Regulation of SOS functions: purification of E. coli LexA protein and determination of its specific site cleaved by the RecA protein.

The LexA protein of Escherichia coli was purified to more than 96% purity from cells harboring a recombinant plasmid carrying the lexA gene with the lacZ promoter sequence. The amino acid composition of the LexA protein and its amino-terminal sequence were analyzed. The results are in agreement with the prediction from the nucleotide sequence of the lexA gene. The LexA protein is cleaved into two polypeptides by E. coli RecA protein in the presence of ATP and single-stranded DNA. The site of the specific cleavage was determined by analyzing amino acid sequences of the cleaved products at the amino and carboxyl termini. The cleavage of the LexA protein by the RecA protein was found to occur at a single site between Ala84 and Gly85.