The lac and gal control regions contain two functional overlapping promoters P1 and P2. Point mutations can shift transcription from P1 to P2 and vice versa. We show that the reactivity of DNA fragments towards nucleolytic attack with orthophenanthroline cuprous complex can be used to predict which promoter competes more efficiently for RNA polymerase binding. Furthermore, similar changes in reactivity are observed as closed complexes isomerize to form the final open complexes, provided that the functional start is taken as a reference. We found a correlation between the reactivity pattern of -10 regions in uncomplexed DNA and the rate of open complex formation.