Ribosomal RNA synthesis in rat liver cells.

DNA-dependent RNA polymerase A was analyzed in rat liver cells after partial hepatectomy, which is known to enhance the synthesis of ribosomal RNA. In agreement with previous results, it was found that the extractable enzyme activity increased after partial hepatectomy and could be quenched by the subsequent administration of cycloheximide, which suggests that continuous protein synthesis is required for the maintenance of the observed stimulation. However, administration of alpha-amanitin to hepatectomized rats did not quench the enzyme activity. These results cast doubt on the hypothesis that the increased activity of RNA polymerase A observed after partial hepatectomy is mediated by RNA products synthesized by polymerase B. Moreover, data are reported which indicate that ribosomal RNA may be regulated directly, without the obligatory involvement of HnRNA.