tRNA and its structure

Introduction

• It is called as soluble RNA because of its small size made up of 75-95 nucleotides and sedimentation coefficient of mature eukaryotic t-RNA is 3.8 S.
• There are about 60 different specific t-RNAs in bacterial and 100-110 in mammalian cells.
• t-RNA is synthesized on a DNA template and is an exception to other cellular RNAs in that a part of it as ribonucleotide sequence (-CCA) is added after it comes off DNA template.
• Each amino-acid is carried by specific t-RNA.
• Since 20 amino-acids are coded to form proteins, there must be at least 20 types of t-RNA.
• However, it has been found that there are at least 2 types for each amino-acid signifying their higher number.
• t-RNAs are the adaptors between codons and amino-acids they specify.
• Most t-RNA recognizes more than one codon but such codons should specify a particular amino acid.
• The 3’ terminal has –CCA sequence where amino acid is attached.
• There are various unusual bases in t-RNA which are pseudouridine (ΨU), dihydrouridine, Hypoxanthine, Methylguanine, Dimethyl guanosine, Inosine, Methyl inosine, Ribothymidine, etc.
• Thee bases improve the t-RNA function. E.g., Hypoxanthine play an important role in codon recognition.

Clover leaf: secondary structure

• The secondary structure of t-RNA shows various parts. They are as follows:

1. Acceptor arm

• It is the site of attachment of amino acids.
• There is presence of 5’-CCA-3’ sequence at extreme 3’ end of the molecules.
• The 5’ end consists of guanine or cytosine.

2. TΨc loop

• Two unusual bases ribothymidine (T) and pseudouridine (ΨU) along with cytosine are found in this loop.
• U is found within the sequence 5’-TΨUCG-3’.

3. D loop

• Dihydrouridine is the unusual base in this loop.
• The synthetase which recognizes the amino acid activating enzyme is located on the part of D loop and apart of acceptor stem on 5’.

4. Anti-codon loop

• It has 3 nucleotides long decoding element for recognizing codon in mRNA.
• Its 3’ end has purine and 5’ end has uracil.

5. Variable loop

• It has 3 to 21 bases and is present between anticodon and ΨU loop.
• It is not present in all t-RNAs.

L- shaped: Three-Dimensional Structure

• X-ray crystallography revealed the L-shaped tertiary structure of t-RNA which has acceptor arm at one end and anticodon loop 70A away at other end.
• It is stabilized by 3 interactions. They are:

1. H bonds (non-watson Crick model).
2. Between bases and sugar phosphate
3. Additional base stacking gained from formation of 2 extended region of base pairing.

References: 

i) https://www.nature.com/scitable/definition/trna-transfer-rna-256/

ii) https://study.com/learn/lesson/trna-structure-function-synthesis.html

tRNA and its structure