p1. A system of microsomes and 105000ig/i supernatant from livers of old mice is less able to promote the incorporation of [sup14/supC]phenylalanine into protein than a similar system from livers of young animals. 2. The decrease in [sup14/supC]phenylalanine incorporation is attributable to changes in microsomes from old animals rather than in the cell-sap fraction. 3. Decreased synthetic ability is found in various classes of microsomes from older animals, namely unfractionated, light and heavy microsomes, but not in detergent-washed ribonucleoprotein particles. 4. Deletions of certain detergent-soluble microsomal proteins accompany the decreased synthetic ability of microsomes from older animals. 5. Microsomes from old mice are less responsive to a synthetic messenger RNA, polyuridylic acid, and this is partly due to a higher rate of hydrolysis in the presence of cell sap from animals of extreme age. 6. Other more direct evidence, from the priming of a cell-free protein-synthesizing system from bacteria and the examination of ribonucleoprotein particles on sucrose density gradients, suggests that senescence is accompanied by a decrease in messenger RNA content./p
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