Ectopic HP1 promotes chromosome loops and variegated silencing in Drosophila

A transgene inserted in euchromatin exhibits mosaic expression when targete d by a fusion protein made of the DNA-binding domain of GAL4 and the hetero chromatin-associated protein HP1. The silencing responds to the loss of a d ose of the dominant modifiers of position-effect variegation Sn(var)3-7 and Su(var)2-5, the locus encoding HP1, The genomic environs of the insertion site at 87C1 comprise the dispersed repetitive elements micropia and alpha gamma. In the presence of the GAL4-HP1 chimera, the polytene chromosomes of this line form loops between the insertion site of the transgene and six o ther sections of chromosome 3R, as well as, rarely, with pericentric and te lomeric heterochromatin. In contrast to the insertion site of the transgene at 87C, the six loop-forming sites in the euchromatic arm were each previo usly described as intercalary heterochromatin. Moreover, GAL4-HP1 tethering on one homologue trans-inactivates the reporter on the other. HP1, probabl y together with other partners, could thus facilitate the coalescence of di spersed middle repetitive sequences, and organize the heterochromatic struc ture responsible for the variegated silencing of nearby euchromatic genes.