Transcriptional control of Drosophila bicoid by Serendipity delta: cooperative binding sites, promoter context, and...

Concentration of maternal BICOID (BCD) establishes the anterior pattern in the Drosophila embryo. Successive deletions in the bcd promoter allowed us to localize an enhancer sequence in the 5'-UTR and a down-regulating element downstream of the ATG initiator codon, and identify a 49 bp region sufficient to drive transcription of a reporter gene specifically in nurse cells. This fragment contains two binding sites for the Serendipity (Sry) d zinc finger activator, that mediate its cooperative binding. Both sites (sdbs) are essential for bcd expression. Further analysis showed that the bcd promoter configuration is decisive for Sry d activating function. Replacement of sdbs by binding sites for Sry b, the Sry d paralog, restores bcd transcription in sry d mutant ovaries, demonstrating that the functional divergence between these two proteins during evolution was mainly driven by changes in their DNA-specific recognition properties, resulting in the control of separate developmental pathways.