Together we are on together we are off -a conserved rule for ribosomal RNA (rRNA) gene regulation?
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REVIEW ARTICLE
Together we are on together we are off -a conserved rule for ribosomal RNA (rRNA) gene regulation? Gargi Prasad Saradadevi1 • Neha Priyadarshini1 • Aveepsha Bera1 • Gireesha Mohannath1 Received: 14 July 2020 / Accepted: 15 September 2020 Ó Society for Plant Biochemistry and Biotechnology 2020
Abstract Eukaryotic cells carry several hundreds of ribosomal RNA (rRNA) genes which are clustered as tandem repeats at chromosomal loci called nucleolus organizer regions (NORs). The number of rRNA genes (dosage) that are transcriptionally active at any stage of development depends on the levels of cellular demands for protein synthesis. Although it was known for quite some time that, across eukaryotes, about 50% of rRNA genes are transcriptionally silenced during the development, how the choice is made to selectively silence a subset of rRNA genes was not understood. In this review, we summarize our recent findings from model plant Arabidopsis, which led to a new hypothesis that, rRNA genes are regulated as a cluster at the level of NORs, not at the level of individual genes. And, the chromosomal location, not the sequence, dictates the activity status of rRNA genes. We also briefly summarize findings from other eukaryotic model systems which appear to support the new hypothesis on rRNA dosage control. Keywords Arabidopsis Nucleolus organizer region (NOR) Nucleolar dominance 45S rRNA gene Gene regulation Gene conversion Abbreviations NOR Nucleolus organizer region ND Nucleolar dominance ITS Internal transcribed spacer ETS External transcribed spacer ASC1 Altered subtype content 1 rRNA Ribosomal ribonucleic acid IGS Intergenic spacer SNP Single nucleotide polymorphism ATXR Arabidopsis trithorax related proteins HDA Histone deacetylase CAPS Cleaved amplified polymorphic sequences dCAPS Derived cleaved amplified polymorphic sequences RT-PCR Reverse transcriptase polymerase chain reaction
Gargi Prasad Saradadevi, Neha Priyadarshini and Aveepsha Bera contributed equally. & Gireesha Mohannath [email protected] 1
Department of Biological Sciences, Birla Institute of Technology and Science, Pilani, Hyderabad Campus, Hyderabad, Telangana, India
FANoS RIL
Fluorescence-activated nucleolus sorting Recombinant inbred line
Introduction Ribosomes are non-membranous cellular organelles that perform the essential function of protein synthesis across taxonomic kingdoms. In eukaryotes, they are composed of four catalytic ribosomal RNA (rRNA) and approximately 80 protein subunits (Pikaard 2018). Eukaryotic genomes encode hundreds to thousands of rRNA genes that are tandemly arrayed at chromosomal locations called nucleolus organizer regions (NORs) (Fig. 1). NORs are named so because rRNA transcription and processing causes the nucleolus to form during interphase in each cell cycle (McClintock 1934; Ritossa and Spiegelman 1965; Wallace and Birnstiel 1966; Raska et al. 2006a, b). NORs appear as secondary constrictions during metaphase, the centromere being the primary constriction (McClintock 193
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