LexA protein is a repressor of the colicin E1 gene.

LexA protein is a repressor of several chromosomal genes involved in the SOS response in Escherichia coli. In previous experiments, we found that LexA protein may also be a repressor of the colicin E1 gene. We now present evidence that the purified LexA protein strongly repressed the in vitro transcription of the colicin E1 gene. As determined in DNase I protection experiments, LexA protein bound with a high affinity to the approximately 40-base pair long sequence between the Pribnow box and the start codon of the colicin E1 gene. The sequence of the binding site was composed of two overlapped "SOS boxes" to which the LexA protein bound in a cooperative manner.