 They also appear to be the main contributors to adaptive mutation in "E. coli".Changes in presence of one or the other was shown to affect the SOS response, which in turn affected how the cells were able to process lactose, which should not be confused with the lactose starvation experiment.The key point to understand here is that LexA and RecA both were required for adaptive mutation to occur, and without the SOS response adaptive mutation would not be possible.