Repetitive DNA arrays are important structural features of eukaryotic genomes that are often heterochromatinized to suppress repeat instability. It is unclear, however, whether all repeats within an array are equally subject to heterochromatin formation and gene silencing. Here, we show that in starving Saccharomyces cerevisiae, silencing of reporter genes within the ribosomal DNA (rDNA) array ...

Ribosomal DNA (rDNA) plays a key role in ribosome biogenesis, encoding genes for the structural RNA components of this important cellular organelle. These genes are vital for efficient functioning of the cellular protein synthesis machinery and as such are highly conserved and normally present in high copy numbers. In the baker's yeast Saccharomyces cerevisiae, there are more than 100 rDNA repe...

Tandemly repeated ribosomal DNA (rDNA) arrays are among the most evolutionary dynamic loci of eukaryotic genomes. The loci code for essential cellular components, yet exhibit extensive copy number (CN) variation within and between species. CN might be partly determined by the requirement of dosage balance between the 5S and 45S rDNA arrays. The arrays are nonhomologous, physically unlinked in m...

Epigenetic, transcriptional and signaling processes in the nucleolus regulate rRNA transcription and cell growth. We report here that the tumor suppressor ING1b binds rDNA, regulates rDNA chromatin modifications and affects nucleolar localization of mTOR to modulate rRNA levels. ING1 represses rDNA transcription by recruiting HDAC1 to rDNA loci, increasing its association with the NoRC complex ...

The SNF2h-containing chromatin-remodeling complex NoRC is responsible for silencing a fraction of mammalian rRNA genes (rDNA). NoRC silences transcription by establishing heterochromatic features-including DNA methylation, hypoacetylation of histone H4, and methylation of H3K9-at the rDNA promoter []. We have investigated the mechanism of NoRC-mediated rDNA silencing and show that binding of th...

Replication of the 400 copies of the 43 kb human ribosomal RNA (rDNA) locus spans most of the S phase. To examine the basis for the unusual pattern of rDNA replication, a sensitive strategy was developed to map origins of DNA replication and measure apparent rates of fork progression within a chromosomal locus. This technique, termed differential intragenomic replication timing, revealed that i...

Removal of UV-induced pyrimidine dimers from the individual strands of the rDNA locus in Saccharomyces cerevisiae was studied. Yeast rDNA, that is transcribed by RNA polymerase I(RNA pol I), is repaired efficiently, slightly strand-specific and independently of RAD26, which has been implicated in transcription-coupled repair of the RNA pol II transcribed RPB2 gene. No repair of rDNA is observed...

The saturation values for ribosomal RNA hybridization have been determined for DNA from diploid and polytene tissues of Drosophila melanogaster. These values have been measured in XO and XX larvae which have, respectively, one and two nucleolus organizers in the diploid chromosome set. The results show that (1) in diploid cells the ribosomal (r)DNA is present in amounts proportional to the numb...

The Sir2 protein mediates gene silencing and repression of recombination at the rDNA repeats in budding yeast. Here we show that Sir2 executes these functions as a component of a nucleolar complex designated RENT (regulator of nucleolar silencing and telophase exit). Net1, a core subunit of this complex, preferentially cross-links to the rDNA repeats, but not to silent DNA regions near telomere...

The promoters of poised rRNA genes (rDNA) are marked by both euchromatic and heterochromatic histone modifications and are associated with two transcription factors, UBF and SL1 that nucleate transcription complex formation. Active rRNA genes contain only euchromatic histone modifications and are loaded with all components of transcriptional initiation complex including RNA polymerase I. Couple...