Effect of supercoiling on the abortive initiation kinetics of the RNA-I promoter of ColE1 plasmid DNA

An analysis of the effect of DNA supercoiling on transcription of the RNA-I promoter of ColE1 has been undertaken using the abortive initiation assay developed by McClure (McClure, W. R., Cech, C. L., and Johnston, D. E. (1978) J. Biol. Chem. 253, 8941-8948). A tau plot was constructed, enabling the equilibrium constant for closed complex formation (KI) and the isomerization rate constant (k2) to be determined. The effect of supercoiling on RNA-I is an apparent 100-fold increase in the isomerization rate, k2, with little or no effect on KI. This is the first reported abortive initiation analysis of RNA-I transcription which serves in the regulation of plasmid replication.