Regulation of Ribosomal RNA Production by RNA Polymerase I: Does Elongation Come First?

Ribosomal RNA (rRNA) production represents the most active transcription in the cell. Synthesis of the large rRNA precursors (35–47S) can be achieved by up to 150 RNA polymerase I (Pol I) enzymes simultaneously transcribing each rRNA gene. In this paper, we present recent advances made in understanding the regulatory mechanisms that control elongation. Built-in Pol I elongation factors, such as Rpa34/Rpa49 in budding yeast and PAF53/CAST in humans, are instrumental to the extremely high rate of rRNA production per gene. rRNA elongation mechanisms are intrinsically linked to chromatin structure and to the higher-order organization of the rRNA genes (rDNA). Factors such as Hmo1 in yeast and UBF1 in humans are key players in rDNA chromatin structure in vivo. Finally, elongation factors known to regulate messengers RNA production by RNA polymerase II are also involved in rRNA production and work cooperatively with Rpa49 in vivo

Ultrastructural localization of rRNA shows defective nuclear export of preribosomes in mutants of the Nup82p complex

To study the nuclear export of preribosomes, ribosomal RNAs were detected by in situ hybridization using fluorescence and EM, in the yeast Saccharomyces cerevisiae. In wild-type cells, semiquantitative analysis shows that the distributions of pre-40S and pre-60S particles in the nucleolus and the nucleoplasm are distinct, indicating uncoordinated transport of the two subunits within the nucleus. In cells defective for the activity of the GTPase Gsp1p/Ran, ribosomal precursors accumulate in the whole nucleus. This phenotype is reproduced with pre-60S particles in cells defective in pre-rRNA processing, whereas pre-40S particles only accumulate in the nucleolus, suggesting a tight control of the exit of the small subunit from the nucleolus. Examination of nucleoporin mutants reveals that preribosome nuclear export requires the Nup82p–Nup159p–Nsp1p complex. In contrast, mutations in the nucleoporins forming the Nup84p complex yield very mild or no nuclear accumulation of preribosome. Interestingly, domains of Nup159p required for mRNP trafficking are not necessary for preribosome export. Furthermore, the RNA helicase Dbp5p and the protein Gle1p, which interact with Nup159p and are involved in mRNP trafficking, are dispensable for ribosomal transport. Thus, the Nup82p–Nup159p–Nsp1p nucleoporin complex is part of the nuclear export pathways of preribosomes and mRNPs, but with distinct functions in these two processes

Excessive rDNA Transcription Drives the Disruption in Nuclear Homeostasis during Entry into Senescence in Budding Yeast

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A new function for the yeast trehalose-6P synthase (Tps1) protein, as key pro-survival factor during growth, chronological ageing, and apoptotic stress

Looking back to our recent work that challenged the paradigm of trehalose in stress resistance in yeast, our objective was to revisit the role of this disaccharide in chronological life span (CLS), and in the control of apoptosis. Using a catalytically dead variant of the trehalose-6-phosphate synthase (Tps1) protein, (the first enzyme in the trehalose biosynthetic pathway), and by manipulating intracellular trehalose independently of this pathway, we demonstrated that trehalose has no role in CLS or in the inhibition of acetic acid or H 20 2-triggered cell death. We showed instead that, in the absence of any apoptotic stimulus, the Tps1 protein itself was necessary in preventing massive, spontaneous commitment of yeast cells to apoptosis during growth. Without Tps1p, the life span was shortened and cells were sensitized to acetic acid (AA) and H 20 2, whereas the overexpression of the inactive variant of Tps1p almost abolished AA-triggered apoptosis. Genetic interaction analysis of TPS1 and genes such as YCA1, NUC1 and AIF1 indicated that these key executioners of cell death partially relayed tps1Δ-triggered signaling. Our results suggested that the pro-survival role of Tps1p could be connected with its ability to preserve ATP levels in yeast cells

Retraction notice to “A new function for the yeast Trehalose-6P Synthase (Tps1) Protein, as key prosurvivalfactor during growth, chronological ageing, and apoptotic stress”

Withdrawal announcementThis article has been retracted at the request of Marie-Ange Teste, Isabelle Léger-Silvestre, Jean M François andJean-Luc Parrou. Marjorie Petitjean could not be reached.The corresponding author identified major issues, brought them to the attention of the Journal.These issues span from significant errors in the Material and Methods section of the article and major flaws incytometry data analysis to data fabrication on the part of one of the authors.Given these errors, the retracting authors state that the only responsible course of action would be to retract thearticle, to respect scientific integrity and maintain the standards and rigor of literature from the retracting authors'group as well as the Journal.The retracting authors sincerely apologize to the readers and editors

Structure-function analysis of hmo1 unveils an ancestral organization of hmg-box factors involved in ribosomal dna transcription from yeast to human

Ribosome biogenesis is a major metabolic effort for growing cells. In Saccharomyces cerevisiae, Hmo1, an abundant high-mobility group box protein (HMGB) binds to the coding region of the RNA polymerase I transcribed ribosomal RNAs genes and the promoters of similar to 70% of ribosomal protein genes. In this study, we have demonstrated the functional conservation of eukaryotic HMGB proteins involved in ribosomal DNA (rDNA) transcription. We have shown that when expressed in budding yeast, human UBF1 and a newly identified Sp-Hmo1 (Schizosaccharomyces pombe) localize to the nucleolus and suppress growth defect of the RNA polymerase I mutant rpa49-Delta. Owing to the multiple functions of both proteins, Hmo1 and UBF1 are not fully interchangeable. By deletion and domains swapping in Hmo1, we identified essential domains that stimulate rDNA transcription but are not fully required for stimulation of ribosomal protein genes expression. Hmo1 is organized in four functional domains: a dimerization module, a canonical HMGB motif followed by a conserved domain and a C-terminal nucleolar localization signal. We propose that Hmo1 has acquired species-specific functions and shares with UBF1 and Sp-Hmo1 an ancestral function to stimulate rDNA transcription