[Transcription initiation: influence of ionic strength and actinomycin D on the kinetics of the open promoter complex formation between Escherichia coli RNA-polymerase and T7 DNA].

A membrane filter assay has been devised to study the influence of ionic strength (0--150 mM NaCl) and actinomycin D on the kinetics of the open promoter complex formation between E. coli RNA-polymerase and [3H]DNA of T7 phage. The dependence of the complex formation rate upon the ionic strength is non-monotoneus with a maximum at 75--100 mM. The addition of one actinomycin molecule per 200 base pairs decreases the open promoter complex formation rate at ionic strength less than 100 mM. A promoter site selection model including liner diffusional selection is proposed which is in a good agreement with the experimental results obtained.