Analysis of two closely-linked engrailed Polycomb Response Elements: similarities and differences. J Lesley Brown, Judith Kassis. NICHD, NIH, Bethesda, MD.

   In Drosophila, Polycomb group response elements (PREs) play an essential role in gene regulation by the Polycomb group (PcG) repressor proteins. They are required for the recruitment of and for the maintenance of repression by the PcG proteins. Here we compare and contrast different characteristics of two closely linked yet separable PREs of the Drosophila engrailed (en) gene, PRE1 and PRE2. We define a binding site for an as yet unidentified protein that binds to PRE2. We find that PRE1 and PRE2 have different requirements for the number of binding sites for the DNA binding PcG protein pleiohomeotic (pho). PRE1 requires two Pho binding sites whereas PRE2 requires only one. In addition, for full function, PRE1 requires an AT rich region not seen in PRE2. These two PREs behave differently in an embryonic maintenance assay when inserted at an identical location in the genome. Such differences in PRE activity may be important for regulation of engrailed.