An 84-bp proximal regulatory region (PRR) of the DrosophUa melanogaster s36 chorion gene is sufficient for directing proper temporal and spatial expression of a reporter gene in three domains of the follicle: anterior, posterior, and main body. Here we show that the fidelity of PRR-directed s36 expression is dependent on the proper dorsal-ventral differentiation of the foilicular epithelium, which requires the Drosophila epidermal growth factor receptor homolog. Transgenic analysis of site-directed mutants of the PRR suggests that s36 expression is regulated by the concerted action of multiple positive activators. Several cis-acting transcriptional elements have been identified: some appear to function in a quantitative manner, while others either are essential or appear to regulate expression in particular spatial domains. The approximate locations of these regulatory elements have been defined; some map within sequences that are strongly conserved in widely divergent dipteran species. In fact, the PRR analog of the medfly Ceratitis capitata Ccs36 gene directs expression in a manner similar to the D. melanogaster s36 PRR. We propose a model for transcriptional regulation of s36 based on the prechoriogenic polarization of the follicular epithelium that surrounds the developing egg chamber.Spatially restricted gene expression is an important feature of development in higher eukaryotes. The expression patterns of genes in the developing Drosophila embryo have been studied extensively (reviewed in references 1, 12, and 45); less well understood is spatial control during postembryonic development. A useful system for studying later developmental gene expression are the Drosophila chorion (eggshell) genes, which are regulated in distinct temporal and spatial patterns, exclusively in the somatic follicular epithelium that surrounds each developing oocyte (for reviews, see references 13 and 27).Several mutations that cause aberrant chorion formation also lead to incorrect pattern formation in the embryo [e.g., fs(1)K10, (51-53), dicephalic (7,20), and torpedo and gurken (36)]. Many of these chorion mutations are germ line dependent, suggesting that the pattern of chorion gene expression may depend on signals from the oocyte to the somatic follicular epithelium, where the chorion genes are actually expressed (4, 28, 36). Conversely, some somatic mutations affect both chorion and embryo (36), suggesting that regional specialization of the follicular epithelium may influence patterning in the oocyte and thus in the developing embryo. Therefore, elucidation of chorion gene regulatory mechanisms could contribute to our understanding of both embryonic and postembryonic developmental patterning.During the 5 to 6 h between stages 10b and 14 of late oogenesis (14) developing oocyte (21,22,29,50). The temporal and spatial regulation of chorion genes is reflected in morphological specializations of the eggshell along the anterior-posterior and dorsal-ventral axes (21,22) and has been documented by molecular studies at both the pr...