Structural and Functional Organization of Ribosomal Genes within the Mammalian Cell Nucleolus

Massimo Derenzini ^1*, Gianandrea Pasquinelli ¹, Marie-Françoise O’Donohue ¹, Dominique Ploton ¹ and Marc Thiry ¹

¹ Sezione di Patologia Clinica, Dipartimento di Patologia Sperimentale, Università di Bologna, Italy (MD,GP); Unité MéDIAN, CNRS UMR, Reims, France (M-FO,DP); and Laboratory of Cell and Tissue Biology, Department of Life Sciences, Faculty of Sciences, University of Liège, Liège, Belgium (MT)

^* To whom correspondence should be addressed. E-mail: massimo.derenzini{at}unibo.it.

Submitted on July 12, 2005
Accepted on 20 September 2005

	Abstract

Data on the in situ structural-functional organization of ribosomal genes in the mammalian cell nucleolus are reviewed here. Major findings on chromatin structure in situ come from investigations carried out using the Feulgen-like osmium ammine reaction as highly specific electron-opaque DNA tracer. Intranucleolar chromatin shows three different levels of organization: compact clumps, fibers ranging from 11 to 30 nm and loose agglomerates of extended DNA filaments. Both clumps and fibers of chromatin exhibit a nucleosomal organization which is lacking in the loose agglomerates of extended DNA filaments. In fact, these filaments constantly show a thickness of 2-3 nm, the same, that is, as a DNA double helix molecule. The loose agglomerates of DNA filaments are located in the fibrillar centers, the interphase counterpart of metaphase NORs, being therefore constituted of ribosomal DNA. The extended, nonnucleosomal configuration of this rDNA has been shown to be independent of transcriptional activity and characterizes ribosome genes which are either transcribed or transcriptionally silent. The data reviewed are consistent with a model of control for ribosome gene activity that is not mediated by changes in chromatin structure; presence in mammalian cells of rDNA always structurally ready for transcription might facilitate a more rapid adjustment of the ribosome production in response to the cell’s metabolic needs.

Key Words: ribosomal genes, nucleolus, chromatin structure in situ, ribosomal transcription, electron microscopy, osmium ammine, mammalian cells

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