tRNA: Meaning, Structure and Initiator Transfer | Nucleic Acids

In this article we will discuss:- 1. Meaning of tRNA 2. Structure of tRNA 3. Initiator Transfer RNA.

Meaning of tRNA:

Transfer RNAs (tRNAs), also called “soluble RNAs” (sRNAs) are small molecules varying from 75 to 100 nucleotides in length. They carry amino acids and bring them to ribosomes to form the polypeptide chain. There are 20 amino acids but the number of tRNA types is greater because some amino acids are adopted by more than one form of tRNAs.

Transfer RNAs are designated by the amino acid abbreviation in super script, such as, tRNA^gly (tRNA for glycine). Different tRNA adopting the same amino acid are designated by number in subscript, e.g. tRNA₁^leu, tRNA₂^leu. Such tRNAs are called “iso-acceptingt RNAs”.

Transfer RNAs are produced on DNA templates and the reaction is catalysed by DNA dependent RNA polymerase. Precursor tRNA is produced first and then processing is made to produce the mature tRNA.

During processing, modifications of some bases occurs, such as dihydrouridine (UH₂), pseudouridine (Ψ), inosine (I), 7-methylguanosine (m⁷ G), acetyl cytidine (acC), N²-dimethyl guanosine (m₂G) and 1- methyl adenosine (m¹A) etc. About 10-15% of the bases in tRNA are modified.

Structure of tRNA:

The universally accepted 2-dimensional model of tRNA is the “clover leaf model” formed due to pairing of short complementary sequences and the formation of unpaired loops (Fig. 4.2). The 5′ P end terminates usually into guanine (G), while the 3′ OH end always terminates into a 5′ CCA3′ sequence. Amino acid is carried on the 3′-end, associated with the adenine (A), general structure of clover leaf model is described below (Fig. 4.2).

(1) The 3′-end terminates into 5’CCA3′ sequence that is always unpaired. The terminal A residue is the site at which the amino acid is bound covalently.

(2) Starting from the 3′-end, after the 3′ ACC5′ sequence, the region includes few paired bases.

(3) Then comes the first loop containing 7 unpaired bases. This loop is designated as “TΨ C loop” because it always contains a sequence 5′ ribothymidine- pseudouridine- cytidine 3′. This loop is involved in binding to ribosome.

(4) After the “5′ -T Ψ C-3′ loop”, in the 5′ direction, there occurs a loop of variable size, called the extra loop or the “lump”. The lump may contain 3 to 21 bases.

(5) The third loop contains 7 unpaired bases and it has the “anticodon.” Anticodon consists of 3 bases. At the 3′ -end of the anticodon, there is a purine (A or G) while at the 5′ -end, there is always a uracil (U). At the time of protein synthesis, anticodon pairs with its complementary “codon” on mRNA.

(6) The fourth loop is larger than others and contains 8-12 unpaired bases. It is designated as “D-loop” because it is rich in dihydrouridine (UH₂). The enzyme aminoacylsynthetase binds to this loop.

Initiator Transfer RNA:

Marcker and Sanger in 1964 isolated two different types of methionine specific tranfer RNAs. One type, designated as tRNA_f^met carries the amino acid N-formyl methionine, while the other type, designated as tRNA^met carries the normal methionine. After the formation of aminoacyl-tRNA (methionyl-tRNA), the formate group is added to methionine to form N-formyl methionine, by the enzyme transformylase. The formate group comes from N-formyltetrahydrofolic acid.

The formate group protects the polypeptide chain from peptide bond formation with other amino acids. In prokaryotes the initiator amino acid is the N-f-methionine, while in eukaryotes, it is the normal methionine. The initiator tRNA in eukaryotes also differs from the tRNA^met and is designated as _tRNA_f^met.