The function of insulators is directional, as discovered in fruit fly experiments. gypsy was inserted into the yellow locus y of Drosophila melanogaster, resulting in inactivation of y gene in some tissues, but y gene was still active in some tissues, because there was an insulator sequence at one end of the gypsy. gypsy will have different effects on gene activity when gypsy is inserted at different locations in the y locus. This is because the activity of y gene is regulated by four enhancers. When the insulator is inserted just upstream of the promoter, it blocks the activation of genes in the wing blade and body cuticle tissues (from the upstream enhancer). But did not block the expression of the y gene (from the downstream enhancer) in the bristles and farsal claws.
Since some enhancers are upstream and some downstream of the promoter, the effect of the insulator does not depend on the position of the insulator relative to the promoter. Therefore, the reason for the directivity of the insulator effect is not really understood. Two loci have been found to affect insulator function in a trans-activation manner. The nucleoprotein encoded by the S2J(Hw) gene recognizes the insulator, and only after the insulator binds to it does it have an insulating effect. When this gene is mutated, the insulation effect is lost despite the insertion of an insulator in the y locus, and y is expressed in all tissues. Another locus is mod(mdg 4). After mutation of this gene, its effect is just opposite to Su(Hw), that is, these mutants enhance the insulation effect, making the insulation effect of the insulator no longer directional and expanded, that is, blocking the effect of enhancers on both upstream and downstream sides.
