Age-dependent gene induction in quail oviduct. XV. Alterations of the poly(A)-associated protein pattern and of the poly(A) chain length of mRNA.

The effect of ageing on polyadenylate [poly(A)] metabolism of mRNA was studied in two age groups of female quails: mature (250-320 days' old) and senescent animals (3-3.5 years' old). In introductory experiments it was shown that poly(A)-associated proteins can not be recovered from cytosol by affinity chromatography. We isolated the poly(A)-associated proteins from polyribosomal poly(A)-ribonucleoprotein complex [poly(A)-RNP] and radioactively labeled them with dansyl chloride. Three main protein species were identified with molecular masses of 48000 (P48), 35000 (P35) and 24000 (P24). During ageing the percentage portion of P48 in poly(A)-RNP from liver (mitotic tissue) and from oviduct or heart (post-mitotic tissue) is reduced at the expense of P35 and P24. Quantitative analyses revealed that the amount of poly(A)-RNP in the different organs decreases significantly with age if the values are based on DNA. The protein content in poly(A)-RNP was found to be reduced especially in post-mitotic tissue. From this finding we assume that the number of poly(A)-associated protein molecules per poly(A) stretch drops from approximately 4.7 molecules (mature oviduct) to 1.9 molecules (senescent oviduct). Control experiments revealed that free, non-polyribosomal poly(A)-RNP accounts only for 10% of total poly(A)-RNP. The size of the poly(A) segment of mRNA decreases with age. After labeling with [3H] dimethylsulfate, the poly(A) stretch from mature oviduct was found to consist mainly of 120-180 AMP units, and those from mature liver and mature heart of 110 and 100, respectively. In organs from senescent animals the percentage of shorter poly(A) stretches is enlarged; on the average, poly(A)-70 chains were detected. These results support the assumption that age-dependent changes occur also on the post-transcriptional level during the maturation steps of poly(A)(-) hnRNA to poly(A)-(+) mRNA.