Endogenous and exogenous factors affecting ribosomal RNA release from rat liver nuclei in a cell-free system

rRNA release from isolated liver nuclei has been analyzed in a reconstituted cell-free system using density-gradient analysis and hybridization to a specific recombinant DNA probe to monitor the process. The cell-free system was shown previously to be energy- and cytosol-dependent and to support the formation and release of functional ribosomal subunits. The release of rRNA is now shown to have an absolute dependence on a 70000 dalton cytosol protein. Although in vivo studies suggest that chronic administration of thioacetamide may block formation of a protein involved in the nucleocytoplasmic transfer of ribosomes, the 70000 dalton transport factor is not affected by the treatment. Rather the defect appears to be localized to the nucleus, since it cannot be reversed with normal cytosol from a homologous source. Early stages of nRNA processing appear to be affected by thioacetamide, although additional effects on transport are not ruled out.