Chromosomal Lesion Suppression and Removal in Escherichia coli via Linear DNA Degradation

RecBCD is a DNA helicase/exonuclease implicated in degradation of foreign linear DNA and in RecA-dependent recombinational repair of chromosomal lesions in E. coli. The low viability of recA recBC mutants vs. recA mutants indicates the existence of RecA-independent roles for RecBCD. To distinguish among possible RecA-independent roles of the RecBCD enzyme in replication, repair, and DNA degradation, we introduced wild-type and mutant combinations of the recBCD chromosomal region on a low-copy-number plasmid into a (delta)recA (delta)recBCD mutant and determined the viability of resulting strains. Our results argue against ideas that RecBCD is a structural element in the replication factory or is involved in RecA-independent repair of chromosomal lesions. We found that RecBCDcatalyzed DNA degradation is the only activity important for the recA-independent viability, suggesting that degradation of linear tails of (sigma)-replicating chromosomes could be one of the RecBCDʹs roles. However, since the weaker DNA degradation capacity due a combination of the RecBC helicase and ssDNA-specific exonucleases restores viability of the (delta)recA (delta)recBCD mutant to a significant extent, we favor suppression of chromosomal lesions via linear DNA degradation at reversed replication forks as the major RecA-independent role of the RecBCD enzyme.