Nucleolus: the fascinating nuclear body

Nucleoli are the prominent contrasted structures of the cell nucleus. In the nucleolus, ribosomal RNAs are synthesized, processed and assembled with ribosomal proteins. RNA polymerase I synthesizes the ribosomal RNAs and this activity is cell cycle regulated. The nucleolus reveals the functional organization of the nucleus in which the compartmentation of the different steps of ribosome biogenesis is observed whereas the nucleolar machineries are in permanent exchange with the nucleoplasm and other nuclear bodies. After mitosis, nucleolar assembly is a time and space regulated process controlled by the cell cycle. In addition, by generating a large volume in the nucleus with apparently no RNA polymerase II activity, the nucleolus creates a domain of retention/sequestration of molecules normally active outside the nucleolus. Viruses interact with the nucleolus and recruit nucleolar proteins to facilitate virus replication. The nucleolus is also a sensor of stress due to the redistribution of the ribosomal proteins in the nucleoplasm by nucleolus disruption. The nucleolus plays several crucial functions in the nucleus: in addition to its function as ribosome factory of the cells it is a multifunctional nuclear domain, and nucleolar activity is linked with several pathologies. Perspectives on the evolution of this research area are proposed

Mild temperature shock affects transcription of yeast ribosomal protein genes as well as the stability of their mRNAs.

Shifting the temperature of a yeast culture from 23 degrees to 36 degrees C results in a sudden and severe (greater than 85%) decline in the cellular levels of ribosomal protein (rp-)mRNAs. Recovery during continued growth at 36 degrees C occurs within 1 h. The use of hybrid genes carrying different portions of the region upstream of the gene coding for ribosomal protein L25 revealed that this characteristic, coordinate temperature shock phenomenon does not depend on the presence of specific upstream DNA sequences. Analysis of a heterologous gene carrying a synthetic UASrpg (upstream activation site of yeast ribosomal protein genes) provided conclusive evidence that the rp-characteristic, transient heat shock response is not mediated through the UASrpg elements. The addition of the transcription inhibitor 1,10-phenantroline prior to a 23 degrees to 36 degrees C heat shock inhibited the severe decline of the rp-mRNA levels. The latter observation indicates that transcription is required for the rp-gene- specific response to heat shock. A milder temperature shift, from 23 degrees to 30 degrees C, gave rise to a two-fold decrease in mRNA levels for all genes studied, both ribosomal and non-ribosomal. Together, these results indicate that a temperature shift causes a temporary general transcriptional arrest in yeast cells, resulting in an over-all decrease in mRNA levels. In addition, an enhanced nucleolytic break-down of pre-existing rp-mRNAs accounts for the dramatic drop in the steady state amounts of these mRNAs observed upon a 23 degrees----36 degrees C shift. This enhanced breakdown is caused directly or indirectly by a factor whose synthesis is induced by the heat shock treatment

Fine mapping of 28S rRNA sites specifically cleaved in cells undergoing apoptosis.

Bona fide apoptosis in rat and human leukemia cells, rat thymocytes, and bovine endothelial cells was accompanied by limited and specific cleavage of polysome-associated and monosome-associated 28S rRNA, with 18S rRNA being spared. Specific 28S rRNA cleavage was observed in all instances of apoptotic death accompanied by internucleosomal DNA fragmentation, with cleavage of 28S rRNA and of DNA being linked temporally. This indicates that 28S rRNA fragmentation may be as general a feature of apoptosis as internucleosomal DNA fragmentation and that concerted specific cleavage of intra- and extranuclear polynucleotides occurs in apoptosis. Apoptosis-associated cleavage sites were mapped to the 28S rRNA divergent domains D2, D6 (endothelial cells), and D8. The D2 cuts occurred in hairpin loop junctions considered to be buried in the intact ribosome, suggesting that this rRNA region becomes a target for RNase attack in apoptotic cells. D8 was cleaved in two exposed UU(U) sequences in bulge loops. Treatment with agents causing necrotic cell death or aging of cell lysates failed to produce any detectable limited D2 cleavage but did produce a more generalized cleavage in the D8 region. Of potential functional interest was the finding that the primary cuts in D2 exactly flanked a 0.3-kb hypervariable subdomain (D2c), allowing excision of the latter. The implication of hypervariable rRNA domains in apoptosis represents the first association of any functional process with these enigmatic parts of the ribosomes