Studies were conducted to characterize the
nuclear RNA (nRNA) species that were present in rat liver but absent in the
hepatoma.
Nuclear RNA was compared between Donryu rat liver and AH136B
hepatoma, an
azo dye-induced transplantable cell line, by
DNA-
RNA competitive hybridization. The
hepatoma lacked 13-14% of nRNA according to measurements of radioactivity of the hybridized 32P-labeled liver nRNA, and this loss was shown to be due to the failure to transcribe such
RNA rather than to the deletion of the relevant
DNA in the genome. Characterization of the lost
RNA was first attempted by fractionating liver nRNA by density gradient sedimentation and
polyacrylamide gel electrophoresis. A comparison of the additive effects of the fractionated
RNA's in the competitive hybridization indicated that the pertinent
RNA was present in the large
RNA molecules (greater than 14S), not in the low molecular weight
RNA's. Then
poly(A) nRNA was found to show a strong additive effect in the competitive hybridization while nucleolar
RNA showed little additive effect, indicating that the pertinent
RNA was present in the heterogeneous nRNA, not in the ribosomal precursor. Further characterization was done by fractionating
DNA with regard to the repetition in the genome. A comparison of the competitive hybridizations on the fractionated
DNA's showed that the loss occurred mostly in
RNA transcribed from highly repetitive
DNA. In conclusion, the
RNA species lost in the
hepatoma were components of heterogeneous nRNA transcribed from highly repetitive
DNA.