In addition, we conceive a suicide gene circuit to ensure these recombinanational bacteria can only survive in laboratories and not cause horizontal gene transfer. We use Lac operon as a skeleton, ccdB gene to cause cell death, and ccdA to prevent bacteria in laboratory from death.

    As you can see, ccdB and LacI continuingly transcribe in bacteria. LacI protein can inhibit ccdA's transcription, which can combine with ccdB to neutralize ccdB's cell toxicity. IPTG can bind with the LacI protein, so when we add IPTG into culture medium, ccdA gene can translate and bacteria finally survive. If we use our E.coli outside the laboratory to sense the contamination, there are scarcely IPTG or lactose in soil or river water, so ccdA can't transcribe anymore. At last, bacteria will die after a few hours. The short-lived bacteria can lower the risk of unexpected gene transfer. However, we only ligate some of these genes and did not test because of limited time