| Summary: | <p>Abstract</p> <p>Background</p> <p>Cell-specific expression of a subset of <it>Enhancer of split (E(spl)-C) </it>genes in proneural clusters is mediated by synergistic interactions between bHLH A (basic Helix-Loop-Helix Activator) and <it>Notch</it>-signalling transcription complex (NTC) proteins. For a some of these <it>E(spl)-C </it>genes, such as <it>m8</it>, these synergistic interactions are programmed by an "SPS+A" transcription code in the <it>cis</it>-regulatory regions. However, the molecular mechanisms underlying this synergistic interaction between NTCs and proneural bHLH A proteins are not fully understood.</p> <p>Findings</p> <p>Using cell transcription assays, we show that the N-terminal region of the Daughterless (Da) bHLH A protein is critical for synergistic interactions with NTCs that activate the <it>E(spl)-C m8 </it>promoter. These assays also show that this interaction is dependent on the specific inverted repeat architecture of Suppressor of Hairless (Su(H)) binding sites in the SPS+A transcription code. Using protein-protein interaction assays, we show that two distinct regions within the Da N-terminus make a direct physical interaction with the NTC protein Su(H). Deletion of these interaction domains in Da creates a dominant negative protein that eliminates NTC-bHLH A transcriptional synergy on the <it>m8 </it>promoter. In addition, over-expression of this dominant negative Da protein disrupts Notch-mediated lateral inhibition during mechanosensory bristle neurogenesis <it>in vivo</it>.</p> <p>Conclusion</p> <p>These findings indicate that direct physical interactions between Da-N and Su(H) are critical for the transcriptional synergy between NTC and bHLH A proteins on the <it>m8 </it>promoter. Our results also indicate that the orientation of the Su(H) binding sites in the SPS+A transcription code are critical for programming the interaction between Da-N and Su(H) proteins. Together, these findings provide insight into the molecular mechanisms by which the NTC synergistically interacts with bHLH A proteins to mediate <it>Notch </it>target gene expression in proneural clusters.</p>
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