Redundant PREs act together to maintain en/inv gene expression. Sandip De, Judith Kassis. NICHD, NIH, BETHESDA, MD.

   In Drosophila, the engrailed (en) and invected (inv) genes are required for segmentation, development of the specific cells in the nervous system, and in the posterior compartment in imaginal disks. en and inv are co-regulated genes juxtaposed in a chromatin domain marked by H3K27me3. It is well established that en/inv gene expression is very dynamic throughout development and is regulated by different DNA regulatory elements and trans-acting proteins. With the purpose of identifying the role of Polycomb group Response Elements (PREs) in setting up en/inv domain, we deleted ~1.5kb (en^1.5) and ~24kb (inv²⁴) containing the major en and inv PREs respectively. Surprisingly, both en^1.5 and inv²⁴ flies are homozygous viable and fertile. In comparison to wild type, we did not observe any significant difference in H3K27me3 accumulation within en/inv domain in either mutant. ChIP-seq analysis with anti-Pho antibody identified 6 additional potential weak PREs, present between the en and tou genes. We observe increased accumulation of Pho proteins in these weak PREs and also at a PRE present in the 5 end of E(Pc), the gene next to inv. We believe these weak PREs act to maintain the epigenetic mark in the en/inv domain recruiting PcG proteins in the absence of the major PREs. Further research is under progress.