The cell nucleus was first described by Antonie van Leeuwenhoek more than 300 years ago. To date, however, the subnuclear organization, and the functional significance of this compartmentalization, is still poorly understood.

Many nuclear factors are localized in distinct structures, such as Cajal bodies and nucleoli. On page 605, Angus Lamond and David Spector discuss speckles — dynamic nuclear structures that are enriched in pre-messenger RNA splicing factors. The authors propose a model for the basal exchange rate of speckle factors, in which the movement of components between the speckles and the nucleoplasm is regulated by reversible protein phosphorylation.

Another striking example of the functional significance of nuclear compartmentalization is provided by ribosomal RNA gene transcription. This process leads to the formation of nucleoli — the sites of ribosome biogenesis. The various ways in which rRNA gene transcription can be silenced is tackled by Ingrid Grummt and Craig Pikaard (page 641). They describe mechanisms that involve DNA methylation and histone modifications, which cause chromatin remodelling and control the ratio of active and inactive rRNA genes.

There are approximately two metres of DNA in a human cell. So how is this DNA compacted into the nuclear compartment? The machinery that is responsible for the assembly of DNA into chromatin consists of histone chaperones and ATP-driven motor proteins. On page 613, Karl Haushalter and James Kadonaga discuss possible models for how the energy of ATP hydrolysis might be used to assemble periodic arrays of nucleosomes.

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