Age-dependence of nuclear RNA processing.

Following a 3 hour in vivo labelling of cytoplasmic RNA in rat liver with orotic acid-6-14C under conditions where ribosomal RNA synthesis was suppressed, the proportion of labelled messenger-like RNA released to the cytoplasm which contained polyadenylate (poly(A)) tracts was about 3.0 times higher in the livers of juvenile (50 day) as compared to adult (180 day) rats. This discrepancy was confirmed in a cell-free system which consisted of isolated prelabelled nuclei in fortified cytosol. Thus under conditions where approximately 80% of the released labelled RNA was messenger-like, the proportion of polyadenylated labelled RNA transported to the homologous cytosol was 3.4-fold greater in the systems derived from juvenile as compared to adult rat liver. Through comparisons of homologous and heterologous systems it was determined that the age-dependent change in the metabolism of polyadenylated messenger RNA resides in the nucleus and not in the cytoplasm. This change, furthermore does not involve the known ATP-dependence of nuclear RNA release. Rather it must involve other age-dependent changes in the processing or transport of polyadenylated messenger RNA.