Short-range repression permits multiple enhancers to function autonomously within a complex promoter

Transcriptional repressors play a key role in establishing localized patterns of gene expression in the early Drosophila embryo. Several different modes of repression have been implicated in previous studies, including competition and direct interference with the transcription complex. Here, we present evidence for 'quenching,' whereby activators and repressors co- occupy neighboring sites in a target promoter, but the repressor blocks the ability of the activator to contact the transcription complex. This study centers on a zinc finger repressor, snail (sna), which represses the expression of neuroectodermal regulatory genes in the presumptive mesoderm. We show that sna can mediate efficient repression when bound 50-100 bp from upstream activator sites. Repression does not depend on proximity of sna- binding sites to the transcription initiation site. sna is not a dedicated repressor but, instead, appears to block disparate activators. We discuss the importance of quenching as a means of permitting separate enhancers to function autonomously within a complex promoter.