The scs and scs 9 Insulator Elements Impart a cis Requirement on Enhancer –

sites from the Drosophila gypsy transposon and the The Xenopus rRNA enhancer activates its cognate chicken b-globin insulator, have been suggested. These promoter when the two elements are placed on oppoinclude higher order chromatin structure (Udvardy et al., site rings of dimeric catenanes. Here we show that 1985; Kellum and Schedl, 1991; Vasquez and Schedl, when scs elements flank either the enhancer or pro1994), tethering to restrict DNA movement (Chung et moter in catenanes, the enhancer cannot activate the al., 1993), blocking a process that facilitates enhancer promoter on the ring in trans. A series of catenanes activation (Chung et al., 1993; Morcillo et al., 1996), and containing differentpermutations of the insulators, enacting as a decoy or trap for the enhancer (Geyer, 1997). hancer, and promoters shows that when insulators Although the available data do not unambiguously disare present, the enhancer is permitted to activate the tinguish among these models, it is clear that the insulator promoter only when both elements are on the same elements do not form a repressive chromatin structure piece of DNA with no intervening insulator. These rethat prevents accessibility of transcription factors or sults suggest that insulators have the potential to recombinases to the DNA (Cai and Levine, 1995; Scott block enhancer–promoter interactions between chroand Geyer, 1995; Dunaway et al., 1997). mosomes and between independent topological doDimeric catenane analysis addresses whether a conmains within a chromosome. tinuous path of DNA is required between two DNA ele-