Stability of Polysome-Associated mRNA in Potato Tuber Cells during Aging of Tissue Discs

The stability of polysome-associated mRNA in potato tuber discs in the early stage of aging was examined by pulse-chase labeling experiments and the change in the translational capacity of the RNA was studied using a wheat germ translation system. The incorporation of pulse-fed 3H-uridine into polysomal RNA was not arrested immediately after the addition of actinomycin D to the tissue, but increased by 25% during 4 hr of chasing. The radioactivity in the polysomal RNA then decreased by only 30% of the value at the 4th hr during the next 9 hr in the presence of actinomycin D. The remaining radioactivity in the polysomal RNA was stable at least for 18 hr. The proportion of radioactivity in polyadenylated RNA to that in non-polyadenylated RNA did not vary appreciably during the chasing period. Non-polyadenylated RNA of high molecular weight degraded faster than that of low molecular weight, but polyadenylated RNA did not show such size-selective degradation. The translational capacity of the polysomal RNA also decreased by about 23% within 9 hr during the period of inhibited RNA synthesis. In vivo experiments of 14C-leucine incorporation into proteins in the absence of RNA synthesis suggested that stable polysome-associated mRNA was actually functioning in the cells. SDS-polyacrylamide gel electrophoresis of the in vitro translation products indicated that mRNA coding for polypeptides with relatively high molecular weights turned over slightly faster than those for low molecular weight polypeptides.