Transfer RNA (tRNA) or Soluble RNA (sRNA)

After rRNA the second most common RNA in the cell is transfer RNA. It is also called soluble RNA because it is too small to be precipitated by ultracentrifugation at 100,000g. Transfer RNA is a relatively small RNA having a molecular weight or about 25,000 to 30,000 and the sedimentation coefficient of mature eukaryote tRNA is 3.8S. It is made up of 73-93 nucleotides. Each bacterial cell probably contains about a hundred or more different types of tRNA.

The function of tRNA is carry amino acids to mRNA during protein synthesis. Each amino acid is carried by a specific tRNA. Since 20 amino acids are coded to form proteins, it follows that there must be at least 20 types of tRNA. It was formerly thought that only 20 tRNA molecular types exist, one for each amino acid. It has, however, been shown that in several cases there are at least two types of tRNA for each amino acid. Thus there are many more tRNA molecules than amino acid types. These are probably coded by one gene.

Transfer RNA is synthesized in the nucleus on a DNA template. Synthesis of tRNA occurs near the end of cleavage stages. Transfer RNA is an exception to other cellular RNAs in that a part of its ribonucleotide sequences (-CCA) is added after it comes off the DNA template. Like rRNA, tRNA is also formed from only a small section of the DNA molecule. Therefore, it does not show any obvious base relationships to DNA.

The tRNA molecule consists of a single strand looped about itself. The 3’ end always terminates in a -C-C-A (cytosine-cytosine-adenine) sequence. The 5’ end terminates in G (guanine) or C (cytosine). Many of the bases are bonded to each other, but there are also unpaired bases.

Structure. The acceptor stem consists of 7 base pairs and 4 unpaired nucleotide units. The latter include a constant 3’ terminal –CCA sequences and a fourth nucleotide which is a variable purine (A or G).

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.

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 nucleoptides of which the middle three form the anticodon.

The variable arm (the “lump”, the miniloop, Loop III) is of two types. In one type there is a loop containing 4-5 bases but not stem. In the other type, the arm consists of 13-21 residues, and both the stem and the loop can be distinguished.