A Drosophila rRNA gene located in euchromatin is active in transcription and nucleolus formation.

Karpen, Gary H.; Schaefer, John E.; Laird, Charles D.

doi:10.1101/gad.2.12b.1745

Cited by 159 publications

(91 citation statements)

References 93 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Using a yeast rDNA deletion mutant, Oakes et al (1998) found that transcription of rDNA by Pol I from a plasmid induced multiple small dense structures, "mininucleoli," whereas transcription of rDNA by Pol II formed an aggregated larger structure. Insertion of an actively transcribing rDNA repeat into Drosophila polytene chromosomes resulted in dense mininucleoli around the transcription sites (Karpen et al, 1988). These findings demonstrate that transcription of rDNA alone generates dense structures that do not resemble typical nucleoli with well-organized and classically defined structural features.…”

Section: Introductionmentioning

confidence: 64%

Initiation of Nucleolar Assembly Is Independent of RNA Polymerase I Transcription

Dousset

Wang

Verheggen

et al. 2000

MBoC

125

122

View full text Add to dashboard Cite

This report examines the distribution of an RNA polymerase I transcription factor (upstream binding factor; UBF), pre-rRNA processing factors (nucleolin and fibrillarin), and pre-rRNAs throughout mitosis and postmitotic nucleologenesis in HeLa cells. The results demonstrate that nucleolin, fibrillarin, and pre-rRNAs synthesized at G2/M phase of the previous cell cycle are directly recruited to UBF-associated nucleolar organizer regions (NORs) early in telophase before chromosome decondensation. Unlike the fusion of prenucleolar bodies to the nucleoli, this early recruitment of processing factors and pre-rRNAs is independent of RNA polymerase I transcription. In the absence of polymerase I transcription, the initial localization of nucleolin, fibrillarin, and pre-rRNAs to UBF-associated NORs generates segregated mininucleoli that are similar to the larger ones observed in interphase cells grown under the same conditions. Pre-rRNAs are juxtaposed to UBF-nucleolin-fibrillarin caps that may represent the segregated nucleoli observed by electron microscopy. These findings lead to a revised model of nucleologenesis. We propose that nucleolar formation at the end of mitosis results from direct recruitment of processing factors and pre-rRNAs to UBF-associated NORs before or at the onset of rDNA transcription. This is followed by fusion of prepackaged prenucleolar bodies into the nucleolus. Pre-ribosomal ribonucleoproteins synthesized in the previous cell cycle may contribute to postmitotic nucleologenesis. INTRODUCTIONThe nucleus is a complex structure within which various functions and components are spatially and temporally organized and interregulated (reviewed by Lamond and Earnshaw, 1998;Dreyfuss and Struhl, 1999). Nucleoli represent the most prominent example of this organization. The major function of the nucleolus is ribosome biogenesis (reviewed by Busch and Smetana, 1970). rRNAs are synthesized, processed, cleaved, and assembled with ribosomal proteins in the nucleolus before export to the cytoplasm. The nucleolus is composed of three structurally distinguishable constituents: fibrillar centers, dense fibrillar components, and granular components, as observed by electron microscopy (reviewed by Busch and Smetana, 1970;Hadjiolov, 1985). Although the spatial and temporal relationship between rRNA synthesis and the three structural constituents remains to be clarified, it is generally thought that transcription of rDNA takes place in a transient area between the fibrillar centers and the dense fibrillar components and that rRNA processing and preribosomal particle assembly progress from the dense fibrillar components to the surrounding granular components (reviewed by Spector et al., 1993;Shaw and Jordan, 1995;Scheer and Hock, 1999). More recent evidence (reviewed by Pederson, 1998;Garcia and Pillus, 1999) suggests that nucleoli have additional functions related to the cell cycle Visintin et al., 1999), cellular aging (Straight et al., 1999), signal recognition particle biosynthesis (Jacobson and Pederson, 1998),...

show abstract

Section: Introductionmentioning

confidence: 64%

Initiation of Nucleolar Assembly Is Independent of RNA Polymerase I Transcription

