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Journal of Microscopy, Vol. 198, Pt 3, June 2000, pp. 199±207. Received 27 September 1999; accepted 31 January 2000

The nucleus: a highly organized but dynamic structure
P. GONZALEZ-MELENDI, A. BEVEN, K. BOUDONCK, R. ABRANCHES, B. WELLS, L. DOLAN & P. SHAW
John Innes Centre, Colney, Norwich NR4 7UH, U.K.

Key words. Chromosome territory, coiled body, green fluorescent protein, fluorescencein situ hybridization, immunogold, nucleus, transcription sites.

Summary
The nucleus in plants and animals is a highly structured organelle containing several well-defined subregions or suborganelles. These include the nucleolus, interphase chromosome territories and coiled bodies. We have visualized transcription sites in plants at both light- and electronmicroscopy level by theincorporation of BrUTP. In the nucleolus many dispersed foci are revealed within the dense fibrillar component, each of which probably corresponds to a single gene copy. In the nucleoplasm there are also many dispersed foci of transcription, but not enough to correspond to one site per transcribed gene. We have shown that in wheat, and probably many other plant species, interphase chromosome territories areorganized in a very regular way, with all the chromosomes in the Rabl configuration, all the centromeres clustered at the nuclear membrane and all the telomeres located at the nuclear membrane on the opposite side of the nucleus. However, despite this regular, polarized structure, there is no sign of polarization of transcription sites, or of any preferred location for them with respect tochromosome territorial boundaries. The nucleus is also highly dynamic. As an example, we have shown by the use of a green fluorescent protein fusion to the spliceosomal protein U2B H H that coiled bodies move and coalesce within the nucleus, and may act as transport structures within the nucleus and nucleolus.

Introduction
It is clear that the nucleus is the site of a multitude of complex biochemicalprocesses including DNA replication, gene transcription and transcript processing. It is also clear that the nucleus has a complex, and still poorly understood, ultrastructure. However, progress is now being made in unravelling nuclear structure and in relating nuclear organization to biochemical functions.
Correspondence to: Dr Peter Shaw. Fax: 1 44 (0)1603 56844; e-mail: shaw@ bbsrc.ac.uk
q2000 The Royal Microscopical Society

Several types of subnuclear compartment are now known, and others probably remain to be discovered (for recent reviews see Strouboulis & Wolffe, 1996; Lamond & Earnshaw, 1998). The most obvious is the nucleolus ± the site of transcription of the ribosomal rDNA and of the biosynthesis of preribosomal particles. The nucleolus in turn is highly organized intodistinct functional regions, which in most plant nucleoli can be interpreted as a multilayered organization of the subsequent steps of prerRNA maturation and ribosome assembly (Shaw & Jordan, 1995; Brown & Shaw, 1998). Mitotic chromosomes have been visualized by microscopy for a very long time, but interphase chromatin structure is not easily interpretable. However, there is accumulating evidencethat interphase chromosomes occupy distinct regions of the nucleus, often referred to as chromosome territories. This has been demonstrated in mammalian cells by the use of specific chromosome in situ `paints' (Cremer et al., 1988; Lichter et al., 1988). It has been further suggested that the division of the intranuclear space into chromosome territories and intrachromosomal spaces is important innuclear activity (Cremer et al., 1993; Cremer et al., 1995). Sites of transcription, and thus active genes, have been proposed to be located at the outside of the chromosome territories, so that RNA transcripts are produced near the intrachromosomal spaces, which would provide an efficient route for export from the nucleus (Kurz et al., 1996). Finally, there are several, as yet poorly understood,...