The proximal enhancer of the <i>snail</i> gene mediates negative autoregulatory feedback in <i>Drosophila melanogaster</i>

Autoregulatory feedback is a mechanism in which a gene product regulates its own expression, stabilizing gene activity amid noise and environmental changes. In Drosophila melanogaster, the gene snail encodes a key transcriptional repressor that regulates the expression of many genes during early embryogenesis, including its own expression. This study focuses on Snail occupancy at both distal and proximal enhancers of the snail gene to understand the cis-regulatory mechanisms involved in autoregulatory control. The coordinated action of these enhancers results in precisely constrained levels of snail expression during early embryogenesis. Using genome editing by CRISPR/Cas9, we found that deletion of each enhancer individually is compatible with embryonic viability under normal conditions. However, the double mutant is lethal, suggesting a functional interplay between the 2 enhancers. To gain further insight, we assayed snail gene expression levels in fixed embryos. Our results revealed that negative autoregulation of snail relies on the proximal enhancer. Moreover, increasing the affinity of binding sites for Dorsal, a transcriptional activator, in the proximal enhancer impaired this autoregulation, suggesting that Snail acts locally to counterbalance Dorsal's input. A mathematical model of snail autoregulatory control further supports our findings, reinforcing the view that the proximal enhancer mediates negative autoregulatory feedback, and implicating the distal enhancer in positive autoregulatory feedback. In summary, Snail's role at the proximal enhancer is pivotal for negative autoregulatory control and essential for balancing the activation mediated by the distal enhancer.