Drug-DNA dissociation kinetics. In vitro transcription and sodium dodecyl sulphate sequestration

The rate of dissociation of actinomycin D from DNA was measured by sodium dodecyl sulphate (SDS) sequestration (37 degrees) from calf thymus DNA and a 24 base pair (bp) synthetic DNA containing one high affinity AGCT site for the drug. The time constants were 276 and 142 sec, respectively, and suggest a stabilising effect though positive cooperativity in heterogenous DNA, or from specific neighbouring sequences. The time constant for dissociation of actinomycin D from the AGCT site was 2900 sec as measured by an in vitro transcription assay at 37 degrees, and suggests that, under conditions of active transcription of the DNA, the drug-DNA complex has additional stabilising contributions, possibly by a cage effect from RNA polymerase, or by additional drug-RNA polymerase contacts.