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content.text= "One approach to regulate protein activity is to use posttranslational modification, especially by splitting off an inactivating part from a translated pro-peptide. A change in conformation leads to the functional protein. The cleaving can be effected by enzymatic proteolysis, autoproteolysis or surrounding conditions (e.g. pH).<sup><a href='#1.1'>1.1</a></br> A simple example is the activation of pepsinogen which is caused by a low pH (fig. 1). </br> It is a powerful system to activate proteins, which is frequently used in nature and science. Unfortunately posttranslational cleaving can usually not be reversed. Once activated the inactive pro-form of the Protein cannot be restored.<h2>References</h2><a name='4.1'>1.1</a> <a href='http://www.ncbi.nlm.nih.gov/books/NBK22589/'> Berg JM, Tymoczko JL, Stryer L.: &quot;Biochemistry. 5th Edition&quot;. New York: W H Freeman 2002.</a>";  

content.summary= "Another irreversible method of protein regulation in bacteria is the Gene knock-in or –out. Here the genomic DNA is manipulated to gain or lose a function, in other words whether a protein is produced or not";

content.text= "To insert the gene of interest in the bacteria genome recombineering is often used. Utilising this system, it is possible to insert or delete DNA. It is based on the homologous recombination of small engineered DNA parts with a target gene. Recombination then occurs in the defined region of the genome, resulting in the insertion of the artificial sequence.</br>One possible method is using bacteriophage proteins from bacteriophage lambda, which mediates the insertion of DNA cassettes ^{<a href=#5.1>[5.1]</a>} ^{<a href=#5.2>[5.2]</a>}.</br> Gene expression, i.e protein production, after a knock-in can be regulated by a promoter. </br> The disadvantage of this particular way of protein regulation is the time needed from the beginning of the induction until protein levels are reached. </br>A knock out causes a complete loss of function. When a gene sequence is interrupted, it may still be translated, but the resulting protein will be non-functional. In this case it is not possible to regulate the protein activity at all since the protein does not come into effect at all and therefore cannot be regulated. </br></br> <p><a name=5.1>[5.1] </a> <a href=http://www.ncbi.nlm.nih.gov/pmc/articles/PMC107131/ > Kenan Murphy, Use of bacteriophage &lambda recombination functions to promote gene replacement in Escherichia coli (1998) </a> </p> <p> <a name=5.2> [5.2] </a> <a href=http://www.ncbi.nlm.nih.gov/pmc/articles/PMC148353/> Rapid modification of bacterial artificial chromosomes by ET- recombination (1998) </a> </p>";

content.text= "<h1>Applications</h1><p>Our system offers several different applications sicne the ssrA tag can be added theoretically to every protein. Hence it allows the user to any protein and eliminate it just by switching on blue light!</p><p>A possible application is a ssrA-labeled toxin. In this case the cells would only be viable if blue light is present and the toxin is degraded.  Alternatively one could create a system in which an antitoxin is degraded in the presence of blue light while the toxin is encoded on another plasmid. The cells would be killed in the present of blue light. These so called &quot;kill-switches&quot; could be used on the one hand for lab security as cells could be created which could only survive in a laboratory where only red light is present, however, they would die in nature since daylight is energetic enough to trigger antitoxin degradation and thus kill escaped bacteria. On the other hand a kill-switch can be used in safe environmental applications using bacteria which can only survive under daylight exposure and die at night when no light is present anymore.</b>Besides our system can be used to investigate the function of a certain gene. By switching degradation of a gene product on and off, the cell&apos;s activity and phenotype can be studied with and without the protein of interest.</p><p>The advantages in contrast to other protein degradation systems are that our system is fast and has little unwanted side effects.</b>Our systems is faster since it only takes milliseconds until the blue light penetrates the cells. Comparable protein degradation systems are induced by small activator molecules which need time to reach their target.</b>The ssrA tag consists of only 15 amino acids and hence changes the protein sequence only very little and an effect on the conformation is unlikely. This creates a high variety of proteins which can be degraded. Additionally there are no external activator molecules in our system which could have an effect on the results.</p>";