Transfer RNA (abbreviated tRNA) is a small RNA chain (74-93 nucleotides) that transfers a specific amino acid to a growing polypeptide chain at the ribosomal site of protein synthesis during translation. It has sites for amino-acid attachment and codon (a particular sequence of 3 bases) recognition. The codon recognition is different for each tRNA and is determined by the anticodon region, which contains the complementary bases to the ones encountered on the mRNA. Each tRNA molecule binds only one type of amino acid, but because the genetic code is degenerate, more than one codon exists for each amino acid.
Transfer RNA is the "adaptor" molecule hypothesized by Francis Crick, which mediates recognition of the codon sequence in mRNA and allows its translation into the appropriate amino acid.
Structure of tRNA
tRNA has primary structure (the order of nucleotides from 5' to 3'), secondary structure (usually visualized as the cloverleaf structure), and tertiary structure (all tRNAs have a similar L-shaped 3D structure that allows them to fit into the P and A sites of the ribosome).
Features
- The 5'-terminal phosphate.
- The acceptor stem (also called the amino acid stem) is a 7-bp stem that incudes the 5'-terminal nucleotide and the 3'-terminal nucleotide with the 3'-terminal OH group (which can bind the amino acid). The acceptor stem may contain non-Watson-Crick base pairs.
- The CCA tail is a CCA sequence added to the 3' end of the tRNA molecule. This sequence is important for the recognition of tRNA by enzymes critical in translation.
- The D arm is a 4 bp stem ending in a loop that often contains dihydrouridine .
- The anticodon arm is a 5-bp stem containing the anticodon. Each tRNA contains a specific anticodon triplet sequence that can base-pair to one or more codons for an amino acid. For example, one codon for lysine is AAA; the anticodon of a lysine tRNA might be UUU (some anticodons can pair with more than one codon due to a phenomenon know as wobble).
- The T arm is a 5 bp stem containing the sequence TψC.
- Modified bases are several bases contained in tRNA that are not "canonical" bases, i.e. that are modified forms of the standard adenine, guanine, cytosine, and uracil bases.
Aminoacylation
Aminoacylation is the process of adding an aminoacyl group to a compound.
Each tRNA is aminoacylated (or charged) with a specific amino acid by an aminoacyl tRNA synthetase. There is often just one aminoacyl tRNA synthetase for each amino acid, despite the fact that there can be more than one tRNA, and more than one anticodon, for an amino acid. Recognition of the approriate tRNA by the synthetases in not mediated solely by the anticodon, and the acceptor stem often plays a prominent role.
Reaction:
- amino acid + ATP → aminoacyl-AMP + PPi
- aminoacyl-AMP + tRNA → aminoacyl-tRNA + AMP
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