Together we are on together we are off -a conserved rule for ribosomal RNA (rRNA) gene regulation?

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.