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content.titleLong = "A kill-switch system using the stress-induced toxin-antitoxin module MazEF in Escherichia coli";

content.summary= "The toxin-antitoxin system MazEF is composed by an upstream gene <i>mazE</i>, encoding a labile antitoxin, and a downstream gene <i>mazF</i>, that encodes a stable toxin. Connecting our light inducible protein degradation system to the antitoxin MazE allows light inducible cell death, as predominance of MazF in a bacterium activates cell death pathway.";

content.text= "Our system of light inducible protein degradation can be utilized to degrade any specific protein and is therefore usable to realize a light inducible kill-switch system. A connection of protein degradation to a cell death pathway is represented by the stress-induced toxin-antitoxin module <i>mazEF</i> in Escherichia coli. <i>mazEF</i> is located on the chromosome in E.coli which is associated with programmed cell death. The toxin-antitoxin system is composed by an upstream gene <i>mazE</i>, encoding a labile antitoxin, and a downstream gene mazF, that encodes a stable toxin. </br>The product of <i>mazF</i> cleaves mRNAs and tmRNAs at a specific site, which leads to an inhibition of translation. MazF shows a specific cleaving mechanism, which is not well understood yet, but shows that there is also protein synthesis which is unaffected by MazF. These proteins are presumably part of a cell death pathway. </br>The effect of <i>mazF</i> is suppressed by the Product of <i>mazE</i> which is degraded by the Protease ClpAP in bacteria. As a result of stressful conditions expression of the chromosomal <i>mazEF</i> module is reduced which leads to an imbalance between the products of <i>mazF</i> and <i>mazE</i>: When expression is lowered the stable toxin of <i>mazF</i> still persists while the labile antitoxin of <i>mazE</i> is degraded and can no longer suppress the effect of <i>mazF</i> leading to acute toxicity and cell death. </br><i>MazEF</i>-mediated cell death in E. coli can be caused by:<ul><li>extreme amino acid starvation^{<a href = '#1'>[17.1]</a>}</li><li> inhibition of transcription and/or translation by antibiotics such as rifampin, chloramphenicol, and spectinomycin under specific growth conditions^{<a href = '#1'>[17.1]</a>}</li><li>inhibition of translation by the Doc protein of prophage P1^{<a href = '#1'>[17.1]</a>}</li><li>DNA damage caused by thymine starvation as well as by mitomycin C, nalidixic acid, and UV irradiation^{<a href = '#1'>[17.1]</a>}</li><li>oxidative stress (H2O2)^{<a href = '#1'>[17.1]</a>}</li></ul>Amitai et al. tested in 2004 the Hypothesis of Pedersen et al.^{<a href = '#2'>[17.2]</a>}, that chromosomal toxin-antitoxin systems may rather cause a state of reversible bacteriostasis than programmed cell death^{<a href = '#1'>[17.1]</a>}.Therefore E.coli strain MC4100 &#916<i>mazE</i>F relA1 lacIq was cotransformated with:<ul><li>pBad-<i>mazF</i></li><li>pQE-&#916his-<i>mazE</i></li></ul><i>mazF</i>-expression can be induced by the addition of Arabinose via the pBad promoter of the first plasmid. The transformation of the second plasmid results firstly in the repression of <i>mazE</i> expression, whereas when IPTG is added <i>mazE</i> production is induced.</br><div class='content-image' align='center' height=501 width=410><a href='https://static.igem.org/mediawiki/2013/a/ac/Team_Bonn_MazF_1.png'><img src='https://static.igem.org/mediawiki/2013/a/ac/Team_Bonn_MazF_1.png' height=491 width=400></a></br><i>Ability of E. coli cells that had been ectopically overexpressing MazF in liquid medium to form colonies when ectopically overexpressing MazE on plates. The cultures were grown in LB medium (A) or M9 minimal medium with 0.5% glycerol (B) at 37°C to midlogarithmic phase (OD600, 0.5)^{<a href = '#1'>[17.1]</a>}.</i></div>Using these tools, Amitai et al. tested the effect of MazE overproduction on MazF-overproducing bacteria during growth in liquid medium.</br>The E.coli strain was incubated in LB medium. After <i>mazF</i> expression was induced by adding arabinose two samples were taken at several time points. To repress <i>mazF</i> expression to both of them glucose was added. In addition IPTG was added to one culture to induce <i>mazE</i> expression. The two cultures were compared via the level of protein synthesis and OD600.</br>Finally Amitai et al. confirmed the assumption that the overproduction of MazE after until 6h under overproduction of MazF could resuscitate E.coli cells in LB medium (Fig. 1A)^{<a href = '#1'>[17.1]</a>}, but the longer MazF was induced the less cells could be resuscitated by MazE.</br>Whereas MazE overproduction can reverse the inhibitory effect of MazF on translation, it cannot reverse the effect of MazF on colony formation, which is shown in figure 2. Only 1h after the induction of MazE expression, the rate of translation was restored to nearly 100% (Fig.2 Aa, Ab, Ac) but the bacteriocidic effect could not be reversed (Fig.2 Ba, Bb, Bc).