Some Properties of Excision-defective Recombination-deficient Mutants of Escherichia coli K-12

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Strains of Escherichia coli that carry the mutation uvrA6 show no measurable excision of pyrimidine dimers and are easily killed by ultraviolet (UV) light, whereas strains that carry recA13 are defective in genetic recombination and are also UV-sensitive. An Hfr strain carrying uvrA6 was crossed with an F− strain carrying recA13. Among the recombinants identified, one carrying uvrA recA proved to be of exceptional sensitivity to UV light. It is estimated from the UV dose (0.2 erg/mm2 at 253.7 nm) required to reduce the number of colony-forming cells by one natural logarithm that about 1.3 pyrimidine dimers were formed in a genome of 5 × 106 base pairs for each lethal event. This double mutant is 40 times more UV-sensitive than the excision-defective strain carrying uvrA6. The replication of one pyrimidine dimer is generally a lethal event in strains carrying recA13. Spontaneous breakdown and UV-induced breakdown of the deoxyribonucleic acid (DNA) of cells of the various genotypes were estimated by growing the cells in medium containing 3H-thymidine and measuring both acid-precipitable and acid-soluble radioactivity. The UV-induced degradation in strains with recA13 did not require the uvr+ genes and hence appears to depend upon a mechanism other than dimer excision. The greater level of survival after irradiation in Rec+ as compared to Rec− bacteria may be due to a recovery mechanism involving the reconstruction of the bacterial chromosome through genetic exchanges which occur between the newly replicated sister duplexes and which effectively circumvent the damaged bases remaining in the DNA.

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