Abstract: Provided are methods and compositions for regulating gene expression in a cell. The invention provides recombinant genetic toggle switches which contain a first constitutive promoter-regulatory gene operon and a second constitutive promoter-regulatory gene operon. Expression of the regulatory gene from the first operon inhibits expression from the promoter in the second operon, and expression of the regulatory gene from the second operon inhibits expression from the promoter in the first operon. By use of the toggle switch and various switching agents it is possible to reversibly switch the expression of a gene of interest between a stable “on” state and stable “off” state or vice versa via transient exposure to a switching agent.

Abstract: Provided are methods and compositions for regulating gene expression in a cell. The invention provides recombinant adjustable-threshold switches which contain a first inducible promoter-regulatory gene operon and a second constitutive promoter-regulatory gene operon. A threshold amount of activating agent is required to switch the inducible promoter “on” and the constitutive promoter “off”. An adjustable-threshold switch is useful to reversibly switch the expression of a gene of interest between a stable “on” state and stable “off” state in response to an activating agent at or above a threshold concentration.