Sparkling insights into enhancer structure, function, and evolution

This review focuses on a single cis-regulatory element: the sparkling eye enhancer of the Drosophila dPax2 gene. sparkling responds to Notch and EGFR signaling, along with other direct regulatory inputs, to drive gene expression that is restricted to cone cells of the developing fly retina. Functional, genetic, biochemical, evolutionary, and bioinformatic analyses have revealed surprising properties of sparkling, which may provide new insights into cis-regulatory logic and mechanisms of transcriptional activation. These properties include: a very high density of regulatory information and a correspondingly low "junk" content; an unexpectedly complex combinatorial code; tight functional constraints on enhancer organization, paradoxically coupled with high turnover of DNA sequence and binding site position; a requirement for weak binding of the transcription factor Su(H) to low-affinity sites in order to maintain a cell-type-specific response to Notch signaling; and multiple specialized regulatory sequences conferring functionally distinct activation activities, all of which are required in concert to achieve proper gene expression in vivo.