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| | <partinfo>BBa_K3634018 short</partinfo> | | <partinfo>BBa_K3634018 short</partinfo> |
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| − | The ccdAB toxin-antitoxin (TA) module is a type II TA module where the toxic ccdB protein, poisons the enzyme DNA gyrase, required for negative supercoiling of DNA (Bernard and Couturier, 1992). Through ccdB-gyrase complex formation, DNA cleavage results as well as inhibition of transcription by the formation of RNA polymerase roadblocks. The activity of the unstable ccdA antitoxin separates the ccdB-gyrase complex if present (Vandervelde et al, 2017).
| + | (see twin - BBa_K899008) |
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| − | Expression of ccdA and ccdB within the type II TA module is self-regulated by low specificity and affinity of ccdA for individual binding sites of the regulatory region upstream of both ccdA and ccdB genes. This 113bp ccdAB promoter/operator sequence extends into the first ccdA gene and is suspected to have a total of 8 ccdA operator binding sites (Tam & Kline, 1989). The antitoxin binds the operator DNA sites, with some sites overlapping the promoter, functioning as a repressor of ccdAB transcription. The toxin then functions as a co-repressor or de-repressor depending on the molar T:A ratio (Vandervelde et al., 2017).
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| − | At low T:A ratio, the operator is repressed however as the ratio is increased, repression is relieved by preferential formation of a V-shaped non-repressing heterohexamer (ccdB-ccdA-ccdB). Specifically, at the moment the molar ratio of T:A > 1, repression is rapidly lost (Vandervelde et al., 2017). Therefore the ccdA antitoxin has dual functionality: to prevent ccdB-gyrase formation and allow for precise transcriptional control of a desired output gene.
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