A number of E. coli genes exhibit increased expression when the cellular DNA is damaged. In undamaged cells, lexA repressor limits the extent of their transcription, whereas, in damaged cells, the repressor is cleaved by a cellular protease, the product of the recA gene. We have sequenced 943 base pairs of cloned E. coli DNA containing the lexA gene. A regulatory region has been identified, followed by a translational open reading frame which encodes a polypeptide of 202 amino acids with a molecular weight of 22,300. The protein contains a single alanyl-glycyl peptide near its middle. This peptide is also found in certain phage repressors which are cleaved by the recA protease and has been shown to be the site of cleavage in these repressors. We have determined the nucleotide sequence of a portion of the lexA3 gene, whose product is 100-fold less susceptible to recA protease than the wild type repressor. We report a single base change (G to A) which alters the unique alanine-glycine sequence to alanine-aspartic acid.