Dousset

Wang

Verheggen

et al. 2000

MBoC

125

122

View full text Add to dashboard Cite

show abstract

“…The presence of fibrillarin around ectopic rDNAs suggests that transcription and processing of ribosomal RNA occurs in ectopic nucleoli. Similarly, ectopically integrated rDNA forms functional `mini-nucleoli' 10 and rescues defects in X-Y pairing in male meiosis caused by endogenous rDNA deletion35. These observations suggest that increased nucleolar volumes and ecc rDNA do not cause significant growth abnormalities.…”

Section: Discussionmentioning

confidence: 99%

“…In combined IF-FISH experiments, immunofluorescence was performed before the FISH treatment. FISH probes were made by nick translation and terminal labelling, using previously described materials 10 .…”

Section: Fish and If-fishmentioning

confidence: 99%

“…The nucleolus organizer region (NOR) contains tandemly repeated ribosomal DNAs (rDNAs), and is embedded in heterochromatin in most eukaryotes. Single rDNA genes inserted into euchromatin are able to form mini-nucleoli through a self-assembly process that is likely to be initiated by rRNA transcription 10 . Epigenetic regulation of rDNA transcription is well documented in yeast, plant and mammalian systems -for example, a mammalian nucleolar remodelling complex (NoRC) regulates heterochromatin formation and rRNA expression by controlling histone H4 deacetylation, H3K9 dimethylation and de novo DNA methylation at rDNA 11 .…”

mentioning

confidence: 99%

See 1 more Smart Citation

H3K9 methylation and RNA interference regulate nucleolar organization and repeated DNA stability

2006

View full text Add to dashboard Cite

Investigations aimed at identifying regulators of nuclear architecture in Drosophila demonstrated that cells lacking H3K9 methylation and RNA interference (RNAi) pathway components displayed disorganized nucleoli, ribosomal DNA (rDNA) and satellite DNAs. The levels of H3K9 dimethylation (H3K9me2) in chromatin associated with repeated DNAs decreased dramatically in Su(var)3-9 and dcr-2 (dicer-2) mutant tissues compared with wild type. We also observed a substantial increase in extrachromosomal circular (ecc) repeated DNAs in mutant tissues. The disorganized nucleolus phenotype depends on the presence of Ligase 4 and ecc DNA formation is not induced by removal of cohesin. We conclude that the structural integrity and organization of repeated DNAs and nucleoli are regulated by the H3K9 methylation and RNAi pathways, and other regulators of heterochromatin-mediated silencing. In addition, repeated DNA stability involves suppression of non-homologous end joining (NHEJ) or other recombination pathways. These results suggest a mechanism for how local chromatin structure can regulate genome stability, and the organization of chromosomal elements and nuclear organelles.Nuclei and chromosomes maintain specific and dynamic architectures that are required for many essential functions 1 . Nuclear bodies are involved in diverse biological processes and exhibit dynamic mobility, and individual chromosomes occupy distinct domains within interphase nuclei2. Chromosomes in the metazoan interphase nucleus are comprised of two types of cytologically and functionally distinct chromatin, euchromatin and heterochromatin3. Patterns of posttranslational histone modifications associated with these domains are strongly correlated with functions such as gene regulation, chromosome inheritance and replication timing4. For example, regions that display heterochromatinmediated gene silencing are rich in histone H3K9 methylation and lack many histone acetylations, whereas histones in transcriptionally active euchromatic regions are highly acetylated and methylated at H3K4 (ref. 5).The first indication that chromosome organization can affect gene expression stems from the discovery of position effect variegation (PEV) in Drosophila 6 . PEV describes the epigenetic inactivation or silencing of a euchromatic gene that has been positioned close to or within ©2007 Nature Publishing Group 3 Correspondence should be addressed to G.H.K. (karpen@fruitfly.org). AUTHOR CONTRIBUTIONS I.P. performed all the experiments, and J.P. and G.K. collaborated on data analysis and project planning.Note: Supplementary Information is available on the Nature Cell Biology website. COMPETING FINANCIAL INTERESTSThe authors declare that they have no competing financial interests. The nucleolus, the site of ribosome assembly, is an example of an essential nuclear organelle. The nucleolus organizer region (NOR) contains tandemly repeated ribosomal DNAs (rDNAs), and is embedded in heterochromatin in most eukaryotes. Single rDNA genes inserted into euchromatin are able...

