Stability of linear DNA in recA mutant Escherichia coli cells reflects ongoing chromosomal DNA degradation.

To study the fate of linear DNA in Escherichia coli cells, we linearized plasmid DNA at a specific site in vivo and monitored its behavior in recA mutant cells deficient in recombinational repair. Earlier, we had found that in wild-type (WT) cells linearized DNA is degraded to completion by RecBCD nuclease. We had also found that in WT cells chi sites on linear DNA inhibit RecBCD degradation by turning off its nucleolytic activities. Now we report that chi sites do not work in the absence of the RecA protein, suggesting that RecA is required in vivo to turn off the degradative activities of the RecBCD enzyme. We also report that the degradation of linearized plasmid DNA, even devoid of chi sites, is never complete in recA cells. Investigation of this linear DNA stability indicates that a fraction of recA cells are recBC phenocopies due to ongoing chromosomal DNA degradation, which titrates RecBCD nuclease. A possible role for RecBCD-promoted DNA degradation in controlling chromosomal DNA replication in E. coli is discussed.