Upstream operators enhance repression of the lac promoter.

To study regulation of transcription by distant elements, a wild-type lac operator was inserted upstream of a promoter-constitutive operator control region. The upstream operator is shown to aid in repression of transcription from the mutant control region. The effectiveness of the upstream operator as a function of its distance from the mutant control region parallels the length dependence observed for DNA cyclization. A quantitative model is proposed for action-at-a-distance of DNA control sites in which protein-protein and protein-DNA interactions are mediated by DNA looping. In this model, the effective concentrations of interacting proteins that are tethered by DNA are determined by the length of the intervening DNA and by its inherent bending and torsional stiffness. This model makes a number of predictions for both eukaryotic and prokaryotic control sequences located far from their sites of action.