Polysomes elongation rate

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. 

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. 

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. 

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. 

The response to the dietary challenge varies in these animals depending upon whether it is investigated in muscle or liver. Protein synthesis is decreased in muscle but increased in liver. However, the increased rate of protein synthesis in liver is selective. For example, the level of serum albumin synthesized by liver is unchanged, whereas that of glucoproteins is markedly increased. This increase in protein synthesis in associated with an elongation of the size of polysome chains, acceleration of RNA synthesis, and an increase in RNA polymerase activity. The meaning of these observations remains to be understood. 

---