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J. Bacteriol. doi:10.1128/JB.00613-06
Copyright (c) 2006, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

sbcB15 and sbcB mutations activate two types of RecF recombination pathway in Escherichia coli

Ksenija Zahradka, Sanela imi, Maja Buljubai, Mirjana Petranovi, Damir Ðermi, and Davor Zahradka^*

Division of Molecular Biology, Ruðer Bokovi Institute, Zagreb, Croatia

^* To whom correspondence should be addressed. Email: zahradka{at}irb.hr.

	Abstract

Escherichia coli cells mutated in recBC genes are defective for the main, RecBCD pathway of recombination and show a severe reduction in conjugational and transductional recombination as well as in recombinational repair of double-stranded DNA breaks. This phenotype can be corrected by suppressor mutations in sbcB and sbcC(D) genes, which activate an alternative, RecF pathway of recombination. It was previously suggested that sbcB15 and sbcB mutations, which both inactivate exonuclease I, are equally efficient in suppressing recBC phenotype. In the present work we have re-examined the effects of sbcB15 and sbcB mutations on DNA repair after UV and irradiation, on conjugational recombination, and viability of recBC (sbcC) cells. We found that the sbcB15 mutation is a stronger recBC suppressor than sbcB, suggesting that some unspecified activity of the mutant SbcB15 protein may be favorable to recombination in the RecF pathway. We also showed that the xonA2 mutation, a member of another class of ExoI mutations, had the same effect on recombination as sbcB, suggesting that it is an sbcB null mutation. In addition, we demonstrated that recombination in a recBC sbcB15 sbcC mutant is less affected by recF and recQ mutations than in recBC sbcB sbcC and recBC xonA2 sbcC strains, indicating that SbcB15 alleviates the requirement for the RecFOR complex and RecQ helicase in recombination processes. Our results suggest that two types of sbcB-sensitive RecF pathways can be distinguished in E. coli: one that is activated by the sbcB15 mutation, and the other activated by sbcB null mutations. Possible roles of SbcB15 in recombination reactions on the RecF pathway are discussed.

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