The nucleolus. Playing by different rules?

The nucleolus, although an integral part of all eukaryotic nuclei, plays by its own rules in many respects. Cytologically, it is the most prominent subnuclear domain; functionally, it is the site of transcription of 3 of the ribosomal RNAs from the tandemly repeated rDNA, and subsequently of ribosome biogenesis; biochemically, it possesses transcriptional and post-transcriptional machinery not shared with the rest of the nucleus. Making the huge number of ribosomes required by the cell represents an enormous investment of metabolic activity, and so nucleolar function can easily become a de facto limit to cell growth: nucleolar activity must also be actively regulated, but the detailed regulatory networks linking the nucleolus with cellular metabolism are still unclear. Several recent reports have now shown that segregation of the rDNA in yeast, along with telomeric DNA, is also controlled differently from the rest of the genome. This short review describes some of the features of the nucleolus and highlights recent progress in understanding this important, but enigmatic, nuclear structure.