1.8.4 tRNA

Unit 1 triplex DNA could disrupt gene expression. This approach to controlling cellular metabolism is of growing commercial interest for its potential application in medicine and agriculture.

1.8.4 tRNA

Transfer ribonucleic acid (tRNA) is a type of RNA molecule that helps decode a messenger RNA (mRNA) sequence into a protein. tRNAs function at specific sites in the ribosome during translation, which is a process that synthesizes a protein from an mRNA molecule. Proteins are built from smaller units called amino acids, which are specified by three-nucleotide mRNA sequences called codons. Each codon represents a particular amino acid, and each codon is recognized by a specific tRNA. The tRNA molecule has a distinctive folded structure with three hairpin loops that form the shape of a three-leafed clover. One of these hairpin loops contains a sequence called the anticodon, which can recognize and decode an mRNA codon. Each tRNA has its corresponding amino acid attached to its end. When a tRNA recognizes and binds to its corresponding codon in the ribosome, the tRNA transfers the appropriate amino acid to the end of the growing amino acid chain. Then the tRNAs and ribosome continue to decode the mRNA molecule until the entire sequence is translated into a protein.

 tRNA itself is an RNA molecule with a conserved inverted L structure.  One end of the tRNA contains an anticodon loop which pairs with a mRNA specifying a certain amino acid. The other end of the tRNA has the amino acid attached to the 3' OH group via an ester linkage.  tRNA with an attached amino acid is said to be "charged". The enzyme that attaches the amino acid to the 3'-OH is called an aminoacyl tRNA synthetase (aaRS).  There is a specific tRNA for each amino acid, 20 in all. Similarly, there is a specific aaRS for each tRNA.  Only the first 2 nucleotides in the tRNA anticodon loop are strictly required for the decoding of the mRNA codon into an amino acid. The third nucleotide in the anticodon is less stringent in its base-pairing to the codon, and is referred to as the "wobble" base.  Since the genetic code is degenerate, meaning that more than one codon can specify a single amino acid, the anticodon of tRNA can pair with more than one mRNA codon and still be specific for a single amino acid.  To understand how tRNAs can serve as adaptors in translating the language of nucleic acids into the language of proteins, we must first examine their structure in more detail.  Transfer RNAs are relatively small and consist of a single strand of RNA folded into a precise three-dimensional structure. The tRNAs in bacteria and in the cytosol of eukaryotes have between 73 and 93 nucleotide residues, corresponding to molecular weights of 24,000 to 31,000.

Figure 1.8.11 Nucleotide sequence of yeast tRNAAla. This structure was deduced in 1965 by Robert W. Holley and his colleagues; it is shown in the cloverleaf conformation in which intrastrand base pairing is maximal. The following symbols are used for the modified nucleotides : , pseudouridine; I, inosine; T, ribothymidine; D, 5,6dihydrouridine; m1I, 1-methylinosine; m1G, 1methylguanosine; m2G, N2-dimethylguanosine The anticodon can recognize three codons for alanine (GCA, GCU, and GCC). Note the presence of two GUU base pairs, signified by a blue dot to indicate non-Watson-Crick pairing. In RNAs, guanosine is often basepaired with uridine, although the GUU pair is not as stable as the Watson-Crick GmC pair