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

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 lambda repressors to their respective operators that were inserted within the bgl upstream silencer. Efficient activation of bgl operon transcription by Lac and lambda repressors was independent of the position and phasing of the operators with respect to the promoter. Activation by Lac and lambda 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. Copyright 1998 Academic Press.