Some properties of mutants of Escherichia coli K12 defective in DNA polymerase I and Exonuclease V

Abstract

The double mutant polA1 recB21 appears to be viable, although it grows poorly. By contrast, double mutants of either recB21 or recC22 with the temperature sensitive mutation polA12 are inviable at non-permissive temperatures. This suggests that some function of DNA polymerase I, the polA+ gene product, is more deficient in polA12 cells at the nonpermissive temperature than it is in polA1 cells. Differences between polA12 and polA1 mutations are also evident when ultraviolet light and γ-ray survival curves of polA1 recB21 strains are compared with those of polA12 recB21 and polA12 recC22 at various temperatures. Unlike polA1 recB21 double mutants in which the sensitivity associated with each mutation is not additive polA12 recB21 and polA12 recC22 double mutants are approximately twice as radiation sensitive as recB21 and recC22 single mutants at 37°. Both polA12 recB21 and polA12 recC22 mutants grow normally at 41° if they acquire an sbcA suppressor mutation. This mutation results in the appearance of a new deoxyribonuclease which can apparently substitute for the Exonuclease V missing in recB and recC mutants. The viability of the polA1 recB21 double mutant does not appear to be due to the presence of suppressors of either mutation. However, this strain grows much better if it, too, acquires an sbcA suppressor mutation.