Genomic context modulates insulator activity through promoter competition

Chromatin insulators regulate gene expression by preventing inappropriate enhancer-promoter interactions. Our previous study showed that insulators do not merely function as rigid blockers, rather their activities are quantitative and selective. We have investigated the factors and mechanisms that determine the effectiveness of the suHw insulator in transgenic Drosophila. We show that the suHw-mediated blockage of the AE1 enhancer from a downstream promoter depends on the ability of the promoter to compete for AE1. Promoters that are highly competitive for the enhancer are blocked less effectively. Moreover, blockage of AE1 from its cognate ftz promoter can range from virtually complete to non-detectable, depending on the property of the neighboring upstream promoter. A highly competitive neighboring promoter enhances the suHw-mediated blockage, whereas a less competitive promoter reduces the insulator effectiveness. The influence on insulator effectiveness by both the interacting and the neighboring competing promoters correlates with their ability to compete for the enhancer, which was previously shown to depend on core promoter sequences. Our findings suggest a mechanism at the level of gene organization that modulates insulator effectiveness through promoter competition. The dependence of insulator function on its cis contexts may provide it with more regulatory flexibility while imposing organizational restraints on eukaryotic gene complexes.