Functional dissection of an early Drosophila chorion gene promoter: expression throughout the follicular epithelium is under spatially composite regulation.

We have fused various DNA sequences located upstream of the Drosophila melanogaster s36 chorion gene TATA box to a heterologous basal promoter and reporter gene (hsp70/lacZ). The expression of these constructs, following P-element-mediated germline transformation, was examined in 144 independent lines by histological staining of dissected ovaries for beta-galactosidase activity. A short 84 bp segment of the proximal 5' flanking DNA was sufficient to confer a wild-type gene expression pattern, including temporal specificity for early choriogenic follicles. Surprisingly, initial expression was very localized at the anterior and posterior poles of the follicle. The downstream half of that DNA segment permitted expression at both poles, but especially at the anterior tip, while the upstream half only favored expression in the posterior pole; these results suggested the existence of multiple, spatially specific cis-regulatory elements. When the proximal 84 bp segment was placed 1.5 kb upstream of the basal promoter, beta-galactosidase activity was observed in an altered spatial pattern, indicating that the cis-regulatory element(s) that favor expression in the posterior half of the follicle are position independent, while the element(s) that favor expression elsewhere in the follicle are position sensitive. A distal regulatory segment containing redundant DNA element(s) specific for expression in the anterior pole was identified much further upstream of s36. Thus, the expression of this chorion gene throughout the follicular epithelium is actually composite, occurring in distinct spatial domains under the control of corresponding DNA elements.