Role of CtBP in transcriptional repression by the Drosophila giant protein

The giant protein is a short-range transcriptional repressor that refines the expression pattern of gap and pair-rule genes in the Drosophila blastoderm embryo. Short-range repressors including knirps, Krüppel, and snail utilize the CtBP cofactor for repression, but it is not known whether a functional interaction with CtBP is a general property of all short-range repressors. We studied giant repression activity in a CtBP mutant and find that this cofactor is required for giant repression of some, but not all, genes. While targets of giant such as the even-skipped stripe 2 enhancer and a synthetic lacZ reporter show clear derepression in the CtBP mutant, another giant target, the hunchback gene, is expressed normally. A more complex situation is seen with regulation of the Krüppel gene, in which one enhancer is repressed by giant in a CtBP-dependent manner, while another is repressed in a CtBP-independent manner. These results demonstrate that giant can repress both via CtBP-dependent and CtBP-independent pathways, and that promoter context is critical for determining giant-CtBP functional interaction. To initiate mechanistic studies of the giant repression activity, we have identified a minimal repression domain within giant that encompasses residues 89-205, including an evolutionarily conserved region bearing a putative CtBP binding motif.