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The eukaryotic genome is highly organized in the nucleus. Genes can be localized to specific nuclear compartments in a manner reflecting their activity. A plethora of recent reports has described multiple levels of chromosomal folding that can be related to gene-specific expression states. Here we discuss studies designed to probe the causal impact of genome organization on gene expression. The picture that emerges is that of a reciprocal relationship in which nuclear organization is not only shaped by gene expression states but also directly influences them. [ABSTRACT FROM AUTHOR]

Control of gene expression involves the concerted action of multiple regulatory elements some of which can act over large genomic distances. Physical interaction among these elements can lead to looping of the chromatin fiber. Although posttranslational modifications of chromatin are thought to play a role in the conveyance of epigenetic information, it is largely unknown whether higher order chromatin organization such as looping contributes to epigenetic memory. A related unresolved question is whether chromatin loops are the cause or the effect of transcriptional regulation. Recent work on diverse organisms suggests a memory function for long-range chromatin interactions. It is proposed that higher order folding of the chromatin fiber can serve to maintain active and repressed states of gene expression. [Copyright &y& Elsevier]