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 chro mosome revealed a new gene (rdgC) encoding a function required for gro wth in recombination-deficient recBC sbcBC strains. Deletion of rdgC d oes not reduce viability, conjugational recombination, or DNA repair i n rec(+), recA, recB, recF, or recJ mutants. However, it makes the gro wth of recBC sbcBC strains reliant on the RecA, RecF, and RuvC protein s and, to a large extent, on RuvAB. The recBC sbcBC Delta rdgC ruvAB c onstruct forms colonies, but cell viability is reduced to <5%. A recBC sbcBC a rdgC derivative carrying the temperature-sensitive recA200 al lele grows at 32 degrees but not 42 degrees. Multicopy rdgC(+) plasmid s reduce the growth rate of recBC sbcBC strains, while multicopy sbcC( +) plasmids that reactivate SbcCD nuclease cannot be maintained withou t RdgC protein. The data presented are interpreted to suggest that exo nuclease-depleted recBC sbcBC strains have difficulty removing the dis placed arm of a collapsed replication fork and that this problem is co mpounded in the absence of RdgC. Recombination then becomes necessary to repair the fork and allow chromosome duplication to be completed. T he possibility that RdgC is an exonuclease is discussed.