DIRECT CONTROL OF TRANSCRIPTION OF THE DROSOPHILA MORPHOGEN BICOID BYTHE SERENDIPITY-DELTA ZINC-FINGER PROTEIN, AS REVEALED BY IN-VIVO ANALYSIS OF A FINGER SWAP

Determination of the anterior structures of the Drosophila embryo is u nder control of the maternal gene product Bicoid (bcd), which specifie s distinct domains of embryonic gene expression in a concentration-dep endent manner. We show here that bcd transcription is controlled by se rendipity delta (sry delta), a zygotic-lethal zinc finger protein gene . This sry delta germ-line function was revealed by transgenic express ion of a modified Sry delta protein, Sry DB56, carrying a two-finger s wap. Although it almost fully rescues sry delta lethality, Sry DB56 do es not substitute for the wild-type protein in activating bcd transcri ption. Two overlapping sites binding the Sry delta protein were identi fied in the bcd promoter region, a few base pairs upstream of the puta tive TATA box. Mutating one site impairs bcd transcription in vivo, in dicating that Sry delta acts directly upstream of bcd. The specific re quirement of sry delta for bcd transcription in the female germ line c onstitutes an unexpected link between a zygotic gene with pleiotropic functions and the establishment of coordinates of the Drosophila egg. It highlights the fundamental role of ubiquitous transcription factors in bringing about a specific developmental program.