Repair and recombination of nonreplicating UV-irradiated phage DNA in E. coli III. Enhancement of excision repair in UV-treated bacteria.

The question of whether induction of the SOS response in Escherichia coli increases the efficiency of excision repair was addressed by measuring repair of UV-damaged nonreplicating lambda phage DNA in previously irradiated bacteria. Prior UV irradiation of lex+ bacteria enhanced both the rate of regeneration of infective phage DNA (about 10-fold) and the rate of cyclobutane dimer removal early in repressed infections. Indirect induction of SOS-regulated repair activities by the nonreplicating irradiated phage DNA itself seemed negligible. Prior bacterial irradiation reduced the frequency of recombination (loss of a tandem chromosomal duplication) of nonreplicating UV-irradiated DNA. In this respect UV-stimulated recombination of nonreplicating DNA differs from RecF-dependent recombination processes that are stimulated by increased SOS expression. Surprisingly, prior UV irradiation of lexA3 bacteria caused a small but reproducible increase in the regeneration of infective phage DNA.