Abstract
The transfecting efficiency of P22 DNA on “rough” strains of Salmonella typhimurium or non-restricting mutants of Escherichia coli K12 approaches 3 × 10 −8 plaques/genome equivalent. It increases 20-fold upon complete erosion of the terminally redundant regions of the DNA molecule with either λ exonuclease or exonuclease III. Eroded DNA molecules form circles and linear oligomers upon annealing. The circular monomers display transfecting activity about ten times higher than that of eroded linear monomers or hydrogen-bonded oligomers. recB recC sbcB strains of E. coli K12 are transfected with P22 DNA with an efficiency of 1.5 × 10 −6 plaques/genome equivalent. The activity of DNA molecules on these strains is not augmented by erosion. This suggests that the activation by erosion, seen in assays on rec + genotypes, is due to the formation of hydrogen-bonded circular molecules, which more readily escape degradation by the recBC nuclease.
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