EFFECTS OF THE SU(HW) INSULATOR PROTEIN ON THE EXPRESSION OF THE DIVERGENTLY TRANSCRIBED DROSOPHILA YOLK PROTEIN GENES

The suppressor of Hairy-wing [su(Hw)] protein mediates the mutagenic e ffects of the gypsy retrotransposon by blocking enhancer activity. The se repressive effects are general, can occur over long distances and r equire that the su(Hw) protein is bound between the affected enhancer and promoter, The effects of the su(Hw) binding region on yolk protein (yp) gene expression were determined, These genes are regulated by sh ared enhancers in the intergenic region, which provided a method to ex amine whether an enhancer blocked by the su(Hw) protein remained funct ional, We demonstrate that a blocked enhancer is completely active, su pporting the proposal that the su(Hw) protein is an insulator protein that acts by forming a new boundary in a pre-existing chromatin domain , thereby preventing the interaction of regulatory elements located up stream of the insertion site with the promoter, In addition, we found that yp promoter function is not diminished by sharing enhancers, sugg esting that these enhancers are not rate limiting for transcriptional activation, Lastly, our data indicate that yp promoter activity is int erdependent, such that transcription from one promoter influences the level of activity of the linked promoter.