Role of the recF gene of Escherichia coli K-12 in λ recombination

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Abstract
When Escherichia coli K12(λ) lysogens are infected with heteroimmune λ phage, which are unable to replicate, general recombination between phage and prophage depends on the bacterial recF gene. It has been shown that in E. coli K12 postconjugational recombination, the RecF pathway only works with full efficiency if exonuclease I is absent (Clark 1973). However, results presented in this paper indicate that under conditions in which λ replication is blocked, the recombination pathway dependant on the recF gene is fully active in producing viral recombinants even, if the phage is Red+, in the presence of exonuclease I. In contrast, removal of λ exonuclease and β protein requires elimination of exonuclease I for an efficient RecF pathway. It is concluded that the Red system cooperates with the RecF pathway and that this cooperation involves overcoming the inhibitory effects of exonuclease I. In the absence of λ exonuclease, β protein stimulates recF-dependent recombination but does not suffice to prevent the negative effect of exonuclease I. In the presence of β protein, full efficiency of the RecF pathway can be obtained either via cooperation with λ exonuclease I or, if the viral exonuclease is defective, via inactivation of exonuclease I. Since activity of λ exonuclease appears necessary to overcome the inhibitory effects of exonuclease I, it is proposed here that λ exonuclease diverts material from the RecF pathway in a shunt reaction which allows completion of recF-initiated recombinational intermediates via a mechanism insensitive to exonuclease I. When λ replication is allowed, the Rec system produces viral recombinants mainly via a recF-independent mechanism. However, a major contribution of the RecF pathway to λ recombination is observed after removal of the Red system and exonuclease I.