Genetic dissection of the Mcp regulatory element from the BX-C. Mario A Metzler¹, Daryl Gohl², Paul Schedl³, Martin Müller¹, Markus Affolter¹. 1) Biozentrum, Universität Basel, Basel, Switzerland; 2) Stanford University, Stanford, USA; 3) Princeton University, Princeton, USA.

   Miscadestral pigmentation (Mcp) refers to a few dominant Abd-B gain-of-function alleles. Karch et al showed that they consist of small overlapping deletions defining an interval of less than 1 kb located between regulatory regions iab4 and iab5. It was proposed that Mcp functions as a boundary or insulator element which enables iab4 and iab5 to function independently of each other. Over the past 20 years, Mcp function has been dissected using various transgene assays. Several labs have demonstrated that Mcp (1) contains a Polycomb Response Element (PRE), (2) can act as an enhancer blocker, (3) can mediate long-distance interactions between Mcp elements located several megabases apart or even on different chromosomes. More recently, Pirrotta and Georgiev et al have shown that the latter two activities might co-localize and be separable from PRE function. We have established a C31-dependent assay system which allows us to easily study the function of many mutated Mcp fragments. They are introduced ~400bp upstream of the apterous promoter, separating it from its wing enhancers. Similar to the well-studied su(Hw) insulator, wild-type Mcp interferes efficiently with enhancer-promoter interaction which results in characteristic apterous wing phenotypes. Furthermore, this Mcp insert also mediates efficient long-distance interactions with other Mcp-containing insertions on the second chromosome. Transgenic lines for many small deletions in the context of a ~800bp Mcp fragment have been established. Data on their activities in the enhancer blocker and the long-distance interaction assays will be presented. We hope that these experiments may give further insights into the working mechanism of the Mcp insulator.