MECHANISM AND BICOID-DEPENDENT CONTROL OF HAIRY STRIPE 7 EXPRESSION IN THE POSTERIOR REGION OF THE DROSOPHILA EMBRYO

Pair-rule gene hairy (h) expression in seven evenly spaced stripes, al ong the longitudinal axis of the Drosophila blastoderm embryo, is medi ated by a modular array of separate stripe enhancer elements. The mini nal enhancer element, which generates reporter gene expression in plac e of the most posterior h stripe 7 (h7-element), contains a dense arra y of binding sites for factors providing the trans-acting control of h stripe 7 expression as revealed by genetic analyses, The h7-element m ediates position-dependent gene expression by sensing region-specific combinations and concentrations of both the maternal homeodomain trans criptional activators, Caudal and Bicoid, and of transcriptional repre ssors encoded by locally expressed zygotic gap genes, Caudal and Bicoi d, which form complementing concentration gradients along the longitud inal axis of the embryo, function as redundant activators, indicating that the anterior determinant Bicoid is able to activate gene expressi on in the most posterior region of the embryo, The spatial limits of t he h stripe-7 domain are brought about by the local activities of repr essors which prevent activation, The results suggest that the gradient s of Bicoid and Caudal combine their activities to activate segmentati on genes along the entire axis of the embryo.