In the sequence upstream from qnrB (but not qnrA or qnrS) is a LexA binding site. qnrB was shown to be under SOS control by demonstrating that quinolone susceptibility decreased with increasing temperature in a strain with a recA441(Ts) allele, whereas qnrB expression increased in response to ciprofloxacin or mitomycin C in strains with an intact lexA gene.