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. 2014 May 3;13:63. doi: 10.1186/1475-2859-13-63

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Copyright © 2014 Dong and Zhang; licensee BioMed Central Ltd.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

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Operator-repressor system-based genetic engineering strategy. (A) In the presence of the R gene, the constitutively expressed R protein specifically binds to the P_r promoter and represses promoter activity. (B) When the R gene is excised from the genome, the P_r promoter is activated and mutants can be selected by antibiotic resistance. (C) Diagrams of selection and counter-selection marker cassettes. R: Repressor, AR: Antibiotic resistance, P_r: Promoter. [AR1^R], [AR1^S], [AR2^R], and [AR2^S] are AR1-resistant, AR1-sensitive, AR2-resistant, and AR2-sensitive bacterial phenotypes, respectively.