CHROMATIN INSULATOR ELEMENTS BLOCK THE SILENCING OF A TARGET GENE BY THE DROSOPHILA POLYCOMB RESPONSE ELEMENT (PRE) BUT ALLOW TRANS INTERACTIONS BETWEEN PRES ON DIFFERENT CHROMOSOMES

Polycomb response elements (PREs) can establish a silenced state that affects the expression of genes over considerable distances. We have t ested the ability of insulator or boundary elements to block the repre ssion of the miniwhite gene by the Ubx PRE. The gypsy element and the scs element interposed between PRE and miniwhite gene protect it again st silencing but the scs' is only weakly effective. When the PRE-miniw hite gene construct is insulated from flanking chromosomal sequences b y gypsy elements at both ends, it can still establish efficient silenc ing in some lines but not others. We show that this can be caused by i nteractions in trans with PREs at other sites. PRE-containing transpos ons inserted at different sites or even on different chromosomes can i nteract, resulting in enhanced silencing. These trans interactions are not blocked by the gypsy insulator and reveal the importance of nonho mologous associations between different regions of the genome for both silencing and activation of genes. The similarity between the behavio r of PREs and enhancers suggests a model for their long-distance actio n.