Recombination-dependent growth in exonuclease-depleted recBC sbcBC strains of Escherichia coli K-12.

Analysis of the aroLM-sbcCD interval of the Escherichia coli K-12 chromosome revealed a new gene (rdgC) encoding a function required for growth in recombination-deficient recBC sbcBC strains. Deletion of rdgC does not reduce viability, conjugational recombination, or DNA repair in rec+, recA, recB, recF, or recJ mutants. However, it makes the growth of recBC sbcBC strains reliant on the RecA, RecF, and RuvC proteins and, to a large extent, on RuvAB. The recBC sbcBC delta rdgC ruvAB construct forms colonies, but cell viability is reduced to < 5%. A recBC sbcBC delta rdgC derivative carrying the temperature-sensitive recA200 allele grows at 32 degrees but not 42 degrees. Multicopy rdgC+ plasmids reduce the growth rate of recBC sbcBC strains, while multicopy sbcC+ plasmids that reactivate SbcCD nuclease cannot be maintained without RdgC protein. The data presented are interpreted to suggest that exonuclease-depleted recBC sbcBC strains have difficulty removing the displaced arm of a collapsed replication fork and that this problem is compounded in the absence of RdgC. Recombination then becomes necessary to repair the fork and allow chromosome duplication to be completed. The possibility that RdgC is an exonuclease is discussed.