Composite insulators: exchanging sincerity for cooperation

The directional effect of insulators was discovered in fruit fly experiments. When the transposon gypsy is inserted into the yellow locus y in fruit flies (D. melanogaster), the y gene is inactivated in some tissues, but remains active in others. This is because the transposon gypsy has an insulator sequence at one end. When gypsy is inserted at different positions within the locus, it has different effects on gene activity. This is because the activity of the y gene is regulated by four enhancers. When the insulator is inserted upstream of the promoter, it blocks gene activation in the wing blade and body cuticle tissues (from the upstream enhancer), but does not block y gene expression in the bristles and farsal claws tissues (from the downstream enhancer).

Since some enhancers are located upstream of the promoter and some downstream, the effect of the insulator does not depend on its relative position to the promoter. Therefore, the reason for the directionality of the insulator effect is not yet fully understood. Two loci have been identified that affect insulator function through transactivation. The S2J(Hw) gene encodes a nucleoprotein that recognizes insulators; the insulator only functions after binding to the protein. When this gene mutates, although an insulator is inserted into the y locus, the insulator loses its insulating function; y is expressed in all tissues. The other locus is mod(mdg4). Mutations in this gene have the opposite effect to Su(Hw); these mutants enhance the insulating effect, making the insulator's insulating effect non-directional and extended, effectively blocking the effects of the upstream and downstream enhancers. One explanation is that Su(Hw) first binds to the insulator DNA, giving the insulator its insulating effect. mod(mdg4) then binds to Su(Hw), causing the insulator to lose its insulating effect; the mutated mod(mdg4) cannot bind to Su(Hw), thus enhancing the insulator's insulating effect again.