Relationship between the extended, non-nucleosomal intranucleolar chromatin in situ and ribosomal RNA synthesis

In the present study we have investigated the relationship between the structure of intranucleolar chromatin in situ and the synthesis of rRNA. Using thin sections selectively stained for DNA we observed that intranucleolar chromatin of cortisol-stimulated rat hepatocytes consisted of clumps and fibres, both showing a nucleosomal configuration, and by loose agglomerates of extended DNA filaments, with a thickness of 2-3 nm, which never formed nucleosomal structures. After inhibition of rRNA synthesis by actinomycin D, the agglomerates of extended DNA filaments without nucleosomal configuration were noticed to be still present even at 3 h after drug treatment. In human resting lymphocytes, with a very low rate of rRNA synthesis, a large roundish, loose agglomerate of DNA filaments without nucleosomal configuration was found in the central zone of the nucleolar body. After a 16-fold increase in rRNA synthesis induced by a 48-h stimulation with phytohemagglutinin (PHA) the loose agglomerates appeared to be more numerous, but markedly reduced in size. We concluded that the extended non-nucleosomal configuration is a permanent feature of intranucleolar loose chromatin agglomerates and not a consequence of transcriptional activity.