Effect of lig-7 on strand joining in repair of damaged DNA and on cutting of intact homologous DNA (cutting in trans) in Escherichia coli

Genetic recombination in Escherichia coli depends on the recA⁺ gene and can be increased in frequency by certain treatments that damage DNA. In previous studies (Ross &; Howard-Flanders, 1977a,b), E. coli (λ) cells were infected with undamaged λ phages and then with λ phages that were either undamaged, or had interstrand crosslinks produced in their DNA by treatment with psoralen and light. When the superinfecting DNA contained psoralen crosslinks, the intact DNA was cut. This cutting, referred to as cutting in trans, occurred only in DNA genetically homologous to the damaged DNA, required recA⁺ and behaved as expected of a step in damage-induced genetic recombination.In the present studies, we investigated the effect on cutting in trans of lig-7, a thermosensitive allele of the structural gene for E. coli polynucleotide ligase and also of uvrA, which controls the excision of damaged bases from DNA. The ligase deficiency caused gaps due to the action of the uvrA⁺ endonuclease on damaged DNA to remain open for at least 25 minutes. For low levels of damage, cutting in trans was also enhanced in the lig-7 cells at non-permissive temperatures but was not increased in wild-type cells. The enhanced cutting in trans depended upon genetic homology, as expected if it reflected elevated levels of damage-induced genetic recombination. Presumably, the unrepaired gaps in the damaged DNA made it a good substrate for the enzymes that promote cutting in trans of its homologs.