Initiation of transcription from the σ-70 rep promoter of plasmid pBR322 was measured by abortive transcription assays at various concentrations of potassium, rubidium, and sodium acetate. When linear and negatively supercoiled templates were compared, each salt generated a characteristic response. Increasing the salt concentration decreased transcription from a linear template but produced an increase (potassium) or a bell-shaped response (rubidium) with a supercoiled template. In the case of sodium ions, increasing concentration inhibited transcription initiation from both linear and supercoiled templates. These results are discussed with respect to effects of monovalent cations on DNA twist.
- DNA supercoiling
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