show abstract

“…[rib7] contains one complete rDNA transcription unit flanked by IGS regions containing 11 total 240-bp repeats, 8 repeats on the 59 side and 3 repeats on the 39 side (Karpen et al 1988;Figure 1, A and B).…”

Section: Generation Of Clones and Transgenesmentioning

confidence: 99%

Meiotic Pairing and Disjunction of Mini-X Chromosomes in Drosophila Is Mediated by 240-bp rDNA Repeats and the Homolog Conjunction Proteins SNM and MNM

Thomas

McKee

2007

Genetics

View full text Add to dashboard Cite

In most eukaryotes, segregation of homologous chromosomes during meiosis is dependent on crossovers that occur while the homologs are intimately paired during early prophase. Crossovers generate homolog connectors known as chiasmata that are stabilized by cohesion between sister-chromatid arms. In Drosophila males, homologs pair and segregate without recombining or forming chiasmata. Stable pairing of homologs is dependent on two proteins, SNM and MNM, that associate with chromosomes throughout meiosis I until their removal at anaphase I. SNM and MNM localize to the rDNA region of the X-Y pair, which contains 240-bp repeats that have previously been shown to function as cis-acting chromosome pairing/segregation sites. Here we show that heterochromatic mini-X chromosomes lacking native rDNA but carrying transgenic 240-bp repeat arrays segregate preferentially from full-length sex chromosomes and from each other. Mini-X pairs do not form autonomous bivalents but do associate at high frequency with the X-Y bivalent to form trivalents and quadrivalents. Both disjunction of mini-X pairs and multivalent formation are dependent on the presence of SNM and MNM. These results imply that 240-bp repeats function to mediate association of sex chromosomes with SNM and MNM. P AIRING of homologous chromosomes is an essential step in meiosis, setting the stage for the segregation of homologs to opposite poles (Page and Hawley 2003;McKee 2004). In most eukaryotes, pairing is accompanied by high frequencies of recombination and by formation of synaptonemal complexes, proteinaceous structures that connect homologs from end to end during the pachytene stage of meiotic prophase. Segregation of homologs requires not only that they pair but also that they undergo at least one crossover. Crossovers generate cytologically visible linkers known as chiasmata, which are essential to maintaining bivalent integrity throughout late prophase I and metaphase I (Hawley 1988).However, synaptonemal complexes, crossing over, and chiasmata are not universal prerequisites for meiotic chromosome segregation. Achiasmate segregation, i.e., segregation of homologs without chiasmata, is found in one sex or the other in several groups of eukaryotes (Wolf 1994). In Drosophila male meiosis, crossing over is normally completely absent and the homologs fail to form either synaptonemal complexes or chiasmata, yet homologs are stably conjoined during late prophase I and metaphase I and segregate from one another with great regularity during anaphase I.Recent data show that stable conjunction and accurate segregation of all four homolog pairs in Drosophila male meiosis depends upon two proteins: stromalin in meiosis (SNM), a homolog of the SCC3/SA cohesin proteins, and Mod(mdg4) in meiosis (MNM), a BTB domain protein (Thomas et al. 2005). SNM and MNM colocalize to meiotic chromosomes throughout prophase I and metaphase I but disappear at the onset of anaphase I, suggesting that they function directly in stabilization of pairing, i.e., as substitutes for ch...

show abstract

A Drosophila rRNA gene located in euchromatin is active in transcription and nucleolus formation.

Cited by 159 publications

References 93 publications

Initiation of Nucleolar Assembly Is Independent of RNA Polymerase I Transcription

Initiation of Nucleolar Assembly Is Independent of RNA Polymerase I Transcription

H3K9 methylation and RNA interference regulate nucleolar organization and repeated DNA stability

Meiotic Pairing and Disjunction of Mini-X Chromosomes in Drosophila Is Mediated by 240-bp rDNA Repeats and the Homolog Conjunction Proteins SNM and MNM

Contact Info

Product

Resources

About