Direct control of transcription of the Drosophila morphogen bicoid by the 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 under control of the maternal gene product Bicoid (bcd), which specifies distinct domains of embryonic gene expression in a concentration-dependent manner. We show here that bcd transcription is controlled by serendipity delta (sry delta), a zygotic-lethal zinc finger protein gene. This sry delta germ-line function was revealed by transgenic expression of a modified Sry delta protein, Sry DB56, carrying a two-finger swap. Although it almost fully rescues sry delta lethality, Sry DB56 does not substitute for the wild-type protein in activating bcd transcription. Two overlapping sites binding the Sry delta protein were identified in the bcd promoter region, a few base pairs upstream of the putative TATA box. Mutating one site impairs bcd transcription in vivo, indicating that Sry delta acts directly upstream of bcd. The specific requirement of sry delta for bcd transcription in the female germ line constitutes 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.