INSERTING THE FTZ HOMEODOMAIN INTO ENGRAILED CREATES A DOMINANT TRANSCRIPTIONAL REPRESSOR THAT SPECIFICALLY TURNS OFF FTZ TARGET GENES IN-VIVO

The Engrailed homeodomain protein is an 'active' or dominant transcrip tional repressor in cultured cells, In contrast, the Fushi Tarazu home odomain protein is an activator, both in cultured cells and in Drosoph ila embryos, where it activates several known target genes, including its own gene, This auto-activation has been shown to depend on targeti ng to a fushi tarazu enhancer by the Fushi Tarazu homeodomain, We comb ined Fushi Tarazu targeting and Engrailed active repression in a chime ric regulator, EFE. When EFE is ubiquitously expressed, it overrides e ndogenous Fushi Tarazu and causes a fushi tarazu mutant phenotype, Nor mal Fushi Tarazu target genes are affected as they are in fushi tarazu mutants, One such target gene is repressed by EFE even where Fushi Ta razu is not expressed, suggesting that the repression is active. This is confirmed by showing that the in vivo activity of EFE depends on a domain that is required for active repression in culture, A derivative that lacks this domain, while it cannot repress the endogenous fushi tarazu gene, can still reduce the activity of the fushi tarazu autoreg ulatory enhancer, suggesting that it competes with endogenous Fushi Ta razu for binding sites in vivo, However, this passive repression is mu ch less effective than active repression.