Short-range transcriptional repressors mediate both quenching and direct repression within complex loci in Drosophila.

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Abstract
The early Drosophila embryo provides a unique system for the analysis of transcriptional repression since a broad spectrum of repressors are distributed in spatially distinct patterns. Krüppel (Kr) and snail (sna), two zinc finger repressors, are essential for segmentation and for the establishment of the mesoderm/neuroectoderm boundary, respectively. Both repressors were examined in the context of synthetic gene complexes containing modular promoters and divergently transcribed reporter genes. These studies indicate that Kr and sna function as short-range repressors, which can mediate either quenching or direct repression of the transcription complex, depending on the location of repressor sites. When located within an upstream enhancer, the repressor locally quenches nearby activators and permits other enhancers to interact with the transcription complex (enhancer autonomy). In contrast, when bound to promoter-proximal regions the repressor functions in a dominant fashion and blocks multiple enhancers. Local quenching and dominant repression require close linkage (<100 bp) of the repressor with either upstream activators or the transcription complex. These studies establish short-range repression as a flexible form of gene regulation and suggest that the key distinction among repressors is their range of action.