A transposon insertion in the Escherichia coli uvrC gene; UvrC protein is absolutely required for the incision step in excision repair.

The formation of single-stranded breaks in DNA following UV irradiation is assessed in uvrC34 mutants. By altering the SOS DNA-repair system, either by additional mutations or by using drugs affecting transcription or translation, it is shown that such single-stranded breaks require one or more DNA-damage-inducible functions. A UV-sensitive strain is characterized as carrying a Tn10 insertion into the uvrC gene. The absence of post-irradiation incision in this strain demonstrates that uvrC function is absolutely required in vivo for the incision stage of excision repair, and suggests that other uvrC mutants are 'leaky'.