Thg1 catalyzes a templated polymerization reaction acting in the reverse direction of that of canonical DNA and RNA polymerases. Surprisingly, Thg1 can also readily use dNTPs for nucleotide addition. These results suggest that 3′-5′ polymerization represents either an uncharacterized role for Thg1 in RNA or DNA repair or metabolism, or it may be a remnant of an earlier catalytic strategy used in nature.