Interactions between su(Hw)-binding regions in neighboring y2 and scD1 alleles hinder trans-activation of the y2 promoter by yellow enhancers locatedon a homologous chromosome

The phenomenon of transvection has been well characterized for the yellow l ocus in Drosophila. Enhancers of a promoterless yellow locus in one homolog ous chromosome can activate the yellow promoter in the other when its own e nhancers are blocked by the su(Hw) insulator introduced by the gypsy retrot ransposon. Insertion of another gypsy into the neighboring scute locus hind ers transvection presumably owing to disruption of chromosomal synapsis bet ween the yellow alleles. We determined the sequences of gypsy required for inhibition of transvection. Two partial revertants of the sc(D1) mutation w ere obtained in which transvection between the yellow alleles was restored. Both sc revertants were generated by deletion of nine of the twelve su(Hw) -binding sites of gypsy inserted into the scute locus. This result suggests that the su(Hw) region is required for an interaction between two gypsy el ements that disrupts trans activation of the yellow promoter by enhancers l ocated on the homologous chromosome.