▁others ▁in ▁one ▁small ▁way : ▁this ▁experiment ▁is ▁concerned ▁with ▁the ▁path ways ▁leading ▁to ▁an ▁adapt ive ▁mut ation ▁while ▁the ▁others ▁tested ▁the ▁changing ▁environment ▁micro organ isms ▁were ▁exposed ▁to . ▁ ▁Sim ply ▁put , ▁the ▁S OS ▁response ▁in ▁E . ▁col i ▁is ▁a ▁response ▁to ▁DNA ▁damage ▁that ▁must ▁be ▁rep a ired . ▁The ▁normal ▁cell ▁cycle ▁is ▁put ▁on ▁hold ▁and ▁mut agen esis ▁may ▁begin . ▁ ▁This ▁means ▁that ▁mut ations ▁will ▁occur ▁to ▁try ▁to ▁fix ▁the ▁damage . ▁This ▁hyper mut ation , ▁or ▁increased ▁rate ▁of ▁change , ▁response ▁has ▁to ▁have ▁some ▁regul atory ▁process , ▁and ▁some ▁key ▁mole cules ▁in ▁this ▁process ▁are ▁Rec A , ▁and ▁Lex A . ▁These ▁are ▁prote ins ▁and ▁act ▁as ▁stop light s ▁for ▁this ▁and ▁other ▁processes . ▁They ▁also ▁appear ▁to ▁be ▁the ▁main ▁contrib utors ▁to ▁adapt ive ▁mut ation ▁in ▁E . ▁col i . ▁Ch anges ▁in ▁presence ▁of ▁one ▁or ▁the ▁other ▁was ▁shown ▁to ▁affect ▁the ▁S OS ▁response , ▁which ▁in ▁turn ▁affected ▁how ▁the ▁cells ▁were ▁able ▁to ▁process ▁la ct ose , ▁which ▁should ▁not ▁be ▁confused ▁with ▁the ▁la ct ose ▁star v ation ▁experiment . ▁The ▁key ▁point ▁to ▁understand ▁here ▁is ▁that ▁Lex A ▁and ▁Rec A ▁both ▁were ▁required ▁for ▁adapt ive ▁mut ation ▁to ▁occur , ▁and ▁without ▁the ▁S OS ▁response ▁adapt ive ▁mut ation ▁would ▁not ▁be ▁possible . ▁ ▁L