Survival, DNA-breakdown and induction of prophage lambda in aEscherichia coliK12recAuvrBdouble mutant

Abstract

Cellular functions of a double mutant of Escherichia coli K 12 deficient in recombination (recA) and defective in excision of pyrimidine dimers (uvrB) have been compared to those of isogenic recA or uvrB single mutants and ‘wild type’ bacteria. A combined effect of the two mutations on cell survival both under normal conditions of growth and after exposure to ultraviolet light or mitomycin C was demonstrated. The ratio of optical density to the number of colony formers in growing cultures of the double mutant is three times greater than in similar cultures of the recA single mutant and 9 times greater than in either uvrB or in ‘wild type’ cultures. The doubling time in growing recA uvrB cultures is 90 min, compared to 60 min, for the recA single mutant and 40 min for the uvrB single mutant and ‘wild type’ bacteria. Growing cultures of recA uvrB (λcI857) bacteria contain a substantial fraction of cells which are unable to form colonies at 32 °C, but produce phage when heated to 42 °C. No such cells were found in cultures of the single mutants or the ‘wild type’ bacteria lysogenic for λc1857. The double mutant is 10 times more sensitive to ultraviolet light and twice more sensitive to mitomycin C than the recA single mutant. In contrast to recA bacteria, exposure of the double mutant to mitomycin C induces little additional breakdown of cellular DNA. Induction of the prophage by mitomycin C is, however, prevented in both recA uvrB (λ) and recA (λ) bacteria. Exposure to mitomycin C creates conditions which render the prophage inducible by a newly transduced ree A gene. This effect of mitomycin C persists and can be revealed in complete medium at 37 °C after 100 min of incubation. The decay of the prophage, in cells exposed to mitomycin C, proceeds at a similar rate in both the double mutant and the recA single mutant. The inability of recA lysogens to be induced to phage production is discussed in the light of the present findings.