Insight into how the mammalian genome is structured in vivo is key to understanding transcriptional regulation. This is especially true in complex domains in which genes are coordinately regulated by long-range interactions between cis -regulatory elements. The regulation of the H19 / Igf2 imprinted region depends on the presence of several cis -acting sequences, including a methylation-sensitive insulator between Igf2 and H19 and shared enhancers downstream of H19 . Each parental allele has a distinct expression pattern. We used chromosome conformation capture to assay the native three-dimensional organization of the H19 / Igf2 locus on each parental copy. Furthermore, we compared wild-type chromosomes to several mutations that affect the insulator. Our results show that promoters and enhancers reproducibly co-localize at transcriptionally active genes, i.e. the endodermal enhancers contact the maternal H19 and the paternal Igf2 genes. The active insulator blocks traffic of the enhancers along the chromosome, restricting them to the H19 promoter. Conversely, the methylated inactive insulator allows the enhancers to contact the upstream regions, including Igf2 . Mutations that either remove or inhibit insulator activity allow unrestricted access of the enhancers to the whole region. A mutation that allows establishment of an enhancer-blocker on the normally inactive paternal copy diminishes the contact of the enhancer with the Igf2 gene. Based on our results, we propose that physical proximity of cis -acting DNA elements is vital for their activity in vivo . We suggest that enhancers track along the chromosome until they find a suitable promoter sequence to interact with and that insulator elements block further tracking of enhancers.

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