MODULATION OF ENHANCER-PROMOTER INTERACTIONS BY INSULATORS IN THE DROSOPHILA EMBRYO

INSULATOR DNAs, or boundary elements, functionally isolate neighbourin g genes by blocking interactions between distal enhancers and inapprop riate target promoters(1-5). The best-characterized insulators in Dros ophila correspond to a 340-base-pair (bp) fragment from the gypsy retr otransposon(3), and the scs and scs' sequences flanking the 87A1 hsp70 locus(1,2). Here we demonstrate that both insulators block the intera ction of defined even-skipped (eve) stripe enhancers(6,7) when positio ned between the enhancer and the target promoter. The simultaneous use of two stripe enhancers (eve stripes 2 and 3) provides evidence that enhancers lying distal to the insulator are selectively blocked. The i nsertion of stripe- insulator-stripe sequences between two divergently transcribed promoters indicates that enhancers barred from acting on one basal promoter are fully accessible to appropriate regulatory fact ors for activating the other promoter. These results suggest that insu lators do not propagate changes in chromatin structure. Finally, we pr esent evidence that the gypsy insulator does not block interactions be tween a silencer element and a basal promoter, Taken together, these r esults suggest that insulators might not be restricted to the function al isolation of neighbouring genetic loci. Rather, they might function as flexible regulatory elements that modulate enhancer-promoter inter actions within complex promoters and complex genetic loci.