DISTINCT PARASEGMENTAL AND IMAGINAL ENHANCERS AND THE ESTABLISHMENT OF THE EXPRESSION PATTERN OF THE UBX GENE

The expression domain of the Ubx gene in Drosophila embryos is bounded by the product of the hb gene, acting as a repressor. We show that al l Ubx fragments that bind Hb protein in vitro contain parasegmental en hancers active in the embryo in specific parasegmental patterns. We ha ve found three new embryonic enhancer elements in the upstream region, in addition to the two previously identified. Each produces a pattern initially bounded at PS6 by Hb but sooner or later breaks down this b oundary and begins to express in the anterior region. These enhancers do not respond to the long-term maintenance mediated by the Polycomb g roup of genes. They also cease functioning after germ band extension. Expression in imaginal tissues is due to a set of entirely separate an d independent imaginal disc enhancers. These do not contain Hb binding sites and by themselves have no anterior/posterior positional informa tion, although some distinguish between ventral and dorsal discs. A th ird kind of element, the Polycomb Response Element (PRE), has no enhan cer activity but causes long-term maintenance of the expression domain of other enhancers present in the vicinity. The interaction of these elements results in the correct expression of Ubx in imaginal tissues.