A GENETIC LOCUS IN E. COLI K12 THAT CONTROLS THE REACTIVATION OF UV-PHOTOPRODUCTS ASSOCIATED WITH THYMINE IN DNA

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Abstract
Genetic crosses between a UV-sensitive F strain of E. coli K12 and several Hfr strains showed that a genetic locus situated between the arglnine and arabinose loci on the male chromosome can confer UV resistance to the progeny of the zygotes. UV resistance was judged by the survival of colony-forming ability of the bacteria and from the survival of irradiated Tl phage with the bacteria as host. The properties of the UVR phenotype suggest that the UVR locus controls an enzyme system capable of reactivating certain photoproducts in DNA without light. UV-irradiated Tl phage is about 5 times more resistant when plated on UVR cells or when photoreactivated in UVS cells than it is when plated on UVS cells in the dark. There is an overlap in the kinds of photoproducts reactivated, as either UVR reactivation or photoreactivation can render harmless about 80 per cent of the UV photoproducts in the phage DNA that would be lethal in the UVS strain without light. The substitution of 5-bromodeoxyuridine for thymidine in T1 phage DNA inhibits reactivation by the UVR locus. This suggests that the UVR reactivation acts on certain photoproducts associated with thymine in DNA.