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Nearly all tRNA His  species have an additional 5′ guanine nucleotide (G −1 ). G −1  is encoded opposite C 73  in nearly all prokaryotes and in some archaea, and is added post-transcriptionally by tRNA His  guanylyltransferase (Thg1) opposite A 73  in eukaryotes, and opposite C 73  in other archaea. These divergent mechanisms of G −1  conservation suggest that G −1  might have an important cellular role, distinct from its role in tRNA His  charging. Thg1 is also highly conserved and is essential in the yeast  Saccharomyces cerevisiae . However, the essential roles of Thg1 are unclear since Thg1 also interacts with Orc2 of the origin recognition complex, is implicated in the cell cycle, and catalyzes an unusual template-dependent 3′–5′ (reverse) polymerization in vitro at the 5′ end of activated tRNAs. Here we show that  thg1 -Δ strains are viable, but only if histidyl-tRNA synthetase and tRNA His  are overproduced, demonstrating that the only essential role of Thg1 is its G −1  addition activity. Since these  thg1 -Δ strains have severe growth defects if cytoplasmic tRNA His  A 73  is overexpressed, and distinct, but milder growth defects, if tRNA His  C 73  is overexpressed, these results show that the tRNA His  G −1  residue is important, but not absolutely essential, despite its widespread conservation. We also show that Thg1 catalyzes 3′–5′ polymerization in vivo on tRNA His  C 73 , but not on tRNA His  A 73 , demonstrating that the 3′–5′ polymerase activity is pronounced enough to have a biological role, and suggesting that eukaryotes may have evolved to have cytoplasmic tRNA His  with A 73 , rather than C 73 , to prevent the possibility of 3′–5′ polymerization.

The requirement for the highly conserved G−1 residue of Saccharomyces cerevisiae tRNAHis can be circumvented by overexpression of tRNAHis and its synthetase