Abstract
We investigated the ultrastructural distribution of histones within Ehrlich tumor cell nucleoli by means of three cytochemical methods and by a Lowicryl post-embedding immunogold labeling procedure involving anti-histone (H2B, H3, H4) antisera as well as antibodies to synthetic peptides of histones. With the two technical approaches, labeling was particularly concentrated over the perinucleolar chromatin and over its intranucleolar invaginations which penetrate the nucleolar body and come in close contact with the fibrillar centers. Furthermore, the high-resolution immunocytochemical technique revealed the presence of a small amount of the three histones in the fibrillar centers, preferentially located towards their peripheral regions. In addition, colocalization of DNA at all the histone-positive sites could be visualized after double immunogold staining using a monoclonal anti-DNA antibody and the anti-histone antisera. These results appear to indicate that all the DNA detected within the nucleolus was associated with histones. This finding suggests that the ribosomal DNA, including transcriptionally active genes, is bound to histones.