The induction of recombination in wild type and ultraviolet light sensitive mutants of Escherichia coli K12 by chemical mutagens and ultraviolet irradiation

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The frequency of F' histidine dependent homogenotes arising from histidine independent heterogenotes vzas increased by N-methyl-N'-nitro-N-nitrosoguanidine, diethyl sulfate (DES), ethyl methanesulfonate (EMS), methyl methanesulfonate (MMS), nitrous acid, 2-aminopurine, UV irradiation and methotrexate. These agents may stimulate recombination by increasing the number of single-stranded regions in the DMA duplex or preventing the closure of these gaps; thus, increasing the probability of synapse between heterologous DMA molecules. At similar survival, MMS induced higher recombination frequencies than EMS, though EMS induced more auxotrophic mutant clones. Methylated bases are more rapidly depurinated than ethylated bases; therefore, depurination-breakage may increase recombination while decreasing mutagenesis. The frequency of induced histidine dependent homogenotes was significantly higher when the his^- allele was located on the F' factor rather than the chromosome. A mutant of E. coli K12 (uvr C+ ) which is unable to excise pyrimidine dimers had a higher recombination frequency in response to UV irradiation than an isogenic uvr C+ strain. This result supports the hypothesis that it is primarily the gaps formed in the newly synthesized DMA strand, opposite the dimers in the template, that give rise to recombinant molecules. [...]