Saccharomyces cerevisiae rDNA as super-hub: the region where replication, transcription and recombination meet
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Cellular and Molecular Life Sciences
REVIEW
Saccharomyces cerevisiae rDNA as super‑hub: the region where replication, transcription and recombination meet Alessandra Egidi1 · Francesca Di Felice1 · Giorgio Camilloni1 Received: 5 February 2020 / Revised: 4 May 2020 / Accepted: 25 May 2020 © Springer Nature Switzerland AG 2020
Abstract Saccharomyces cerevisiae ribosomal DNA, the repeated region where rRNAs are synthesized by about 150 encoding units, hosts all the protein machineries responsible for the main DNA transactions such as replication, transcription and recombination. This and its repetitive nature make rDNA a unique and complex genetic locus compared to any other. All the different molecular machineries acting in this locus need to be accurately and finely controlled and coordinated and for this reason rDNA is one of the most impressive examples of highly complex molecular regulated loci. The region in which the large molecular complexes involved in rDNA activity and/or regulation are recruited is extremely small: that is, the 2.5 kb long intergenic spacer, interrupting each 35S RNA coding unit from the next. All S. cerevisiae RNA polymerases (I, II and III) transcribing the different genetic rDNA elements are recruited here; a sequence responsible for each rDNA unit replication, which needs its molecular apparatus, also localizes here; moreover, it is noteworthy that the rDNA replication proceeds almost unidirectionally because each replication fork is stopped in the so-called replication fork barrier. These localized fork blocking events induce, with a given frequency, the homologous recombination process by which cells maintain a high identity among the rDNA repeated units. Here, we describe the different processes involving the rDNA locus, how they influence each other and how these mutual interferences are highly regulated and coordinated. We propose that an rDNA conformation as a super-hub could help in optimizing the micro-environment for all basic DNA transactions. Keywords rDNA · Genome stability · DNA topoisomerase I · Copy number · Transcriptional regulation · SIR2 · FOB1
Introduction In Saccharomyces cerevisiae, the ribosomal DNA locus (rDNA) represents the region of chromosome XII where 35S and 5S RNAs are synthesized. These ribosomal genes are present in 150–200 copies (depending on the yeast strain), repeated in tandem in a single cluster [1]. Intriguingly, the rDNA locus hosts all the protein apparatuses responsible for the main transactions to which DNA is subjected (i.e., replication, transcription and recombination), thus making this genetic locus unique and complex compared to any other. rDNA is the substrate for the action of so many different Alessandra Egidi and Francesca Di Felice contributed equally to this work. * Giorgio Camilloni [email protected] 1
Dipartimento di Biologia e Biotecnologie, Università di Roma, Sapienza, Rome, Italy
molecular machineries that it is fundamental for all the different processes in which it is involved to be accurately and fine
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