Translation chain elongation

All polyketides use the same general mechanism for chain elongation. Acetyl coenzyme A provides acetate (C2) units, which are condensed by a ketosynthase (KS). This in turn catalyzes condensation of the growing chain onto an acyl carrier protein (ACP), as generalized in Fig. 1.4. Enzymes such as ketoreductase (KR), enoyl reductase (ER), and dehydratase (DH) establish the oxidation state of caibon during translation, imparting structural diversity. Successive translation of each module leads to a chain of the required length that is eventually passed to thioeste-rase (TE), which releases the chain as a free acid or lactone. 

The translation of the mRNA into proteins is the final step in the biological flow of information (see Fig. 6.1). Similar to other macromolecular polymerizations, protein synthesis can be divided into initiation, chain elongation, and termination. Critical players in this process are the aminoacyl transfer RNAs (tRNAs). These molecules form the interface between the mRNA and the growing polypeptide. Activation of tRNA involves the addition of an amino acid to its acceptor stem, a reaction catalyzed by an aminoacyl-tRNA synthetase. Each aminoacyl-tRNA synthetase is highly specific for one amino acid and its corresponding tRNA molecule. The anticodon loop of each aminoacyl-tRNA interacts... 

Another possible explanation for the lack of observed export block is the difference in rate of translation in vivo and in vitro. The rate of chain elongation in vivo in eukaryotes is about 180 residues/minute, while in vitro translation proceeds at about 30 residues/minute. If an SRP-nascent chain-ribosome complex has a half-life of, e.g., 1 second, it would cause a significant pause in synthesis in vitro, but would probably not be noticed in vivo. Such a short-lived complex may be sufficient to couple translation to translocation in vivo, but not in vitro, as the time required for the ribosome to diffuse to the membrane will depend on how far it has to go. Inside the cell, the ribosome will have a much smaller distance to travel than in an in vitro translocation mixture. 

The correctly positioned eukaryotic SOS ribosome-Met-tRNAj complex is now ready to begin the task of stepwise addition of amino acids by the in-frame translation of the mRNA. As is the case with initiation, a set of special proteins, termed elongation factors (EFs), are required to carry out this process of chain elongation. The key steps in elongation are entry of each succeeding aminoacyl-tRNA, formation of a peptide bond, and the movement, or translocation, of the ribosome one codon at a time along the mRNA. 

Each stage of translation—initiation, chain elongation, and termination—requires specific protein factors including GTP-binding proteins that hydrolyze their bound GTP to GDP when a step has been completed successfully. 

The second stage of translation is chain elongation. This occurs in three steps that are repeated until protein synthesis is complete. We enter the action after a tetrapeptide has already been assembled, and a peptidyl tRNA occupies the P-site (Figure 24.19b). 

The first event is binding of an aminoacyl-tRNA molecule to the empty A-site. Next, peptide bond formation occurs. This is catalyzed by an enzyme on the ribosome called peptidyl transferase. Now the peptide chain is shifted to the tRNA that occupies the A-site. Finally, the tRNA in the P-site falls away, and the ribosome changes positions so that the next codon on the mRNA occupies the A-site. This movement of the ribosome is called translocation. The process shifts the new peptidyl tRNA from the A-site to the P-site. The chain elongation stage of translation requires the hydrolysis of GTP to GDP and Pj. Several elongation factors are also involved in this process. 

The process of protein synthesis is called translation. The genetic code words on the mRNA are decoded by tRNA. Each tRNA has an anticodon that is complementary to a codon on the mRNA. In addition the tRNA is covalently linked to its correct amino acid. Thus hydrogen bonding between codon and anticodon brings the correct amino acid to the site of protein s)mthesis. Translation also occurs in three stages called initiation, chain elongation, and termination. 

It has been shown that the first step in ribosome-dependent peptide synthesis is activation of amino acids to form amino acid adenylates. The amino acids are then transferred to RNA present in the soluble extract of the cell, the so-called transfer RNA (tRNA) to which the amino acids become fixed by an ester linkage. These two steps are usually referred to as the formation of aminoacyl-tRNA. The next step, the translation step of codons in messenger RNA (mRNA), which is associated with ribosomes, to provide a polypeptide includes three stages (1) chain initiation by mutual coordination with initiation factors, (2) chain elongation in aid of elongation factors, and (3) chain termination in support of release factors. 

The details of the chain of events in translation differ somewhat in prokaryotes and eukaryotes. Like DNA and RNA synthesis, this process has been more thoroughly studied in prokaryotes. We shall use Escherichia coli as our principal example, because aU aspects of protein synthesis have been most extensively studied in this bacterium. As was the case with replication and transcription, translation can be divided into stages—chain initiation, chain elongation, and chain termination. 

Regulatory mechanisms at the level of mRNA translation could also lead to gross metabolic changes. The mechanism of protein synthesis has been exhaustively studied [5], and many components have been implicated. Changes in each of these components—ribosomes, factors involved in the ribosomal binding of mRNA, in the initiation and termination of protein synthesis, and in polypeptide chain elongation, tRNA, and the components responsible for its acylation and subsequent transfer to the polysomal complex—could potentially lead to alteration in the rate, extent, or fidelity of protein synthesis. 

As traiislation progresses, the ribosome moves from the S -phosphate end to the 3 -OH end of messenger RNA whilst the polypeptide chain elongates. When the S end of the messenger RNA is sterically free, synthesis of a new polypeptide may be initiated. In this mamm, the translating machinery becomes able to realize... 

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. 

---