CONVERSION OF A SILENCER INTO AN ENHANCER - EVIDENCE FOR A CO-REPRESSOR IN DORSAL-MEDIATED REPRESSION IN DROSOPHILA

The dorsal (dl) protein gradient determines patterns of gene expressio n along the dorsal-ventral axis of the Drosophila embryo. dl protein i s at peak levels in ventral nuclei of the embryo where it activates so me genes (twist and snail) and represses others [zerknullt (zen), deca pentaplegic and tolloid]. It is a member of the rel family of transcri ption factors and interacts with specific DNA sequences in the regulat ory regions of its target genes. These sequences (dl binding sites), w hen taken from the context of either an activated or repressed promote r, mediate transcriptional activation of a heterologous promoter, but not repression. We found that T-rich sequences close to the dl binding sites in the silencer region of the zen promoter are conserved betwee n three Drosophila species. Using this sequence information we defined a minimal element that can mediate repression of a heterologous promo ter. This element interacts with at least two factors present in embry onic extracts, one of which is dl protein. The other factor binds to t he T-rich site. Point mutations in either site abolish ventral repress ion in vivo. In addition, mutations in the T-rich site cause ectopic e xpression in ventral regions indicating that the minimal silencer was converted into an enhancer.