DNA helicases in recombination and repair: construction of a DuvrD DhelD DrecQ mutant deficient in recombination and

DNA helicases play pivotal roles in homologous recombination and recombinational DNA repair. They are involved in both the generation of recombinogenic single-stranded DNA ends and branch migration of synapsed Holliday junctions. Escherichia coli helicases II (uvrD), IV (helD), and RecQ (recQ) have all been implicated in the presynaptic stage of recombination in the RecF pathway. To probe for functional redundancy among these helicases, mutant strains containing single, double, and triple deletions in the helD, uvrD, and recQ genes were constructed and examined for conjugational recombination efficiency and DNA repair profi-ciency. We were unable to construct a strain harboring a DrecQ DuvrD double deletion in a recBC sbcB(C) background (RecF pathway), suggesting that a DrecQ deletion mutation was lethal to the cell in a recBC sbcB(C) DuvrD background. However, we were able to construct a triple DrecQ DuvrD DhelD mutant in the recBC sbcB(C) background. This may be due to the increased mutator frequency in DuvrD mutants which may have resulted in the fortuitous accumulation of a suppressor mutation(s). The triple helicase mutant recBC sbcB(C) DuvrD DrecQ DhelD was severely deficient in Hfr-mediated conjugational recombination and in the repair of methylmethane sulfonate-induced DNA damage. This suggests that the presence of at least one helicase—helicase II, RecQ helicase, or helicase IV—is essential for homologous recombination and recom-binational DNA repair in a recBC sbcB(C) background. The triple helicase mutant was recombination an