A Looping-Based Model for Quenching Repression

Positive and Negative Control of Enhancer-Promoter Interactions by Other DNA Loops Generates Specificity and Tunability

Enhancers are ubiquitous and critical gene-regulatory elements. However, quantitative understanding of the role of DNA looping in the regulation of enhancer action and specificity is limited. We used the Escherichia coli NtrC enhancer-σ54 promoter system as an in vivo model, finding that NtrC activation is highly sensitive to the enhancer-promoter (E-P) distance in the 300-6,000 bp range. DNA loops formed by Lac repressor were able to strongly regulate enhancer action either positively or negatively, recapitulating promoter targeting and insulation. A single LacI loop combining targeting and insulation produced a strong shift in specificity for enhancer choice between two σ54 promoters. A combined kinetic-thermodynamic model was used to quantify the effect of DNA-looping interactions on promoter activity and revealed that sensitivity to E-P distance and to control by other loops is itself dependent on enhancer and promoter parameters that may be subject to regulation.