Effects of Topoisomerase I Inhibition on Nucleolar Structure and Function

Abstract

Using the Feulgen-like osmium-ammine reaction as a selective staining method for DNA in thin sections, bi-dimensional and stereo-pair micrographs have shown that compact chromatin is composed of roundish units, which represent the nucleosomal organization of chromatin in situ. Nucleosome-like particles have never been visualized in ribosomal chromatin of interphasic NORs, which appears to be composed of numerous, interwoven, long filaments with a thickness of about 3nm. Ribosomal chromatin, present in the fibrillar components of the nucleoli was always in an extended non-nucleosomal configuration, independent of the transcriptional activity and the phase of the cell cycle (Derenzini et al., 1981; 1983; 1987; Hernandez-Verdun and Derenzini, 1983). Recently, there has been considerable interest in a group of enzymes, the DNA topoisomerases, which have the ability to convert one topological isomer of DNA to another. In vivo a possible role of topoisomerase I in ribosomal gene transcription has been suggested by studies of inhibition of rRNA synthesis by camptothecin, an inhibitor of topisomerase I activity (Zhang et al., 1988) and by the observation that topoisomerase I and RNA polymerase I have been immuno-cytochemically localized in-situ on the fibrillar components of nucleoli (Rose et al., 1988; Raska et al., 1989). These results led us to investigate the effects of camptothecin (National Cancer Institute, U.S.A.), a specific inhibitor of topoisomerase I, on the structural and functional organization of ribosomal chromatin in TG cells, a human tumor cell line.