THE SCS AND SCS' INSULATOR ELEMENTS IMPART A CIS REQUIREMENT ON ENHANCER-PROMOTER INTERACTIONS

The Xenopus rRNA enhancer activates its cognate promoter when the two elements are placed on opposite rings of dimeric catenanes. Here we sh ow that when scs elements flank either the enhancer or promoter in cat enanes, the enhancer cannot activate the promoter on the ring in trans . A series of catenanes containing different permutations of the insul ators, enhancer, and promoters shows that when insulators are present, the enhancer is permitted to activate the promoter only when both ele ments are on the same piece of DNA with no intervening insulator. Thes e results suggest that insulators have the potential to block enhancer -promoter interactions between chromosomes and between independent top ological domains within a chromosome.