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Molecular and Cellular Biology, October 2004, p. 8323-8331, Vol. 24, No. 19
0270-7306/04/$08.00+0     DOI: 10.1128/MCB.24.19.8323-8331.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Nonhomologous End-Joining Ligation Transfers DNA Regulatory Elements between Cointroduced Plasmids

Toshio Ishikawa, Eun Jig Lee, and J. Larry Jameson^*

Division of Endocrinology, Metabolism, and Molecular Medicine, Northwestern University Feinberg School of Medicine, Chicago, Illinois

Received 14 March 2004/ Returned for modification 20 April 2004/ Accepted 2 July 2004

Cointroduction of plasmids into mammalian cells is commonly used to investigate transcription factor regulation of reporter genes or to normalize transfection efficiency. We report here that cotransfected DNA molecules commonly transfer enhancer elements from one plasmid to another. Using separate Renilla or Firefly luciferase reporters, we found that an estrogen response element (ERE) originally linked to one of the reporters stimulated expression of the non-ERE-containing reporter. Similar enhancer transfer was seen with the cytomegalovirus enhancer. This enhancer transfer effect was not seen when cells were transfected separately with the reporters and the extracts were then combined before luciferase assays. The degree of enhancer transfer increased with transfected plasmid concentration and was greater when linearized rather than circular plasmid DNA was used. We hypothesized that double-strand breaks and heteroligation of cointroduced DNA molecules mediated the transfer of regulatory elements from one molecule to another. PCR of transfected plasmid DNA confirmed nonhomologous end-joining (NHEJ) ligation of DNA fragments originally present in separate plasmids. The NHEJ reaction was enhanced by UV light treatment to introduce double-strand breaks, and it was greater after liposome-mediated transfection than after calcium-phosphate-mediated transfection. NHEJ also occurred after adenoviral transfer of DNA into cells. We conclude that NHEJ mediates the transfer of regulatory DNA elements among cointroduced DNA molecules. These findings indicate the need for caution when interpreting results of transfection experiments containing more than one plasmid and suggest a mechanism whereby viruses or other exogenous DNA might recombine to activate unrelated genes.

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* Corresponding author. Mailing address: Galter Pavilion, Suite 3-150, 251 E. Huron St., Chicago, IL 60611-2908. Phone: (312) 926-9436. Fax: (312) 926-7260. E-mail: ljameson{at}northwestern.edu.

Present address: Third Department of Internal Medicine, Teikyo University Ichihara Hospital, Ichihara, Chiba 299-0111, Japan.

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Molecular and Cellular Biology, October 2004, p. 8323-8331, Vol. 24, No. 19
0022-538X/04/$08.00+0     DOI: 10.1128/MCB.24.19.8323-8331.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

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