The functional role of the Drosophila gypsy insulator in the regulation of gene expression

Chromatin insulators are short DNA sequences that, together with enhancers and silencers, orchestrate gene transcription through DNA-protein interactions in eukaryotic genomes. It has been proposed that insulators operate at the chromatin level by generating functionally independent higher-order chromatin domains. Insulators may maintain the integrity of such domains using two properties: blocking enhancer-promoter interactions and blocking heterochromatin spreading. The gypsy insulator of Drosophila was identified as a region of the gypsy retrovirus responsible for the production of tissue-specific mutations in many genes. The Suppressor of Hairy wing [Su(Hw)] protein contains 12 zinc fingers that specifically bind the gypsy insulator. Upon DNA binding, Su(Hw) recruits a second protein, Modifier of Mdg4 67.2 [Mod(mdg4) 67.2], and the interaction of both proteins is required for insulator function in vivo. We have found that three different arrays of gypsy retrovirus insertions in a yellow transgene result in unique yellow phenotypes, showing that the enhancer-blocking activity of the Drosophila gypsy insulators depends on the relative orientation of the gypsy retroviruses on the chromosome. We also observed from transgenic lines with gypsy retrovirus or insulator insertions that interaction of insulators may be regulated by active enhancers according to the relative positions of the insulators flanking the enhancers. Moreover, we show that gypsy insulators can positively modulate yellow activation and result in wild-type levels of expression when placed upstream of enhancers in yellow transgenes in which enhancers are placed out of context by &#:-DNA spacers and fail to reproduce the expression levels of yellow in wings and body cuticle. Our results provide evidence indicating that this phenomenon is independent of the boundary activity. Genetic analysis using mod(mdg4)67.2 mutant lines containing gypsy retrovirus insertions revealed that the gypsy insulator may be placed close to the yellow promoter region and be intimately involved in transcriptional activation and repression. Therefore, we suggest that insulators may also function by mediating long range interactions between enhancers and promoters.