RNA as an Inhibitor.of Protein Synthesis

One way of searching for the presence of inhibitors of polypeptide initiation in infected cells was to add cytoplasmic fractions from virus infected cells to a cell-free system from rabbit reticulocytes. This system initiates the synthesis of new polypeptide chains at a very high rate. Cytoplasm from poliovirus infected HeLa cells, but not from uninfected cells, inhibited protein synthesis in the reticulocyte lysate (59) The inhibitor was isolated and identified as double-stranded (ds) RNA (60). To study the effect of ds RNA on host and viral protein synthesis, a cell-free system from HeLa cells was developed which initiated translation on endogenous cellular or viral mRNA. When added to this system, the ds RNA was found to inhibit the translation of both cellular and viral mRNAs (61). Furthermore, measurement of the amount of ds RNA present in cells early in infection (61, 62) revealed that an insufficient quantity was present to act as a direct agent of protein synthesis inhibition. 

Interestingly, in reticulocyte lysates, the effect of ds RNA is similar to that resulting from heme deficiency (63), that is, polypeptide chain initiation is inhibited. (For an extensive discussion, see chapter 10 of this book).. The ds RNA appears to induce a protein kinase which phosphorylates initiation factor eIF-2 (63). Experiments in our laboratory (M. Jaye, unpublished data) have failed to detect the presence of any phosphorylated components in the crude initiation factor preparation after virus infection corresponding to those elicited in the presence of ds RNA. 

When added to whole cells, non-infectious bovine enterovirus ds RNA is cytotoxic (64). The cytotoxicity occurred in the presence of inhibitors of protein synthesis, suggesting that translation is not necessaiy for the effect. The ds RNA appeared to be selective, since it inhibited only cellular protein synthesis and not mengovirus protein synthesis (65). These findings are contrary to those found in extracts, where no selectivity of ds RNA was observed. No explanation for this difference is available at this time. 

The general consensus of researchers in the field is that ds RNA is not involved in shut-off, since its effect in extracts showed no specificity between viral and cellular protein synthesis, as would 

In this review I have outlined several theories that have been proposed to explain the mechanism by which picornaviruses inhibit cellular protein synthesis. Some theories seem less likely than others. Inhibition by ds ENA, for example, is no longer thought to be a likely possibility. In cell-free extracts ds ENA inhibits both cellular and viral mRNA translation (61). The inhibitor of cellular protein synthesis would be expected to be selective in its inhibitory activity. It is also apparent that picornavirus infection does not result in the degradation or alteration of cellular mENA (9 27, 29 51). So,.too, experiments demonstrating that protein synthesis inhibition takes place in the absence of significant viral ENA synthesis (I4) tend to weaken the argument that protein synthesis inhibition results from direct competition of viral mENA with cellular ENA for initiation factor eIE-4D (47) As mentioned earlier, superinfection with poliovirus of cells infected with VSV prevents VSY mENA translation (J2, 56). In lysates from uninfected HeLa cells, however, 7SY mENA translation is favored over poliovirus mENA translation when both mENA species are present in equimolar saturating concentrations (55) If competition were a major cause of cellular protein synthesis inhibition, one would have expected poliovirus mENA to out-compete VSV mENA in cell-free translation, not the contrary. 

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Robertson, H. D., and Mathews, M. B., 1973, Double-stranded RNA as an inhibitor of protein synthesis and as a substrate for a nuclease in extracts of Krebs II ascites cells, Proc. Natl. Acad. Sci. USA 70 225. 

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