Lac and λ repressors relieve silencing of the Escherichia coli bgl promoter. activation by alteration of a repressing nucleoprotein complex

Abstract

The Escherichia coli bgl promoter is kept in a repressed state by silencer sequences which flank the promoter and by the histone-like protein H-NS. Silencing of the bgl promoter is likely due to the formation of a repressing nucleoprotein complex of which H-NS is an essential component. Here, we show that silencing is abolished by the binding of Lac or λ repressors to their respective operators that were inserted within the bgl upstream silencer. Efficient activation of bgl operon transcription by Lac and λ repressors was independent of the position and phasing of the operators with respect to the promoter. Activation by Lac and λ repressors as shown here is unprecedented. We conclude that the activation of bgl transcription by both repressors is achieved by a novel mechanism, that is by alteration of the repressing nucleoprotein complex rather than by protein-protein interactions with RNA polymerase and the catabolite activator protein, CAP.