Insulation of enhancer-promoter communication by a gypsy transposon insertin the Drosophila cut gene: Cooperation between suppressor of hairy-wing and modifier of mdg4 proteins

The Drosophila mod(mdg4) gene products counteract heterochromatin-mediated silencing of the white gene and help activate genes of the bithorax complex . They also regulate the insulator activity of the gypsy transposon when gy psy inserts between an enhancer and promoter. The Su(Hw) protein is require d for gypsy-mediated insulation, and the Mod(mdg4)-67.2 protein binds to Su (Hw). The aim of this study was to determine whether Mod(mdg4)-67.2 is a co insulator that helps Su(Hw) block enhancers or a facilitator of activation that is inhibited by Su(Hw). Here we provide evidence that Mod(mdg4)-67.2 a cts as a coinsulator by showing that some loss-of-function mod(mdg4) mutati ons decrease enhancer blocking by a gypsy insert in the cut gene. We find t hat the C terminus of Mod(mdg4)-67.2 binds in vitro to a region of Su(Hw) t hat is required for insulation, while the N terminus mediates self associat ion. The N terminus of Mod(mdg4)-67.2 also interacts with the Chip protein, which facilitates activation of cut. Mod(mdg4)-67.2 truncated in the C ter minus interferes in a dominant-negative fashion with insulation in cut but does not significantly affect heterochromatin-mediated silencing of white. We infer that multiple contacts between Su(Hw) and a Mod(mdg4)-67.2 multime r are required for insulation. We theorize that Mod(mdg4)-67.2 usually aids gene activation but can also act as a coinsulator by helping Su(Hw) trap f acilitators of activation, such as the Chip protein.