Factor which affects the mode of genetic recombination in E. Coli.

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THE normal recombination process in Escherichia coli requires a pathway involving a functional recB and recC gene product (henceforth designated the RecBC pathway)1. The recB and recC genes map as closely linked cistrons near thy on the E. coli chromosome map (54 min)2,3. ATP-dependent DNase (recBC DNase) has been identified as the product encoded by these genes4–6. Another gene essential for recombination proficiency, recA1, is located between cysC and pheA (51 min)8. Genetic transformation studies9–12 showed that the most efficiently transformable E. coli strain (genotype: recB− recC− sbcB−) lacks the DNase but retains recombination proficiency because of the sbcB− allele which acts as an indirect suppressor of the recB− recC− mutations13. This strain uses a recombination pathway (designated as the RecF pathway)1 which does not involve the ATP-dependent DNase but consists of products of several recombination genes13. Restoration of the ATP-dependent DNase to this strain by introduction of the recB+ recC+ alleles reduced the transformation frequency by a factor of approximately five9. In vitro studies demonstrated that this DNase degrades linear DNA molecules extensively, while it has no effect on circular DNA4. The adverse effect of the DNase on transformation may therefore be due to destruction of linear donor DNA molecules before they reach the recombination machinery. These results raise the possibility that in normal cells the DNase affects the mode of recombination by selecting or destroying DNA molecules of a particular structure.