The Problem

Genetic Toggle Switch

The Genetic Toggle Switch was among the first artificially constructed Gene Regulatory Networks. The original Genetic Toggle Switch consisted of the ptrc-2 (lacI repressible) promoter paired with the temperature sensitive lambda repressor and the PLs1con (lambda repressible) promoter paired with the lacI repressor. Subsequently, induction of the ptrc-2 promoter would repress the PLs1con promoter through expression of the lambda repressor, while induction of the PLs1con promoter would repress the ptrc-2 promoter through expression of the lacI repressor. Switching of the toggle switch from one promoter to another was accomplished by addition of either Isopropyl β-D-1-thiogalactopyranoside (IPTG), which causes the lacI to unbind allowing the ptrc-2 to switch on, or a thermal pulse, which causes the lambda repressor to unbind and allow the PLs1con promoter to switch on. In order to characterize the toggle switch, a GFPmut3 structural gene was placed downstream of the Ptrc-2 promoter so that induction of the Ptrc-2 promoter would result in high expression of GFPmut3 while induction of PLs1con would result in low expression of GFPmut3.

Figure 1. Genetic Toggle Switch

Characteristics of Genetic Toggle Switches

In general, Genetic Toggle Switches have several characteristics which make them unique from other Gene Regulatory networks. For example, Genetic Toggle Switches must be capable of bi-stable behavior, meaning that they should only exist in one of two stable states instead of in a variety of intermediate states. Bi-stability also suggests that a certain threshold must be passed before the toggle switch can switch from one state to another. In addition, toggle switches should have low basal transcriptional noise to prevent random switching without outside induction. Finally, a reporter or marker structural gene, such as a fluorescent protein, should be present in order to characterize the effectiveness of the toggle switch.

• • This is from 2011 Team Duke**

Building a Toggle Switch in Fission Yeast

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References

1. Gardener,T. et al. Construction of a genetic toggle switch in Escherichia coli. Nature. 403, 339-342 (2000).