Metabolism of free and membrane-bound ribosomes during aging of Jerusalem artichoke tuber slices.

Changes in ribosomes of artichoke (Helianthus tuberosus L.) tuber cells following excision and aging of tissue slices in water were studied using biochemical techniques. During the first 2 h of aging total rRNA dropped 28% and then remained constant for a subsequent 46 h. Since ribosome synthesis occurs through at least the first 24 h of aging, turnover of ribosomes must take place in this period. Cells of the dormant tuber gave essentially no membrane-bound (mb) ribosomes. On aging, the mb ribosome fraction rose and reached a maximum of 25% of total ribosomes at 24 h. Density gradient analysis showed that the ribosomes of dormant cells were present largely as monosomes. After 4 h aging a significant number of ribosomes in both free and mb populations sedimented as polysomes and the number of polysomes in both populations increased to a maximum at 24 h. The direct polysome analysis was confirmed by estimates of synthetically "active" ribosomes obtained using 0.8 M KCl to isolate monosomes carrying nascent polypeptides. This approach showed that while unaged cells had only 13% of total ribosomes active, on aging the active fraction rose to about 68% at 24 h. Both free and mb populations showed the same percentage of ribosomes active at all times studied. [(3)H]uridine showed significant incorporation into ribosomes during three periods studied; 2-4h, 12-14h, 22-24h. At the two latter periods the specific activity of the free ribosomes was greater than that of the mb ribosomes. Uridine was incorporated into both active and inactive ribosomes of both populations, judged by KCl fractionation, with the inactive fraction having greater specific activity in both cases. These differences in labelling possibly result from relatively slow mixing of different ribosome populations. Uptake of soluble [(3)H]uridine into the tissue increased 4-fold between 4 h and 14 h accounting at least in part for greater overall specific activity of ribosomes at later aging times.