Nucleoli: composition, function, and dynamics.

rDNA structure. 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 multi-molecular complexes. The nucleolus is the most prominent of these structures. It is the site of transcription by RNA polymerase I of the rDNA, tandemly repeated copies of the genes for three of the four ribosomal RNAs (s-rRNA, 5.8s RNA and l-rRNA), which are called the nucleolar organizer regions of the chromosomes or NORs (Raska et al., 2006a; Raska et al., 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 rDNA repeats are separated by untranscribed intergenic sequences, which are much shorter in plants (2-3kb) than in mammals and many other vertebrates (10-30kb) (Hadjiolov, 1985). In many cases the size of the intergenic spacers is heterogeneous, as, for example, in Xenopus laevis (3-9kb). The fourth ribosomal RNA, 5S, is transcribed by RNA polymerase III from tandem repeats elsewhere in the nucleus (Highett et al., 1993). The nucleolus is also the location at which most of the steps of ribosome assembly and maturation occur. This means about 90 ribosomal proteins, as well as many processing and accessory factors, need to be Plant Physiology Preview. Published on November 14, 2011, as DOI:10.1104/pp.111.188052