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Transfer RNA (tRNA)
The RNA which possesses the capacity to combine specifically with only one amino acid in a reaction mediated by a set of amino acid-specific enzymes called aminoacyl-tRNA synthetases ; transfers that amino acid from' the "amino acid pool" to the site of protein' synthesis and recognises the codons of the mRNA is known as the soluble RNA (sRNA) or transfer RNA(tRNA). Thus, tRNA molecule acts as interpreter of genetic code and bas to perform several highly complex functions during protein synthesis-it interacts with a specific synthetase enzyme, possesses a site for binding an amino acid, possesses a second site for interacting with a ribosome, and contain an anti-codon that must be exposed to the condons of mRNA.
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Structure and maturation of tRNA
The tRNA molecules that perform all these functions are relatively small : they contain only 70 to 80 nucleotides so that each has a molecular weight, of about 30,000 and a sedimentation coefficient of 45. Like the rRNA and mRNA molecules, the molecules of tRNA are found to be matured or tailored in the nucleus prior to their movement to the cytoplasm. For example, in E. coli the precursors of tRNA molecules have been isolated each of which has about 40 extra nucleotides, principally at 5' end but also at the 3' end. These extra nucleotides are subsequently cleaved oft' by still unidentified enzymes to yield a molecule of the final 70 to 80 nucleotide size. Following such large scale tailoring each tRNA molecule is more delicately modified before it becomes fully active.
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Three bases, 5'-CCA-3' are added to the 3' end of every tRNA- molecule regardless of its amino acid affinity, by an enzyme called tRNA phosphorylase. In addition, specific nucleotides in a given tRNA are transformed into what are called minor or unusual bases by specific chemical modifications : for example, methyl groups are added at certain positions to yield 3-methyl-cytosine or 1-methyl-guanosine; certain uracils are reduced to dihydrouracil or rearranged into a form, known as pseudouracil; and adenine is deaminated to yield inosine.
The significance of these unusual bases of tRNA was understood well by molecular biologists during the construction of two-dimensional model from the primary-sequences of nucleotides of known tRNA.Thus, it was realized that most bases of tRNA pair according to Watson-Crick's pairing rule, but, unusual bases fait to do so because they carry substitutions or alterations in those positions that usually participate in hydrogen bonding.
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Consequently, the presence of these bases forces the model builder to construct several non-base-paired loops in the tRNA molecule. By working on these lines, R. Holley (1968) first of all proposed a clover leaf model for yeast tRNAala. The clover leaf model of tRNA because accommodated several of the known functions of tRNA, so it gained general acceptance. A typical clover-leaf model of tRNA depicts following structural peculiarities:
(i) All tRNA molecules contain the same terminal sequence of 5'.CCA-3' bases at 3'-end of the polynucleotide chain. The last residue, adenylic acid (A), is the amino acid attachment site.
(ii) All tRNAs have a loop called Tψ arm of seven unpaired bases including pseudouridine. The Tψ arm is involved in the binding of tRNA molecules to molecules to the ribosomes.
(iii) AU tRNA molecules contain a site for the recognition of the amino acid activating synthetase enzymes. This is the function of dihydrouridine loop or DHU arm which is made up of 8 to 12 unpaired bases.
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| A. Aminoacid binding site |
B.Regions of bonding between base pairs |
C.Aminoacid arm |
D.DHU arm |
| E.TψCG |
F.TψCarm |
G.Extra arm |
H. Anticodon arm |
| I.Anticodon |
J.DHU arm |
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(iv) There is one nucleotide triplet in the tRNA molecule which is different in all tRNAs examined. This is the codon recognition site or anti-codon and it is complementary to the corresponding triplet codon of mRNA.
(v) Some tRNA with long chains may form a short, extra arm.
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As the clover leaf model is in two-dimensions, whereas the tRNA-synthetase enzyme interaction occurs between molecules that are three dimensional. Therefore, very recently S. H. Kim and coworkers (1972, 1973) have proposed a three dimensional L-shaped configuration for a phenylalanine tRNA molecule of yeast cells. According to this model, the molecule of tRNA forms two main portions that are more or less at right angles to each other, with each portion doubled over by bonds holding complementry base together.
Genes (or tRNA-There are probably at least 30 to 40 different tRNA genes and tRNA molecules in E. coli. Higher organisms are found to contain 60 tRNA molecules and 60 tRNA genes. Since the cell uses only 20 amino acids in protein synthesis (and probably only 20 synthetase enzymes), it follows that several tRNAs will often have an affinity for the same a6lino acid. For example, E. coli cells contain five species of tRNA for leucine amino acid.
All the tRNA genes constitute for less than 1% of the total genome in both E. coli and eukaryotic cells, yet some 10 to 15% of each cell's RNA may be in the form of tRNA. This discrepancy between the number of tRNA genes and gene transcripts occurs because of following fads-(l) The tRNA molecules are relatively stable compared with many kinds of RNA. (2) The tRNA molecules are transcribed continuously and more quickly by tRNA genes than other RNAs because they are needed in plentiful amounts.
| 1.Actual appearance of the folding of the molecule. The polynucleotide chain is represented as a continuous coiled tube |
2.The way that the clover leaf representation must be transformed in order to show the physical connectins between various parts of the molecule. |
| A.TψC loop |
B.5' end |
| C.DHU loop |
D.Small loop |
| E.Anticodon loop |
F.3'OH acceptor end |
| G TψC loop |
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