Organization of Ribosomal RNA Genes in Eukaryotes

In eukaryotes, ribosomal RNA genes (rDNA) encode the major ribosomal RNA molecules and are typically organized into a distinctive genomic structure involving tandem repetition of a polycistronic coding region. Here, we review eukaryotic rDNA organization, emphasizing the extensive, but not absolute, conservation of the canonical organization across eukaryotes. We describe exceptions to both the canonical gene structure and tandem organization and discuss important questions and hypotheses raised by this conserved organization, which is also tolerant of alternatives. Furthermore, we examine variations in rDNA copy/array number and their implications, including whether variants are likely to be adaptive, a passive consequence of ongoing cellular processes, or the outcome of interrepeat competition within rDNA arrays. Finally, we describe how recent technological advances offer great promise for overcoming the barriers imposed by the repetitive nature of rDNA and thus for improving our understanding of this fascinating region of the genome.