Fig. 2

Passcode and Deadman kill switches [28]. Both systems are built around synthetic molecules which are necessary for the cell’s survival. a The Deadman kill switch uses anhydrotetracycline (ATc) as the synthetic molecule and is based on mutually reinforcing feedback loops. ATc prevents inhibition of lacI by transcription of TetR. Transcription of the toxin is inhibited by LacI, preventing cell death. Furthermore, LacI weakly represses tetR and strongly represses transcription of the protease Mf-Lon, which degrades LacI. Using a combination of the restriction endonuclease EcoRI (the toxin) and the degradation of the essential protein MurC by Mf-Lon, the researchers were able to achieve escape frequencies below the limit of detection. b The Passcode kill switch is based on galactose, cellobiose, and IPTG as three independent ‘input signals’. Input A (galactose) inhibits GalR-LacI, while input B (cellobiose) inhibits CelR-LacI. Both GalR-LacI and CelR-LacI can bind the LacI-ScrR operator, thus inhibiting expression of LacI-ScrR. LacI-ScrR represses the expression of the toxin and can be inhibited by input C (IPTG). The cell only survives in presence of galactose and cellobiose and in absence of IPTG. Escape frequencies below 1 × 10^− 6 were reached using EcoRI and Mf-Lon as toxins