The same dorsal binding site mediates both activation and repression in a context-dependent manner.

Like many DNA binding transcription factors, the Drosophila morphogen encoded by dorsal can both stimulate and repress promoter activity. In particular, this factor activates twist and represses zerknüllt on the ventral side of the early embryo. We find that when multiple copies of a dorsal binding site from the zerknüllt ventral repressor element are fused to a heterologous basal promoter, the resulting construct is activated by dorsal to give a ventral specific expression pattern. Thus, the ability of a dorsal binding site to mediate repression rather than activation is not an intrinsic property of the site, but depends upon its context. We also show that a hybrid promoter containing both the zerknüllt ventral repressor element and the twist ventral activator region is not ventrally active in the early embryo, demonstrating that repression is dominant over activation. Thus, the default mode of action of the dorsal protein is transcriptional activation. Additional factors may modify dorsal activity to bring about repression.