Molecular analysis of the zeste-white interaction reveals a promoter-proximal element essential for distant enhancer-promoter communication.

We have analyzed the eye and testis enhancers located 1 kb upstream of the transcription start site of the white gene. Both enhancers confer the corresponding tissue-specific expression on a heterologous promoter as well as on the white promoter. The eye determinant consists of multiple elements, each able to stimulate eye-specific expression. It also contains five binding sites for the zeste protein while the immediately adjacent testis element contains none. Site-directed mutation of these zeste binding sites abolishes the zeste-white interaction but does not significantly affect the eye enhancer activity, indicating that they are not important for the eye enhancer activity per se. Other zeste binding sites just upstream of the promoter are not necessary for the zeste-white interaction. We conclude that the overlap of the eye enhancer with the zeste binding sites is responsible for the zeste-white interaction and explains why this interaction affects eye but not testis expression. Sequence deletion or substitution experiments suggested that the white promoter is internal to the transcription start site; the zeste protein is not required for distant enhancer action but a 95-bp promoter-proximal sequence is essential for distant enhancer-promoter interaction. This element may serve as an anchor to stabilize formation of a loop that brings the enhancer to the vicinity of the promoter.