</br>Additionally, Amtai et al. found out, that in M9 medium MazE was less able to reverse the effects of MazF overexpression than in LB medium (Fig.1B vs. 1A). it was concluded that there is a point of no return, when MazE is inable to resuscitate a MazF damaged cell, which occurs earlier in M9 medium than in LB medium.</br>Based on their results a model of the MazEF mechanism was built: A <i>mazF</i>-mediated cascade leads to a cell death pathway, but can nevertheless be stopped at several intermediary steps by e.g. <i>mazE</i>. When a point of no return is reached, the cascade cannot be stopped anymore.<div class='content-image' align='center' height=827 width=784><a href='https://static.igem.org/mediawiki/2013/9/9e/Team_Bonn_MazF_2.png'><img src='https://static.igem.org/mediawiki/2013/9/9e/Team_Bonn_MazF_2.png' height=817 width=764></a></br><i>Effect of MazE overproduction during growth in liquid medium on the ability of MazF-overproducing E. coli cells to synthesize proteins and form colonies. To induce <i>mazE</i> expression, IPTG was added to the bacterial culture at 1 h (Aa and Ba), 4 h (Ab and Bb), and 6 h (Ac and Bc) after <i>mazF</i> induction at time zero. The effects of the ectopic overexpression of MazE were measured at 1 and 3 h after the induction of <i>mazE</i> expression.^{<a href = '#1'>[17.1]</a>}.</i></div></br>Back to our project and to the idea of a light inducible kill-switch system:</br>As we described in the previous paragraph for both of our kill-switch systems using the MazEF module either MazF or MazE could be degraded:<ul><li>Using the degradation of MazF:</b> For that purpose the insertion of a plasmid containing the ssrA-tagged toxin encoding gene is needed. Since the predominance of the toxin activates a cell death pathway in bacteria, a bacterium containing a module that allows light inducible degradation of the MazF would only be viable, when it is degraded. In darkness the MazF overexpression is no longer compensated and aggregation of it leads to cell death. </li><li><b>Using the degradation of the antitoxin:</b> Two plasmids are needed: The first one to express the MazF and the second one to express the ssra-tagged MazE, in such manner that the amounts of the <i>MazF</i> and the <i>MazE</i> are in equilibrium. Once light induces the degradation system, the MazE is degraded and the predominant toxin will kill the bacterium.</li></ul>As we explained in the general kill-switch system text we finally focused on the second system (via the degradation of MazE).</br>With the design of a MazEF kill-switch system the possibility of resuscitating bacteria in the way Amitai et al. showed has to be considered. A predominant MazF could kill a bacterium in LB within about two hours, but it needs to be predominant over a long period (>7h) to induce its death without it being resuscitated by renewed <i>mazE</i> expression (Fig.1A)with more than 50% probability .</br>Certainly these facts seem to be unfavourable for the realization of a kill switch system via MazEF, but fortunately our system of heterodimerization (Lungu et al.) allows long continuous degradation^{<a href = '#3'>[17.3]</a>}, due to the high stability of the light induced hererodimer. Therefore likely a short exposure time will result in prolonged protein degradation sufficing for bacterial death. Additionally, Amitai et al. showed that the less nutrition is available for a bacterium, the earlier the point of no return is reached. If a bacterium escapes the lab, it will likely have less nutrition available than in LB medium. It might reach the point of no return earlier.</br>We described a light inducible MazEF kill-switch system via the insertion of plasmids into bacteria. However a final kill-switch system would have to be implemented in the genomic DNA since plasmids in bacteria can be ejected, for instance via cell division, whereas a genomic DNA mutation is less likely to occur. Nevertheless risk of a loss of function cannot be eliminated , which is why a secure system should countain much more than one kill-switch system to compensate the malfunction of a single kill-switch system. Therefore, we consider the MazEF kill-switch system to be part of a much larger security system for genetically engineered bacteria.<h2>References</h2><a name = '1'>^[17.1]</a> <a href = 'http://www.ncbi.nlm.nih.gov/pmc/articles/PMC532418/'>MazF-Mediated Cell Death in Escherichia coli: a Point of No Return, Shahar Amitai et al., Journal of Bacteriology Vol. 186, No. 24, 2004, p.8295–8300.</a></br><a name = '2'>^[17.2]</a> <a href = 'http://www.ncbi.nlm.nih.gov/pubmed/?term=Rapid+induction+and+reversal+of+bacteriostatic+conditions+by+controlled+expression+of+toxins+and+antitoxins'>Rapid induction and reversal of bacteriostatic conditions by controlled expression of toxins and antitoxins, Pedersen et al., Molecular Microbiology 45, 2002, 501–510.</a></br><a name = '3'>^[17.3]</a> <a href = 'http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3334866/'>Designing Photoswitchable Peptides Using the AsLOV2 Domain, Oana I. Lungu et al., Chem Biol. 2012, 19(4):507-17.</a>";