Long-distance interactions between enhancers and promoters.

Abdominal-B (Abd-B) is a complex homeotic gene with a difficult task: one transcript determines the identity of four different abdominal segments throughout development in Drosophila. Although an increasing amount of information is available about the structure and the functioning of the regulatory regions that determine the expression pattern of Abd-B, it is still not clear how these regulatory regions can contact the distantly located (several tens of kilobases away) promoter in the nucleus, what mechanism restricts promiscuous enhancers to this specific interaction, and how different regulatory regions replace one another at the same promoter in subsequent abdominal segments. Moreover, several of these regulatory regions have to act over chromatin domain boundaries and extensive inactive chromatin domains, similarly to the situation found in the chicken beta-globin cluster. In this minireview we survey mechanisms and factors that may be involved in mediating specific interactions between the Abd-B promoter and its regulatory regions.