Mutants of Escherichia coli K-12 defective in DNA repair and in genetic recombination.

BACTERIA are readily killed by exposure to ultraviolet light (UV) , presumably as the result of the formation of UV photoproducts in the bacterial DNA. It is also known that pyrimidine dimers are among the photoproducts formed with a high yield, and that bacteria normally contain an efficient mechanism for the repair of DNA containing these products. Pyrimidine dimers are excised from DNA during incubation and can be recovered, still contained within a short oligonucleotide. This excision appears to involve the interruption of single DNA strands, and to be followed by local DNA breakdown and repair synthesis (SETLOW and CARRIER 1964; BOYCE and HOWARD-FLANDERS 1964; PETTIJOHN and HANAWALT 1964). These observations are of interest not only because they reveal the mechanism of an effective and possibly widespread process for the repair of injured DNA, but also because they suggest that local repair can occur in one strand of a double helix. This mechanism for DNA repair may be related to the mechanism of genetic exchange. An insight into the mechanisms of genetic recombination was gained through the discovery that h bacteriophage recombinants can be formed by joining fragments of preexisting phage DNA molecules (MESELSON and WEIGLE 1961). As continuity in the base sequence of the phage genome must be preserved, base pairing between overlapping single strands from each parentel molecule is presumably required as a prelude to the formation of a recombinant ( LEVINTHAL 1959). It has been suggested that the process of recombination may be completed by local DNA repair synthesis on either side of the overlap (MESELSON 1964) and that certain of the enzymes involved may serve in both genetic recombination and in repair after irradiation ( HOWARD-FLANDERS and BOYCE 1964). Further evidence in support of these concepts has been obtained through the isolation of mutants of E. coZi K-12 that have lost the ability to form recombinants when mated with suitable donor strains. As these mutants are able to accept genetic material normafly, it appears that they may be defective in the process of integrating the donor DNA into the recipient chromosome (phenotype symbol Kec-) . Because of the suggested relationship between repair and recombination, the strains were tested for ability to survive exposure to UV, and were found to be highly radiosensitive (CLARK and MARGULIES 1965). After exposure to UV, these mutants degrade their DNA excessively and fail to incorporate labeled thymidine ( CLARK, CHAMBERLIN, BOYCE, and HOWARD-FLANDERS 1966).