Elongation of Polypeptide Chains

The molecular mechanism of translation in eukaryotes is very similar to that in bacteria. The activation of amino acids and their attachment to tRNA molecules, and the steps of initiation, elongation, and termination of polypeptide chains, are essentially the same in overall terms. The small and large ribosomal subunits of bacteria and eukaryotes, although different is size and composition, are equivalent with respect to their roles in initiation and elongation of polypeptide chains. Differences lie mainly in the details of some steps, particularly initiation, and in the greater munher of accessory proteins involved in each step. 

Elongation. The polypeptide chain is now lengthened by covalent attachment of successive amino-add units, each of which has been carried to the ribosome by a tRNA, which is base paired to the corresponding codon. Two molecules of GTP are required for each amino-acid residue added to the peptide... 

The chemical polymerization of even a moderately sized protein of a hundred amino acids in the laboratory is extremely laborious, and the yields of active product can often be low to zero (Kent and Parker, 1988). Cells accomplish this task by using an intricate mechanism which involves catalytic machinery composed of proteins, nucleic acids and their complexes, and synthesize polypeptide chains that are composed of hundreds of amino acids. This process is depicted in Fig. 2.4, and is described in the sections below. The basic components of the cellular protein synthesis apparatus, in all known biological systems, are ribosomes, which are aggregate structures containing over fifty distinct proteins, and three distinct molecules of nucleic acid known as ribosomal ribonucleic acid (ribosomal RNA or rRNA). The amino acids are brought to the ribosomes, the assembly bench , by an RNA molecule known appropriately as transfer RNA . Each of the twenty amino acids is specifically coupled to a set of transfer RNAs (discussed below) which catalyze their incorporation into appropriate locations in the linear sequence of polypeptide chains. Several other intracellular proteins known as init iation and elongation factors a re also required for protein synthesis. 

Figure 2. Schematic representation of the cycl ic and repetitive event of polypeptide chain elongation. Numbers in O represent various elongation factors, o represents aminoacyl-tRNA, and a represents the amino acid. (From Johansen and Rattan, 1993).

The previous sections have introduced the major participants in protein synthesis—mRNA, aminoacylated tRNAs, and ribosomes containing large and small rRNAs. We now take a detailed look at how these components are brought together to carry out the biochemical events leading to formation of polypeptide chains on ribosomes. Similar to transcription, the complex process of translation can be divided into three stages—initiation, elongation, and termination—which we consider in order. We focus our description on translation in eukaryotic cells, but the mechanism of translation is fundamentally the same in all cells. 

To this point, you have become familiar with the molecules that participate in protein synthesis and the genetic code, the language that directs the synthesis. We now investigate the actual process by which polypeptide chains are assembled. There are three major stages in protein synthesis initiation of the polypeptide chain, elongation of the chain, and termination of the completed polypeptide chain. 

MATHEWS, M.B. and OSBORE, M. The rate of polypeptide chain elongation in a cell-free system from Krebs II ascites cells. Biochem. Biophys. Acta. (1974)> 340t 147-152. 

To eliminate exonucleolytic attack as a cause of mRNA degradation we followed the fate of mRNA stably attached to ribosomes. If an inhibitor of polypeptide chain elongation is added to the incubations to prevent ribosome movement along mRNA, breakdown of polysomes can only result from endonucleolytic cleavage of the mRNA strand connecting ribosomes. Figure 2 shows... 

Blasticidins pyrimidine antibiotics synthesized by Streptomyces griseochromogenes (see Nucleoside antibiotics). They inhibit the growth of fungi, e.g. the rice fungus, Piricularia oryzae, and a few bacteria. The antibiotic effect is due to suppression of polypeptide chain elongation during protein biosynthesis. 

Figure 1 summarizes the current knowledge of polypeptide chain elongation reactions in E. coli. The three protein factors, EF-Tu, EF-Ts and EF-G, are involved in these reactions. The complex of EF-Tu and EF-Ts, the EF-Tu EF-Ts complex, reacts with GTP... 

Arai, K., Arai, T., Kawakita, M. and Kaziro, Y. (1975) Conformational transitions of polypeptide chain elongation factor Tu. I. Studies with hydrophobic probes. J. Biochem. Tokyo), 77, 1095-1106. 

Nuss, D. L., and Koch, G., 1976 , Variation in the relative synthesis of immunoglobulin G and non-immunoglobulin G proteins in cultured MFC-11 cells with changes in the overall rate of polypeptide chain initiation and elongation, J. Mol. Biol. 102 601. 

Thomas, G. P., and Mathews, M. B., 1982, Control of polypeptide chain elongation in the stress response A novel translational control, in Heat Shock, from Bacteria to Man (M. J. Schlesinger, M. Ashburner, and A. Tissieres, eds.), pp. 207-213, Cold Spring Harbor Laboratory, Cold Spring Harbor, New York. 

Silverstein and Engelhardt (1979) measured the protein-synthesizing activity of polysomes and rates of chain elongation of polypeptides, and found that the translation rate did not alter during the time that the polysomes were declining. They also concluded that a substantial proportion of the larger polysomes that formed after the early breakdown were inactive in protein synthesis and suggested a second block which resulted in inhibition of translation but not breakdown of polysomes and affected specifically cellular and early viral protein synthesis. 

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