tRNAHis-guanylyltransferase establishes tRNAHis identity

Area of Science:

• Molecular Biology
• Biochemistry
• RNA Biology

Background:

• Histidine transfer RNA (tRNA) possesses a unique 5' terminal guanosine residue (G(-1)).
• This G(-1) is a critical identity element for recognition by histidyl-tRNA synthetase, ensuring accurate His-tRNA(His) formation.
• The addition of G(-1) is a post-transcriptional maturation step mediated by tRNA(His) guanylyltransferase (Thg1).

Purpose of the Study:

• To biochemically, structurally, and phylogenetically characterize Thg1 homologs across life's domains.
• To understand the evolutionary origins and functional diversification of Thg1 enzymes.
• To elucidate the mechanism of tRNA(His) guanylylation and potential additional roles of Thg1.

Main Methods:

• Biochemical assays to characterize enzyme activity and substrate specificity.
• Structural biology techniques, including X-ray crystallography, to determine enzyme structure.
• Phylogenetic analysis to classify Thg1 homologs and trace their evolutionary history.

Main Results:

• Thg1 homologs are classified into eukaryal- and archaeal-types with distinct cofactor requirements and specificities.
• Yeast Thg1 can perform limited 3' → 5' RNA polymerization on mutant tRNA substrates.
• Archaeal-type Thg1, found in bacteria and some eukaryotes, exhibits broader substrate specificity and potential roles in tRNA editing.
• The crystal structure of human Thg1 reveals similarity to 5' → 3' polymerases, suggesting an evolutionary link.

Conclusions:

• Thg1 enzymes are crucial for tRNA(His) processing, with distinct evolutionary paths and functional adaptations.
• Thg1's structural resemblance to polymerases highlights its specialized evolution from a common ancestor.
• Understanding Thg1 provides insights into RNA maturation, tRNA identity, and the evolution of RNA-modifying enzymes.