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Molecular and Cellular Biology, June 2003, p. 3990-3999, Vol. 23, No. 11
0270-7306/03/$08.00+0     DOI: 10.1128/MCB.23.11.3990-3999.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

CtBP-Independent Repression in the Drosophila Embryo

Division of Genetics and Development, Department of Molecular and Cell Biology, University of California, Berkeley, California 94720,¹ Department of Cell and Developmental Biology, Weill Medical College of Cornell University, New York, New York 10021²

Received 16 September 2002/ Returned for modification 3 January 2003/ Accepted 21 February 2003

There are three mechanisms of transcriptional repression in eukaryotes. The first is quenching, whereby repressors and activators co-occupy closely linked sites and then the repressor inhibits adjacent activators. The second is direct repression, in which repressors block the function of the core transcription complex. The third is competition, in which repressors compete with activators for a common DNA-binding site. Previous studies have shown that the Drosophila melanogaster CtBP corepressor (dCtBP) is essential for the quenching activity of three short-range sequence-specific repressors in the early Drosophila embryo: Krüppel, Knirps, and Snail. Here we demonstrate that dCtBP is dispensable for target enhancers that contain overlapping activator and repressor binding sites. However, it is essential when Krüppel and Knirps repressor sites do not overlap activator sites but are instead located adjacent to either activators or the core promoter. These findings provide evidence that competition is distinct from quenching and direct repression. Quenching and direct repression depend on dCtBP, whereas competition does not.

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