Final answer:
LexA functions as a repressor for SOS gene expression, which is alleviated following DNA damage to allow for the repair of the DNA.
Step-by-step explanation:
The function of LexA in the SOS response pathway is to act as a repressor of SOS gene expression under normal conditions. When DNA damage occurs, the SOS response is initiated, leading to the proteolytic cleavage of LexA, which in turn allows the expression of DNA repair genes. The cleavage of LexA is regulated through the RecA protein, which senses the presence of single-stranded DNA indicative of damage. As a result, LexA repression is removed, and the SOS genes are transcribed to assist in the repair and tolerance to DNA damage.