Reconstitution of an SOS response pathway: Derepression of transcription in response to DNA breaks

E. coli responds to DNA damage by derepressing the transcription of about 2 0 genes that make up the SOS pathway. Genetic analyses have shown that SOS induction in response to double-stranded DNA (dsDNA) breaks requires LexA r epressor, and the RecA and RecBCD enzymes - proteins best known for their r ole as initiators of dsDNA break repair and homologous recombination. Here we demonstrate that purified RecA protein, RecBCD enzyme, single-stranded D NA-binding (SSB) protein, and LexA repressor respond to dsDNA breaks in vit ro by derepressing transcription from an SOS promoter. Interestingly, derep ression is more rapid if the DNA containing the dsDNA break has a chi recom bination hot spot (5'-GCTGGTGG-3'), suggesting a novel regulatory role for one of the most overrepresented octamers in the E. coli genome.