Increased binding of actinomycin D to halodeoxyuridine-substituted DNAs.

We have used equilibrium dialysis and other techniques to examine the actinomycin D binding sites on unsubstituted and halodeoxyuridine-substituted DNAs. Some of the actinomycin D binding sites present on halodeoxyuridine-substituted DNAs have an enhanced affinity for actinomycin D compared to the standard binding sites present on unsubstituted DNA. Kinetic studies indicate that actinomycin D dissociates from these high affinity sites at a slower rate than it does from the standard sites on unsubstituted DNA. Increasing levels of bromodeoxyuridine substitution are associated with increased binding of actinomycin D to the substituted DNA. Substitution with iododeoxyuridine causes a greater increase in actinomycin D binding than do equivalent levels of substitution with chlorodeoxyuridine or bromodeoxyuridine. Some halodeoxyuridine-substituted DNAs appear to have additional binding sites for actinomycin D which are not available on unsubstituted DNA. The finding, that poly(dA5BrdU).poly(dA-5BrdU) as well as poly(dA-dT) l