GENETIC-ANALYSIS OF EMBRYONIC CIS-ACTING REGULATORY ELEMENTS OF THE DROSOPHILA HOMEOTIC GENE SEX COMBS REDUCED

The homeotic gene Sex combs reduced (Scr) of Drosophila melanogaster i s expressed in the labial and prothoracic segments of the ectoderm, in parasegments two and three of the CNS, and in the visceral mesoderm o f the anterior and posterior midgut. The mutationally defined function of Scr is to specify the identity of the labial and prothoracic segme nts and to control the development of the gastric caeca. The Scr locus occupies a chromosomal region of similar to 80 kb within the Antennap edia complex (ANT-C). To understand how Scr's spatiotemporal expressio n pattern is generated in the embryo, we have mapped its transcription al regulatory elements using three approaches. First, we examined the expression pattern of Scr in embryos containing chromosomal rearrangem ents that remove potential Scr regulatory elements. Second, we made an d analyzed a set of Scr minigene transformants. Third, we analyzed a s et of Scr-lacZ enhancer tester constructs. Using more sensitive anti-S CR antisera, we discovered that Scr is expressed in tissues that were not previously thought to accumulate SCR: a stripe of ectodermal cells in the parasegment 2 region of stage 5 embryos, the embryonic salivar y glands, and the dorsal ridge. Four DNA fragments that had previously been shown in an analysis of Scr-lacZ reporter constructs to contain putative Scr enhancer elements were found to have functional enhancers ; similarly, another Scr fragment was found to contain a functional re pressor. Our results suggest that regulation of Scr in the labial segm ent and the CNS requires the apparently synergistic action of multiple , widely spaced enhancer elements. Regulation in the prothorax also ap pears to be controlled by multiple enhancers: one complete pattern ele ment and one subpattern element. In contrast, Scr regulation in the vi sceral mesoderm is controlled by an enhancer(s) located in only one DN A fragment.