Structural and functional organization of ribosomal genes within the mammalian cell nucleolus.
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 a 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 that is lacking in the loose agglomerates of extended DNA filaments. In fact, these filaments constantly show a thickness of 2-3 nm, the same as a DNA double-helix molecule. The loose agglomerates of DNA filaments are located in the fibrillar centers, the interphase counterpart of metaphase NORs, therefore being constituted by ribosomal DNA. The extended, non-nucleosomal configuration of this rDNA has been shown to be independent of transcriptional activity and characterizes ribosome genes that are either transcribed or transcriptionally silent. Data reviewed are consistent with a model of control for ribosome gene activity that is not mediated by changes in chromatin structure. The presence of rDNA in mammalian cells always structurally ready for transcription might facilitate a more rapid adjustment of the ribosome production in response to the metabolic needs of the cell.
Derenzini
Massimo
M
Dipartimento di Patologia Sperimentale, Via S. Giacomo 14, 40126 Bologna, Italy. massimo.derenzini@unibo.it
Pasquinelli
Gianandrea
G
O'Donohue
Marie-Françoise
MF
Ploton
Dominique
D
Thiry
Marc
M
D000818:Animals; D002466:Cell Nucleolus / Q000235:genetics* / Q000378:metabolism / Q000648:ultrastructure; D002843:Chromatin / Q000235:genetics / Q000648:ultrastructure; D004275:DNA, Ribosomal / Q000737:chemistry; D006801:Humans; D008322:Mammals; D008854:Microscopy, Electron; D009690:Nucleic Acid Conformation; D012322:RNA Precursors / Q000096:biosynthesis; D012335:RNA, Ribosomal / Q000096:biosynthesis / Q000235:genetics; D012270:Ribosomes / Q000235:genetics*; D014158:Transcription, Genetic
D002843:Chromatin; D004275:DNA, Ribosomal; D012322:RNA Precursors; D012335:RNA, Ribosomal