Dividing the empire: boundary chromatin elements delimit the territory of enhancers

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Abstract
The scs elements and the suppressor of the Hairy‐wing [su(Hw)] protein‐binding region of the gypsy retrotransposon are the best‐characterized insulators. The scs and scs′ elements are defined by two closely spaced nuclease‐hypersensitive (HS) sites arranged around a central nuclease‐resistant segment. Characteristic changes occur in the fine structure of this complex HS region in response to heat induction (Udvardy et al., 1985). Additionally, there are localized topoisomerase II sites in the HS complex, and a redistribution of topoisomerase II takes place on heat induction (Udvardy and Schedl, 1993). When DNA fragments carrying either the scs or the scs′ region were placed between the yolk protein‐1 (yp‐1) enhancer and the hsp70 promoter–LacZ fusion gene, the stage‐, sex‐ and tissue‐specific activation of the reporter gene by the yp‐1 enhancer was completely blocked. The blocking activity was independent of the orientation of the scs elements. No insulation occurred, however, when the scs elements were inserted upstream of the enhancer (Kellum and Schedl, 1992). Furthermore, the expression of a transgene became copy number‐dependent and independent of the site of integration when it was flanked at the 5′ end by an scs element and at the 3′ end by an scs′ element (Kellum and Schedl, 1991). On these criteria, the scs elements are bona fide insulators, because they can fulfill the two most important functions required for a boundary element: insulation of a gene from the influence of an enhancer and protection of a transgene from chromosomal position effect. Insulators, however, do not have enhancer activity.