Practice question

SOS response encompasses global induction of more than forty DNA repair and tolerance genes triggered by extensive single-stranded DNA at stalled replication forks. Central negative regulator is LexA repressor, homodimer binding SOS boxes with consensus CTGTN8ACAG in promoter regions, repressing transcription including own lexA gene and recA, uvr, umuDC operons. Under steady growth, LexA maintains low expression. After DNA damage, activated RecA nucleofilament stimulates LexA autocleavage at Ala84-Gly85, causing dissociation from operators and derepression. Temporal induction occurs as LexA affinity varies across promoters, coordinating error-free and error-prone repair sequentially.