Viability of rep recA mutants depends on their capacity to cope with spontaneous oxidative damage and on the DnaK chaperone protein

Replication arrests due to the lack or the inhibition of replicative helica ses are processed by recombination proteins. Consequently, cells deficient in the Rep helicase, in which replication pauses are frequent, require the RecBCD recombination complex for growth. rep recA mutants are viable and di splay no growth defect at 37 or 42 degreesC. The putative role of chaperone proteins in rep and rep recA mutants was investigated by testing the effec ts of dnaK mutations, dnaK756 and dnaK306 mutations, which allow growth of otherwise wild-type Escherichia coli cells at 40 degreesC, are lethal in re p recA mutants at this temperature. Furthermore, they affect the growth of rep mutants, and to a lesser extent, that of recA mutants. We conclude that both rep and recA mutants require DnaK for optimal growth, leading to low viability of the triple (rep recA dnaK) mutant, rep recA mutant cells form colonies at low efficiency when grown to exponential phase at 30 degreesC. Although the plating defect is not observed at a high temperature, it is no t suppressed by overexpression of heat shock proteins at 30 degreesC. The p lating defect of rep recA mutant cells is suppressed by the presence of cat alase in the plates. The cryosensitivity of rep recA mutants therefore resu lts from an increased sensitivity to oxidative damage upon propagation at l ow temperatures.