ABSTRACT

We have examined coupled reactions with the RecA protein of Escherichia coli, which can mediate DNA strand exchange in vitro between homologous DNA molecules, and the RecJ exonuclease, a 5` to 3` single-stranded DNA exonuclease. In RecA-mediated strand-transfer reactions between circular single-stranded and duplex linear DNA, we have found that RecJ stimulates the rate of heteroduplex product formation. Because RecJ must be present concurrent with strand transfer and RecJ does not detectably stimulate the synapsis stage of the reaction, we believe that RecJ stimulates specifically the branch migration phase of the RecA strand-transfer reaction. RecJ also dramatically enhances the efficiency with which RecA is able to traverse regions of non-homology in the substrates. We propose a model where RecJ degrades the displaced strand produced by strand exchange which competes for pairing with the transferred strand, thus driving forward the unidirectional branch migration mediated by RecA protein. This suggests a new role for exonucleases in genetic recombination, facilitating the strand-transfer reaction itself.

FOOTNOTES

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This work was supported by Grant GM43889 from the National Institutes of Health. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore by hereby marked ``advertisement'' in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

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To whom correspondence and reprint requests should be addressed. Tel.: 617-736-2497; Fax: 617-736-2405; lovett{at}hydra.rose.brandeis.edu.

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The abbreviations used are: ssDNA, single-stranded DNA; dsDNA, double-stranded DNA; SSB, single-strand DNA binding protein; bp, base pair(s); kb, kilobase pair(s).

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S. Kowalczykowski, personal communication.

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S. T. Lovett, unpublished experiments.

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S. Corrette-Bennett, unpublished experiments.

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S. Lovett and R. D. Kolodner, unpublished experiments.

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