Ribosome elongation rate

Aminoacyl-tRNA dissociates from the ribosome with rate constant Ici, thereby aborting the elongation step. 

These results indicated that thiaisoleucine may be incorporated in place of isoleucine into polypeptides,in competition with isoleucine,and that its incorporation does not impair the polypeptide synthesis. This was confirmed by the analysis of the polysomal pattern and of the polysomal-bound radioactivity. Rabbit reticulocyte lysates were incubated,in the presence of all the factors necessary for protein synthesis,with labelled isoleucine or leucine, and with or without thiaisoleucine. As shown in Fig.3 the polysomal profile remained unchanged in the presence or absence of thiaisoleucine,while the polysomal-bound radioactivity due to isoleucine was highly reduced in the presence of thiaisoleucine.On the other hand the polysomal-boiind radioactivity due to leucine was unaffected by the presence of thiaisoleucine. These results showed that the incorporation of thiaisoleucine into the polypeptide chain does not modify the ribosome run-off nor the elongation rate of the growing polypeptide chain. 

The principal drawbacks to the technique are its labor-intensive nature and low protein yields, which in the best cases reach only a milligram or less. Competition with release factors at the amber stop codon often results in truncated protein as the primary product [16, 17]. Suppression efficiency rates are also affected by the character of the amino acid, which determines whether it is a good substrate for the ribosome and protein elongation factors. In addition, context effects variously ascribed to the influence of neighboring mRNA... 

Initiation (Figs. 29-10 and 29-11), elongation (Fig. 29-12), and termination are three distinct steps in the synthesis of a protein. A variety of specialized proteins are required for each stage of synthesis. Their sequential interaction with ribosomes can be viewed as a means of ensuring an orderly sequence of steps in the synthesis cycle. The rate of protein formation will depend upon the concentrations of amino acids, tRNAs, protein factors, numbers of ribosomes, and kinetic constants. The formation of specific proteins can also be inhibited by translational repressors, proteins that compete with ribosomes for binding to target mRNAs.287... 

In perfused liver it also has effects on synthesis of most types of RNA and on amino acid uptake, and these may underlie much of the action on protein synthesis. For most effects on the liver there appears to be a delay of 15-60 min between the application of hormone and the first manifestation of the effect. Ribosomes from liver of hypophysectomized rats possess a lowered ability to carry out protein synthesis [86] and the defect can be reversed by administration of GH in vivo or in vitro. The effect may be at least partly on the rate of elongation of the growing polypeptide chain [85]. 

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. 

Messenger RNA translation may be separated into three stages—initiation, elongation, and termination—with the number of ribosomes traversing a mRNA at any one time a function of the relative rates of these three steps as well as the length of the mRNA. Many different lines of evidence point to initiation as the process most likely to be controlled, either by factors apparently affecting the synthesis of all... 

In poliovims infected cells and presumably in cells infected by other picomaviruses, the polysome complex formed with viral mRNA is large. The average number of ribosomes in the largest polysomes (jSOS) is about 35 per viral mRNA (5)> due at least partially to the large size of the viral mRNA (2.5 x 10 daltons (6) ). The transit time for a ribosome to traverse a molecule of picomavirus mRNA is approximately 10 min (7, 8) early in infection. The rate of chain elongation is about 205 to 225 amino acids per min. This rate is considerably slower than the rate of synthesis of the a and B chains of hemoglobin, namely 600 and 400 amino acids per min 9) respectively. 

It may be that we should be more concerned with why the bacterial machinery is relatively so simple. In rapidly growing bacterial cells a very substantial fraction of the total cellular protein is involved directly in protein biosynthesis (activating enzymes, ribosomal structural proteins, elongation factors etc). Evolutionary pressure to attain high growth rates may therefore have resulted in a streamlining of the bacterial protein synthesis mechanism to the bare minimum of components, in which case we should not look for some subtlety of regulation in every additional feature of complexity in the eukaryotic system. 

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. 

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