Part:BBa_K4743030

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Kill switch’s mechanism

The graph below (Figure 1) shows the first exposure to the normal condition. When there is a low amount of hydrogen sulfide, the translated sqrR protein (red) represses the sqr promoter, preventing the transcription of recombinase DRE or CRE and the antitoxin . Additionally, the terminator labeled by rox sequences halts the transcription of toxin. Consequently, no death occurs because there is no toxin and antitoxin present.

Figure 1: First exposure to the normal condition.

The second condition is exposure to the gut environment(shown in figure 2). When the organism is exposed to the gut, which hydrogen sulfide(Blue) presents , the suppressive effect of the sqrR suppressor decreases, resulting in the transcription of recombinase DRE or CRE and the antitoxin .The recombinase DRE targets the terminator sequence labeled by rox and cleaves it, allowing the toxin to be transcribed. However, the transcribed antitoxin neutralizes the toxin and inhibits its toxic activity. In conclusion, the cell remains alive.

Figure 2: exposure to the gut environment.

The third condition(Figure 3)represents the organism's second exposure to the normal environment, which signifies its escape from the gut or excretion of feces. In this condition, the sqrR suppressor binds to the promoter once again, restricting transcription of antitoxin. However, the terminator that halts toxin transcription is excised, resulting in the transcription of the toxin. In this scenario, the bacteria lose the antitoxin, ultimately leading to cell death.

Figure 3: Second exposure to the normal environment.

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