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

The phenomenon of transvection has been well characterized for the yellow locus in Drosophila. Enhancers of a promoterless yellow locus in one homologous chromosome can activate the yellow promoter in the other when its own enhancers are blocked by the su(Hw) insulator introduced by the gypsy retrotransposon. Insertion of another gypsy into the neighboring scute locus hinders transvection presumably owing to disruption of chromosomal synapsis between the yellow alleles. We determined the sequences of gypsy required for inhibition of transvection. Two partial revertants of the scD1 mutation were 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 elements that disrupts trans activation of the yellow promoter by enhancers located on the homologous chromosome.