Multi-enhancer transcriptional hubs confer phenotypic robustness.

Abstract

We previously showed in <i>Drosophila melanogaster</i> embryos that low-affinity Ultrabithorax (Ubx)-responsive <i>shavenbaby</i> (<i>svb</i>) enhancers drive expression using localized transcriptional environments and that active <i>svb</i> enhancers on different chromosomes tended to colocalize (Tsai et al., 2017). Here, we test the hypothesis that these multi-enhancer 'hubs' improve phenotypic resilience to stress by buffering against decreases in transcription factor concentrations and transcriptional output. Deleting a redundant enhancer from the <i>svb</i> locus led to reduced trichome numbers in embryos raised at elevated temperatures. Using high-resolution fluorescence microscopy, we observed lower Ubx concentration and transcriptional output in this deletion allele. Transcription sites of the full <i>svb cis</i>-regulatory region inserted into a different chromosome colocalized with the <i>svb</i> locus, increasing Ubx concentration, the transcriptional output of <i>svb</i>, and partially rescuing the phenotype. Thus, multiple enhancers could reinforce a local transcriptional hub to buffer against environmental stresses and genetic perturbations, providing a mechanism for phenotypical robustness.