Letter | Published:

Normal Replication of DNA after Repair Replication in Bacteria

Naturevolume 214pages269270 (1967) | Download Citation



AN early step in the dark repair of ultra-violet damaged DNA in bacteria is the excision of the damaged single strand regions1,2. It has been postulated that a later step involves the replacement of the excised segments with undamaged deoxyribonucleotides, utilizing the complementary base pairing with the intact single strand1,2. Direct physical evidence for this repolymerization step has been obtained in our laboratory employing the procedure of DNA density labelling with 5-bromouracil and density distribution analysis in caesium chloride density gradients3 to distinguish semiconservative replication from the non-conservative mode of repair replication4. This non-conservative replication has been observed in bacteria after treatment with ultra-violet4, nitrogen mustard5, nitrosoguanidine (unpublished work of E. Cerda-Olmedo and P. C. Hanawalt), or thymine starvation6. The belief that this is the postulated repair synthesis step is strengthened by the results of a number of control experiments as follows. Non-conservative DNA replication is not observed in bacteria in which ultra-violet induced pyrimidine dimers have been repaired in situ by photo-reactivating conditions before density labelling4. It is not observed after ultra-violet irradiation of a strain, E. coli BS–1, sensitive to ultra-violet light which is unable to perform the excision step in the repair sequence7. Non-conservative replication has also been demonstrated by the use of deuterium oxide and nitrogen-15 as density labels to prevent possible artefacts caused by the pathogenicity of 5-bromouracil8. We report here an additional control experiment which demonstrates that the DNA molecules damaged by ultra-violet irradiation which have undergone non-conservative replication are then capable of normal semiconservative replication.

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  1. Biophysics Laboratory, Stanford University, Stanford, California

    • P. C. HANAWALT


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