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 lineariz ed plasmid DNA at a specific site in vivo and monitored its behavior i n recA mutant cells deficient in recombinational repair. Earlier, we h ad found that in wild-type (WT) cells linearized DNA is degraded to co mpletion by RecBCD nuclease. We had also found that in WT cells chi si tes on linear DNA inhibit RecBCD degradation by turning off its nucleo lytic activities, Now we report that chi sites do not work in the abse nce of the RecA protein, suggesting that RecA is required in vivo to t urn off the degradative activities of the RecBCD enzyme. We also repor t that the degradation of linearized plasmid DNA, even devoid of chi s ites, is never complete in recA cells, Investigation of this linear DN A stability indicates that a fraction of recA cells are recBC phenocop ies due to ongoing chromosomal DNA degradation, which titrates RecBCD nuclease, A possible role for RecBCD-promoted DNA degradation in contr olling chromosomal DNA replication in E. coli is discussed.