A NUCLEAR MATRIX SCAFFOLD ATTACHMENT REGION COLOCALIZES WITH THE GYPSY RETROTRANSPOSON INSULATOR SEQUENCE/

The 5'-untranslated region of the Drosophila gypsy retrotransposon con tains an ''insulator,'' which disrupts the interactions between enhanc er and promoter elements located apart, The insulator effect is depend ent on the suppressor of Hairy-wing (su(Hw)) protein, which binds to r eiterated sites within the 350 base pairs of the gypsy insulator, wher eby it additionally acts as a transcriptional activator of gypsy, Here , we show that the 350 base pair su(Hw) binding site containing gypsy insulator behaves in addition as a matrix/scaffold attachment region ( MAR/SAR), involved in interactions with the nuclear matrix. In vitro e xperiments using nuclear matrices from Drosophila, murine, and human c ells demonstrate specific binding of the gypsy insulator, not observed with any other sequence within the retrotransposon, Moreover, we show that the gypsy insulator, like previously characterized MAR/SARs, spe cifically interacts with topoisomerase II and histone H1, i.e. with tw o essential components of the nuclear matrix, Finally, experiments wit hin cells in culture demonstrate differential effects of the gypsy MAR sequence on reporter genes, namely no effect under conditions of tran sient transfection and a repressing effect in stable transformants, as expected for a sequence involved in chromatin structure and organizat ion, A model for the gypsy insulator, which combines within a short '' compacted'' retroviral sequence three functional domains (insulator, e nhancer, and the presently unraveled MAR/SAR) dispersed within more ex tended regions in other ''boundary'' domains, is discussed in relation to previously proposed models for insulation.