Binding of Escherichia coli RNA polymerase and the abortive initiation of transcription at the A2 promoter ofbacteriophage T7, separately cloned in pBR322, was found to be strongly dependent on the degree of supercoiling of the plasmid. Supercoiling does not seem to play any role in the initiation of transcription at the T7A1 promoter under identical conditions. Plasmid containing T7A2 promoter was found to be less amenable to S1 nuclease in comparison to that having T7A1. Sequence comparison reveals a high G/C content upstream to the −35 region of T7A2 which by extra duplex stability probably renders the initiation of transcription more dependent on the state of supercoiling of the template.
