Localization of nucleic acids in hepatocyte nucleoli of rats upon D-galactosamine-induced block of transcription.

The precise localization of DNA and RNA within rat hepatocyte nucleoli during the process of D-galactosamine-induced nucleolar segregation has been studied by using sensitive methods for their detection: osmium-ammine staining and terminal deoxynucleotidyl transferase reaction for DNA, and immunoelectron microscopy with anti-RNA antibodies, RNase-gold, and autoradiography with tritiated orotic acid for RNA. The blocking of transcription was followed by the disappearance of intranucleolar condensed chromatin. Agglomerates of thin extended DNA filaments were found to change their location to the nucleolar periphery and to coalesce with each other. At the last stage of nucleolar segregation they were concentrated at the pole of the nucleolar fibrillar remnant while the rest of the nucleolus did not contain any DNA. No DNA was found in the dense fibrillar component of both intact and treated hepatocyte nucleoli. During the process of nucleolar segregation the bulk of the nucleolar RNA was found within the so-called spherical bodies. This RNA appeared to be synthesized shortly before or even after drug administration. The results obtained are in agreement with the hypothesis that the fibrillar centers are the site of nucleolar transcription. They also show that uncompleted molecules of pre-rRNA whose synthesis has been blocked are segregated from the rest of nucleolar RNA species into the spherical bodies.