A POLYCOMB RESPONSE ELEMENT IN THE UBX GENE THAT DETERMINES AN EPIGENETICALLY INHERITED STATE OF REPRESSION

Segmentation genes provide the signals for the activation and regulati on of homeotic genes in Drosophila but cannot maintain the resulting p attern of expression because their activity ceases halfway through emb ryogenesis. Maintenance of the pattern is due to the Polycomb group of genes (Pc-G) and the trithorax group of genes (trx-G), responsible fo r the persistence of the active or repressed state of homeotic genes. We have identified a regulatory element in the Ubx gene that responds to Pc-G and trx-G genes. Transposons carrying this element create new binding sites for Pc-G products in the polytene chromosomes. This Pc-G maintenance element (PRE), establishes a repressive complex that keep s enhancers repressed in cells in which they were originally repressed and maintains this state through many cell divisions. The trx-G produ cts stimulate the expression of enhancers in cells in which they were originally active. This mechanism is responsible for the correct regul ation of imaginal disc enhancers, which lack themselves antero-posteri or positional information. The PRE also causes severe variegation of t he mini-white gene present in the transposon, a phenomenon very simila r to heterochromatic position-effect variegation. The significance of this mechanism for homeotic gene regulation is discussed.