Abstract
Transfer RNA (tRNA) is the central molecule in genetically encoded protein synthesis. Most tRNA species were found to be very similar in structure: the well-known cloverleaf secondary structure and L-shaped tertiary structure. Furthermore, the length of the acceptor arm, T-arm, and anticodon arm were found to be closely conserved. Later research discovered naturally occurring, active tRNAs that did not fit the established ‘canonical’ tRNA structure. This review discusses the non-canonical structures of some well-characterized natural tRNA species and describes how these structures relate to their role in translation. Additionally, we highlight some newly discovered tRNAs in which the structure–function relationship is not yet fully understood.
Highlights
Transfer RNAs range in length between 70 and 100 nucleotides. tRNAs are acylated with the cognate amino acid by their cognate aminoacyl-tRNA synthetase, and the resulting aminoacyl-tRNAs are substrates for ribosomal protein synthesis. tRNAs were determined early on to have a highly conserved cloverleaf secondary structure (Figure 1A) (Holley et al, 1965) and an L-shaped tertiary structure (Figure 1B) (Cramer et al, 1969)
The variable loop is the least conserved amongst all tRNAs (Sigler, 1975). tRNAs are classified into two groups based on the size of their variable loops
Other 8/4 and 9/3 allo-tRNAs were found to contain the major identity element for recognition by alanyl-tRNA synthetase (AlaRS), the G3:U70 wobble base pair (Hou and Schimmel, 1988; Mcclain and Foss, 1988). Testing their capabilities in vivo, it was found that Ala and Ser were the main residues incorporated at an amber codon; insertion of other amino acids including Asn, Gln, Lys, Cys, Ile, and Glu were detected. These studies showed that allo-tRNAs derived from other bacterial species could be efficiently used as a substrate in the E. coli translation system, and the nature of the incorporated tRNAs With Non-canonical Structures amino acid is based on the allo-tRNA identity elements rather than on the anticodon present (Mukai et al, 2017)
Summary
Transfer RNAs (tRNAs) range in length between 70 and 100 nucleotides. tRNAs are acylated with the cognate amino acid by their cognate aminoacyl-tRNA synthetase (aaRS), and the resulting aminoacyl-tRNAs are substrates for ribosomal protein synthesis. tRNAs were determined early on to have a highly conserved cloverleaf secondary structure (Figure 1A) (Holley et al, 1965) and an L-shaped tertiary structure (Figure 1B) (Cramer et al, 1969). Both tRNA groups contain the same distinctive tRNASec structural features; a longer variable arm, acceptor stem, and anticodon stem compared to canonical tRNAs (Mukai et al, 2017).
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