Reaching for the off switch in nucleolar dominance

Abstract

SUMMARYNucleolus organizer regions (NORs) are eukaryotic chromosomal loci where ribosomal RNA (rRNA) genes are clustered, typically in hundreds to thousands of copies. Transcription of these rRNA genes by RNA polymerase I and processing of their transcripts results in the formation of the nucleolus, the sub‐nuclear domain in which ribosomes are assembled. Approximately 90 years ago, cytogenetic observations revealed that NORs inherited from the different parents of an interspecific hybrid sometimes differ in morphology at metaphase. Fifty years ago, those chromosomal differences were found to correlate with differences in rRNA gene transcription and the phenomenon became known as nucleolar dominance. Studies of the past 30 years have revealed that nucleolar dominance results from selective rRNA gene silencing, involving repressive chromatin modifications, and occurs in pure species as well as hybrids. Recent evidence also indicates that silencing depends on the NOR in which an rRNA gene is located, and not on the gene's sequence. In this perspective, we discuss how our thinking about nucleolar dominance has shifted over time from the kilobase scale of individual genes to the megabase scale of NORs and chromosomes and questions that remain unanswered in the search for a genetic and biochemical understanding of the off switch.

Functions of RNAi Pathways in Ribosomal RNA Regulation

“…Nucleolar dominance is observed in animal and plant interspecific hybrids, for example, in Arabidopsis suecica, a hybrid of A. thaliana and A. arenosa species [67]. A NOR comprising hundreds of rRNA genes from A. thaliana is repressed, while rRNA genes from the A. arenosa NOR are transcribed [65,68]. There is reason to believe that nucleolar dominance reflects a general mechanism of rRNA expression control, also observed in non-hybrid organisms, providing rRNA level adjustment depending on the need for ribosomes and protein synthesis.…”

Sequence and epigenetic landscapes of active and silent nucleolus organizer regions in Arabidopsis

“…In this line, the transposed NOR2 genes escaped silencing ( 22 ). Collectively, these findings indicated that rRNA gene activity is not dictated by individual gene sequences but, instead, somehow depends on NOR affiliation, highlighting a need to better understand the functional organizations of A. thaliana NORs and eukaryotic NORs in general ( 23 , 24 ). In the case of A. thaliana , complete NOR sequences have proven difficult to assemble, with the longest contigs (overlapping sequencing reads representing a contiguous region) before our study being limited to the edges of the NORs and consisting of fewer than 50 rRNA genes ( 25 – 27 ).…”