The Mcp element from the Drosophila melanogaster bithorax complex mediateslong-distance regulatory interactions

In the studies reported here, we have examined the properties of the Mel, e lement from the Drosophila melanogaster bithorax complex (BX-C). We have fo und that sequences from the Mcp region of BX-C have properties characterist ic of Polycomb response elements (PREs, and that they silence adjacent repo rters by a mechanism that requires trans-interactions between two copies of the transgene. However, Mcp trans-regulatory interactions have several nov el features. In contrast to classical transvection, homolog pairing does no t seem to be required. Thus, trans-regulatory interactions call be observed not only between Mcp transgenes inserted at the same site, but also betwee n Mcp, transgenes inserted at distant sites on the same chromosomal arm, or even on different arms. Trans-regulation can even be observed between tran sgenes inserted on differ ent chromosomes. A small 800-bp Mcp sequence is s ufficient to mediate these long-distance trans-regulatory interactions. Thi s small fragment has little silencing activity on its own and must be combi ned with other Polycomb-Group-responsive elements to function as a "pairing -sensitive'' silencer. Finally, this pairing element can also mediate long- distance interactions between enhancers and promoters, activating mini-whit e expression.