Alteration in cellular RNAs during the in vitro lifespan of cultured human diploid fibroblasts. II. Synthesis and processing of RNA.

Incorporation of tritiated uridine into cellular RNA is decreased in senescent human fibroblast cultures when measured per cellular RNA content. Since early and late passage cells demonstrate similar kinetics of RNA precursor pool labeling and saturation, this decreased tritiated uridine incorporation reflects decreased RNA synthesis. However, cellular RNA contents are markedly elevated in senescent cell cultures. When data are measured per cell number or DNA content, net RNA synthesis is slightly increased in senescent cultures. Separation of labeled RNAs by polyacrylamide gel electrophoresis revealed the same diminished specific activities (CPM/RNA) and increased net synthesis (CPM/cell number) of major ribosomal and transfer RNAs. However, this slight increase in RNA synthesis is probably not of sufficient magnitude to completely explain the observed increase in cellular RNA content in senescent cells.