Template:Team:Bonn:NetworkData

content.text= "The ccd module is a toxin-antitoxin (TA) system similar to the mazE/mazF system. The module is located on the F Plasmid in Escherichia coli bacteria and essential for their survival. Normally the toxin ccdB is inactivated by the presence of the antitoxin ccdA in the form of a ccdAB complex. If ccdA is no longer available, ccdB inhibits DNA gyrase which leads to cell death. Gyrase is a type IIA topoisomerase and is able to produce negative DNA supercoiling by making a double-strand break in the DNA and religating it. The gyrase enzyme consists of two subunits: the C-terminal GyrA domain that wraps around the DNA strand and the N-terminal GyrB domain that catalyses the ATP-dependant supercoiling of the DNA. CcdB stabilizes the gyrase cleavage complex by binding to the GyrA domain and thus blocks the catalytic function of the gyrase. That means that the gyrase remains bound to the DNA and the cleaved DNA is not religated. DNA- and RNA polymerases can’t copy the DNA anymore and cell proliferation as well as protein biosynthesis is stopped. The double-stranded breaks in the DNA initiate cell death.<p>Because gyrases are specific to bacteria such as E. coli it is also a target for some anti-bacterial medications e.g. ciprofloxacin (CFX). As can be seen in the data below, CcdB proves to be as effective as CFX at inducing DNA cleavage ^{<a href=#1>[1]</a>}^{<a href=#2>[2]</a>}</p><div class='content-image'align='center'><img src='https://static.igem.org/mediawiki/2013/2/2f/Sspb_CFX_compared.jpg' width='300'>Comparison of the effect of CcdB and CFX on gyrase activity. N: negatively supercoiled DNA, L: linear DNA, SC: supercoiled DNA. A higher concentration of CcdB/CFX leads to more cleaved (linear) DNA^{<a href= http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1635281/figure/fig2/>[source]</a>}</div><div class='content-image' align='center'><img src='https://static.igem.org/mediawiki/2013/e/e4/Bonn_Ccdb_and_ccda.jpg' width='300'>  A higher concentration of ccdB leads to blocking of gyrase and positively supercoiled DNA is cleaved to linear DNA instead of being processed to negatively supercoiled DNA. ^{<a href=http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3460896/figure/pone-0046499-g003/>[source]</a>}</div><p>We used this system to build a light-induced kill-switch. Therefore we added the ssrA tag to the antitoxin ccdA. When the bacteria a emitted to light, ccdA is degraded and ccdB is set free and can bind to the gyrase. and cell death is initiated.  Like in most TA systems, the toxin ccdB is relatively stable, while the antitoxin ccdA is vulnerable to degradation.</p><h2>References</h2><p><a name='1'>1.</a> <a href='http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1635281/'>A strand-passage conformation of DNA gyrase is required to allow the bacterial toxin, CcdB, to access its binding site, Andrew B. Smith and Anthony Maxwell, Nucleic Acids Res. 2006 October; 34(17): 4667–4676, PMC 1635281</a></p><p><a name='2'>2.</a> <a href='http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3460896/'> A Common Origin for the Bacterial Toxin-Antitoxin Systems parD and ccd, Suggested by Analyses of Toxin/Target and Toxin/Antitoxin Interactions, Andew B. Smith et al, PLoS One. 2012; 7(9): e46499, PMCID: PMC3460896</a></p>";