Index >>Ribose Nucleic Acids >>Structure of Transfer RNA - tRNA

Structure of Transfer RNA - tRNA - The nucleotide sequence (primary structure) of tRNA was first worked out by Holley et al (1965) for yeast alanine tRNA. Since then the sequence of about 75 different tRNAs, ranging from bacteria to mammals, has been established. The different tRNAs are all minor variants of the same basic type of structure.

Several models of the secondary structure of tRNA have been proposed, and of these the cloverleaf model of Holley is the most widely accepted. According to the cloverleaf model the single polynucleotide chain of tRNA is folded upon itself to form 5 arms. As a result of the folding the 3' and the 5' ends of the chain come near each other. An arm consists of a stem and a loop.

The second arm is called the D arm. It consists of 15-18 nucleotides with 3-4 base pairs in the stem and 7-11 unpaired nucleotides in the loop. The loop of the D arm is called Loop I or dihydrouridine (DHU) loop or the D loop. It contains two variable regions, α and β on either side of two constant guanine residues. These regions consist of 1-3 nucleotides, mostly pyrimidines, with a high proportion of DHU.

The synthetase site which recognizes the amino acid activating enzyme is located on a part of the D loop and a part of the acceptor stem on the 5' side. The third arm or anticodon arm consists of an anticodon stem of 5 base pairs and a loop, called Loop II or the anticodon loop. This loop consists of 7 unpaired nucleotides of which the middle three form the anticodon.