Variable rate of polypeptide chain elongation in vitro. Effect of spermidine

Translation of rabbit globin mRNA and tobacco mosaic virus mRNA was studied in a wheat germ system. After a short pulse with labeled methionine, the reaction products were separated by polyacrylamide gel electrophoresis in sodium dodecyl sulfate and visualized by fluorography. Discrete bands corresponding to incomplete products were detected both with globin and tobacco mosaic virus RNA as messenger at a time when no release of peptides from tRNA could be detected. In the absence of spermidine certain intermediate products accumulated during the chase period. With increasing incubation time, the radioactivity of most bands was chased into higher-molecular-weight products. This change in size distribution was more pronounced when magnesium was partially replaced by spermidine. Evidence is presented that some of the incomplete products had in part been released from tRNA. No such release of polypeptides could be detected in experiments using the artificial messenger poly(U), indicating that non-specific peptidyl-tRNA hydrolases were not present in the system. The transient accumulation of discrete bands during the chase period indicates that in the wheat germ system polypeptide chain elongation occurs at discontinuous rates. The fact that spermidine increases the rate of elongation and the yield of full-length translation products may be due to its ability to facilitate the movement of the ribosomes beyond regions where the elongation is retarded.