RNA synthesis by nuclei and chromatin isolated from adult and senescent Wistar rat liver.

Nuclei isolated from senescent (22–26 months) female Wistar rat liver show a decreased RNA synthesis compared to nuclei from adult (12 months) liver. Kinetic analyses demonstrate a decreased rate (25–42%) and maximal incorporation (41–54%) of labeled uridine triphosphate into both rRNA and mRNA in senescent nuclei. Chromatin-bound RNA polymerase activity is decreased by 36%, whereas free RNA polymerase activity, i.e., not bound to chromatin, is increased by 41 % in senescent nuclei, but the total bound plus free activity is the same in senescent and adult nuclei. Isolated senescent chromatin shows reduced transcriptional capacity and requires a higher temperature to initiate melting. A decreased ability of chromatin to bind RNA polymerases appears to underlie the observed decreased RNA synthesis in senescent liver nuclei.