Update on the Nucleolus Nucleoli: Composition, Function, and Dynamics

The nucleus is a highly organized structure. However, unlike cytoplasmic organelles, nuclear substructures are not bounded by membranes, but rather are held together by interactions between their component proteins and nucleic acids, and are thus probably best regarded as giant, extended multimolecular complexes. The nucleolus is themost prominent of these structures. It is the site of transcription by RNApolymerase I (Pol I) of the rDNA, tandemly repeated copies of the genes for three of the four ribosomal RNAs (rRNAs: s-rRNA, 5.8S-rRNA, and l-rRNA), which are called the nucleolar organizer regions (NORs) of the chromosomes (Raska et al., 2006a, 2006b). The rDNA repeats are transcribed as a single precursor RNA, which is edited into the three rRNAs by excising leading, internal, and trailing transcribed spacer sequences. The rDNArepeats are separated by untranscribed intergenic sequences, which are much shorter in plants (2–3 kb) than in mammals and many other vertebrates (10–30 kb; Hadjiolov, 1985). In many cases the size of the intergenic spacers is heterogeneous, as, for example, in Xenopus laevis (3–9 kb). The fourth rRNA, 5S, is transcribed by RNA Pol III from tandem repeats elsewhere in the nucleus (Highett et al., 1993). The nucleolus is also the location at whichmost of the steps of ribosome assembly and maturation occur. Thismeans about 90 ribosomal proteins, aswell asmany processing and accessory factors, need to be imported from the cytoplasm to the nucleolus, and the nearly complete small and large ribosomal subunits separately exported to the cytoplasm. Since a rapidly growing cell may require millions of ribosomes to be synthesized, the nucleolus is by far the major destination and origin of nucleocytoplasmic transport in the most active cells.