Protein Synthesis:- Transfer RNAs Have Characteristic Structural Features
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. Mitochondria and chloroplasts contain distinctive, somewhat smaller tRNAs. Cells have at least one kind of tRNA for each amino acid; at least 32 tRNAs are required to recognize all the amino acid codons (some recognize more than one codon), but some cells use more than 32.
drawn in two dimensions, the hydrogen-bonding pattern of all tRNAs forms a cloverleaf structure with four arms; the longer tRNAs have a short fifth arm, or extra arm (Fig. 1). In three dimensions, a tRNA has the form of a twisted L (Fig. 2). Two of the arms of a tRNA are critical for its adaptor function. The amino acid arm can carry a specific amino acid esterified by its carboxyl group to the 2- or 3-hydroxyl group of the A residue at the 3end of the tRNA. The anticodon arm contains the anticodon. The other major arms are the D arm, which contains the un usual nucleotide dihydrouridine (D), and the TC arm, which contains ribothymidine (T), not usually present in RNAs, and pseudo uridine (Ψ), which has an unusual carbon–carbon bond between the base and ribose . The D and TΨC arms contribute important interactions for the overall folding of tRNA molecules, and the TΨC arm interacts with the large-subunit rRNA. Having looked at the structures of ribosomes and tRNAs, we now consider in detail the five stages of protein syn thesis.
FIGURE 2 General cloverleaf secondary structure of tRNAs. The large dots on the backbone represent nucleotide residues; the blue lines represent base pairs. Characteristic and/or invariant residues common to all tRNAs are shaded in pink. Transfer RNAs vary in length from 73 to 93 nucleotides. Extra nucleotides occur in the extra arm or in the D arm. At the end of the anticodon arm is the anticodon loop, which always contains seven unpaired nucleotides. The D arm contains two or three D (5,6-dihydrouridine) residues, depending on the tRNA. In some tRNAs, the D arm has only three hydrogen-bonded base pairs. In addition to the symbols explained in Figure 27–11: Pu, purine nucleotide; Py, pyrimidine nucleotide; G*, guanylate or 2-O-methylguanylate.
FIGURE 2 Three-dimensional structure of yeast tRNAPhe deduced from x-ray diffraction analysis. The shape resembles a twisted L. (a) Schematic diagram with the various arms identified in Figure 27–12 shaded in different colors. (b) A space-filling model, with the same color coding (PDB ID 4TRA). The CCA sequence at the 3 end (orange) is the attachment point for the amino acid.
