Analysis of the role of recombination and repair in mutagenesis of Escherichia coli by UV irradiation.

Multiple mutant strains have been tested or their mimicry of the UV-mutagenesis deficiency of a recA single mutant. Revertants to histidine prototrophy and clear plaque mutants of lambda were scored to determine capacity for UV-mutagenesis. Nearly normal capacity was shown by a uvr+ recB- recF- strain, which shows almost no recA-dependent recombination, by uvr- recB+ recF- strains, which show almost no recA-dependent repair and by a uvrA- recB- recF- strain, which shows neither recA-dependent recombination nor repair. Since the uvr mutants can be assumed to show additionally no excision repair, these results may mean that UV-mutagenesis occurs during processes other than recombination and repair. Alternative hypotheses are discussed. The slight difference in mutagenic capacity was traced to the recF single mutation, which blocks the production of unmixed bursts of clear-plaque lambda mutants. Since this accounts for only about 10% of the mutations leading to clear-plaque mutants, it is suggested that there is more than one UV-mutagenic process.