Mutant allele gam18 involved in the RecF repair pathway of Escherichia coli K-12

Plasmid pGam18 carrying one of the cloned mutant loci, responsible for enha nced radiation resistance in the strain Escherichia coli Gam(r)444, was sho wn to increase resistance to the lethal effect of gamma-rays with a dose mo dification factor DMF = 2. Enhanced resistance was observed in wild-type ce lls and in the mutant recBC sbcB, but not recBC sbcA. This indicates the in volvement of a product of the gam18 locus in the RecF pathway of recombinat ional repair. The protective effect of plasmid pGam 18 against radiation wa s completely abolished by mutations in the most RecF pathway genes (recF, r ecJ, recR, recO, recQ, recN and ruvB). However, three mutations in the uvrD gene, which encodes DNA helicase II and belongs to the RecF pathway, can b e partially complemented by plasmid pGam18. These data suggest that the mut ant allele gam18 affects the DNA helicase II activity at the presynaptic st age of the RecF pathway-mediated repair of DNA double-stranded breaks induc ed by gamma-irradiation.