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

Abstract

S U M M A R Y 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 doublehelix 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. (J Histochem Cytochem 54:131-145, 2006)

Location of rRNA Transcription to the Nucleolar Components: Disappearance of the Fibrillar Centers in Nucleoli of Regenerating Rat Hepatocytes

“…There is evidence that this silver staining procedure allows the detection of active ribosomal genes in situ (Howell, 1982) by specifically staining a set of acidic proteins, the major being nucleolin (Roussel et al, 1992) which, in fact, is associated with the rDNA (Mongelard and Bouvet, 2007). During interphase, at electron microscopic level, this reaction is confined to the nucleolar fibrillar components (Hernandez-Verdun, 1986) where the silver grains co-localize with the rDNA, in a completely extended, open configuration (Derenzini et al, 2006). After silver staining of histological sections of liver from sham-operated rat, the hepatocyte nucleoli were black-stained (Fig.…”

“…It has been proposed that these changes are the consequence of the fact that the ribosomal genes which are, in the inner portion of the FCs, transcriptionally inactive and nevertheless in a ready-to-be-transcribed configuration, become progressively active in synthesizing the rRNA precursors. These rRNA precursors, by steric hindrance, progressively separate the still inactive ribosomal genes located in the FCs, thus giving rise to progressively smaller FCs (Derenzini et al, 2006). The observation that the stimulation of rRNA synthesis is inversely related to the size of FCs might allow to suggest that when activation of ribosome genes occurs, they, by becoming covered by the rRNA transcripts, constitute a portion of the DFC thus indicating this nucleolar component as the site of transcription.…”

“…Ultrastructural studies have shown that in mammalian cell nucleoli, a part from intranucleolar chromatin, three main components are constantly present: the fibrillar centers (FCs), which are light electron-opaque, roundish structures composed by thin, loosely distributed fibrils, the dense fibrillar component (DFC) constituted by tightly packed fibrils, which surround and are intimately associated to the fibrillar centers, and the granular component (GC), constituted by granules with a diameter of 15-20 nm, which embeds both fibrillar components (Hadjiolov, 1985). There is evidence that ribosomal genes in an extended, ready-to-be-transcribed configuration are mainly located in the FCs and, in part, within the DFC (Derenzini et al, 1993a;Mosgoeller et al, 1993) and all the substances necessary for rRNA transcription are present both in the FCs and in the DFC (Derenzini et al, 2006). In the DFC nascent pre-rRNA molecules undergo to early processing and maturation reactions which continue in the GC where pre-ribosomes in an advanced stage of maturation are located (Scheer and Hock, 1999).…”

Structure of the nucleoli in domestic cattle spermatocytes

“…The process is enabled by the fibrillar centres (FC) in which copies of ribosomal gene precursors are transcribed -dense fibrillar components indispensable for the transformation of precursors into target 18S, 5,8S and 28S rRNA molecules and the granular element that connects with pre-rRNA and rRNA, including the rRNA synthesized outside the nucleolus, which enables the formation of ribonucleoprotein molecules that are precursors of smaller and larger subunits of the mature ribosome. The resultant precursors of ribosome subunits are transported to the cytoplasm where they mature [1][2][3].…”

Age-Dependent Change in the Morphology of Nucleoli and Methylation of Genes of the Nucleolar Organizer Region in Japanese Quail (<I>Coturnix japonica</I>) Model (Temminck and Schlegel, 1849) (Galliformes: Aves)

“…Nucleoli in young cells are regularly shaped, while in the ageing cell their shape becomes irregular (SHAW & JORDAN 1995;K£YSZEJKO-STEFANOWICZ 2002). The shape of nucleoli is also affected by physiological or pathological states in the organism (OLSON et al 2002;DERENZINI et al 2005;HER-NANDEZ-VERDUN 2006). Nucleoli are present in the nuclei of nearly all eukaryotic cells because they contain housekeeping genes, with the exception of sperm cells and mature erythrocytes in all vertebrates (K£YSZEJKO-STEFANOWICZ 2002;RAŠKA et al 2006).…”