The Role of Polyamines in Cell-Free Protein Synthesis in the Wheat-Germ System

Abstract

Polyamines, in particular spermidine and spermine, were shown to have a stimulatory effect on the incorporation of amino acids into protein in the wheat-germ cell-free system programmed with exogenous mRNA. The addition of the optimum concentrations of polyamines results in a lowering in the Mg2+ optimum for protein synthesis. At the optimum combination of spermidine and Mg2+ the rate of peptide chain elongation with tobacco mosaic virus RNA as messenger RNA was twice the rate observed at the optimum concentration of Mg2+ in the absence of polyamines, but there was no significant difference in the rate of chain initiation under these two conditions. The presence of polyamines also increased the yield of full-length translation products from the viral RNA with a corresponding reduction in the short polypeptide products. These short products are shown to be incomplete translation products that accumulate mainly in the form of peptidyl-tRNA associated with 80-S ribosomes which can release the incomplete nascent chains on incubation with puromycin. The origin of these incomplete chains may be due to endonucleolytic cleavage of mRNA, which we have demonstrated occurs at a significant rate in our system. It is proposed that the increased rate of elongation in the presence of polyamines raises the probability of ribosomes completing the synthesis of full-length products before the mRNA is degraded.