Assessing the requirement of a Polycomb Response Element during Drosophila development. H. Xin, A. Lloyd, S. Sakonju. Dept Human Genetics, Univ Utah, Salt Lake City, UT.

   Silencing of homeotic genes by Polycomb proteins is mediated by cis-regulatory elements called Polycomb Response Elements (PREs). We have previously shown that the MCP-PRE from the Abdominal-B gene is recognized by at least two DNA binding proteins, Pleiohomeotic and GAGA factor. These proteins presumably recruit Polycomb protein complexes to the MCP regulatory region. We have also tested whether the PRE element is required throughout development, by excising it from the silenced lacZ reporter during larval stages. The appearance of lacZ expressing cells in wing discs, where the reporter is normally silenced, only after FLP recombinase is induced suggested that the presence of the PRE element is required throughout development. In these experiments, however, we could not assess what fraction of cells that had lost the MCP-PRE was no longer silenced. Any marker placed within the PRE element would likely interfere with the silencing event we are trying to assess. To circumvent this problem, we have developed a new method of marking clones of cells without placing a marker within the fragment to be excised. Using this method, we find that only about 10% of clones that had the MCP-PRE excised during larval stages fails to maintain silencing. These results suggest that the key recruitment of Polycomb complexes via the PRE occurs during embryogenesis and that, once established, silencing by Polycomb proteins can be largely maintained through cell divisions without their repeated recruitment to the PRE.