By this, tailed DNA ends are processed to entry sites for RecBCD. It is concluded that in wild-type cells the tails are effectively removed by single-strand-specific DNases including exonuclease I, RecJ DNase, and SbcCD DNase. The findings suggest that the DNA fragments had single-stranded tails of a length which prevents loading of RecBCD. Further, mutations in xonA, recJ, and sbcCD, particularly in the recJ sbcCD and xonA recJ sbcCD combinations, impeded RecBCD silencing. The silencing was suppressed by induction or permanent derepression of the SOS system or by the overproduction of single-strand DNA binding protein (from a plasmid with ssb +) which is known to inhibit degradation of chromosomal DNA by cellular DNases.
In accord with previous observations, it was concluded that the RecBCD enzyme is silenced during interaction with duplex DNA fragments containing Chi nucleotide sequences. During this time, chromosome fragmentation occurred as demonstrated by pulsed-field gel electrophoresis. The activity decrease depended on excision repair proficiency of the cells and a postirradiation incubation. The activity decrease was seen by an increase in survival of phage T4 2 − of about 200-fold (phage T4 2 − has defective duplex DNA end-protecting gene 2 protein). We found that this activity decreased strongly when cells were irradiated with UV light (135 J/m 2). The RecBCD enzyme has a powerful duplex DNA exonuclease activity in vivo.
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