RELEASE OF ULTRAVIOLET LIGHT-INDUCED THYMINE DIMERS FROM DNA IN E. COLI K-12

The fate of thymine dimers in DNA during incubation after uv light irradiation was studied in two strains of E. coli K-12. One was a multiply auxotrophic strain, AB1157, and the other was a uv-sensitive mutant, AB1886, derived from it. Strain AB1886 is unable to reactivate uv-irradiated Tl phage and is known to have a mutation at the uvrA locus. Cells were labeled in their DNA by growth with H/sup 3/thymidine, exposed to uv light, incubated in enriched minimal medium, and extracted with cold trichloroacetic acid. The acid precipitate and soluble fractions were hydrolyzed in hot acid, and the products were separated by paper chromatography. Thymine dimers were identified in the acid-insoluble fractions from both strains before incubation. During incubation thymine dimers were released into the acid-soluble fraction in the parental strain, AB1157, but not in the uv-sensitive strain AB1886. It is concluded that thymine dimers are excised from the DNA during the reactivation process in the uvr/sup +/ strain and that the sensitive uvr/sup -/ strain cannot do this. These findings suggest that the enzymatic removal of injured bases, including thymine dimers, and the reconstruction of the DNA from information on the complementary strand may be an important biologicalmore » mechanism for the preservation of DNA. (auth)« less