Published online 18 February 2006
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RecJ exonuclease: substrates, products and interaction with SSB
Department of Biology and Rosenstiel Basic Medical Sciences Research Center, Brandeis University Waltham, MA 02454-9110, USA 1Graduate Program in Biophysics and Structural Biology, Brandeis University Waltham, MA 02454-9110, USA 2New England Biolabs, Inc. 240 Country Road Ipswich, MA 01938-2723, USA
*To whom correspondence should be addressed. Tel: +1 781 736 2497; Fax: +1 781 736 2405; Email: lovett{at}brandeis.edu
Received December 23, 2005. Revised January 25, 2006. Accepted January 25, 2006.
The RecJ exonuclease from Escherichia coli degrades single-stranded DNA (ssDNA) in the 5'–3' direction and participates in homologous recombination and mismatch repair. The experiments described here address RecJ's substrate requirements and reaction products. RecJ complexes on a variety of 5' single-strand tailed substrates were analyzed by electrophoretic mobility shift in the absence of Mg2+ ion required for substrate degradation. RecJ required single-stranded tails of 7 nt or greater for robust binding; addition of Mg2+ confirmed that substrates with 5' tails of 6 nt or less were poor substrates for RecJ exonuclease. RecJ is a processive exonuclease, degrading 
1000 nt after a single binding event to single-strand DNA, and releases mononucleotide products. RecJ is capable of degrading a single-stranded tail up to a double-stranded junction, although products in such reactions were heterogeneous and RecJ showed a limited ability to penetrate the duplex region. RecJ exonuclease was equally potent on 5' phosphorylated and unphosphorylated ends. Finally, DNA binding and nuclease activity of RecJ was specifically enhanced by the pre-addition of ssDNA-binding protein and we propose that this specific interaction may aid recruitment of RecJ.
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