Mathematical relationships among DNA supercoiling, cation concentration, and temperature for prokaryotic...

DNA twist has been proposed to affect transcription from some promoters of Escherichia coli, but involvement of twist has been difficult to test because it cannot be measured in transcription reaction mixtures. However, changes in other factors affect both DNA twist and transcription. These parameters are expected to be related when maximum transcription initiation is considered. In the present work, mathematical relationships among supercoiling, cation concentration, and temperature are derived for prokaryotic transcription initiation. The relationships indicate that as DNA becomes more negatively supercoiled, maximal initiation occurs at a higher cation concentration and at a lower temperature. For example, when superhelical density becomes more negative by 0.0025, a 1.6-fold increase in potassium concentration is predicted to be required to maintain transcription initiation at its maximum rate. Experimental verification of the relationships should provide a useful test of the idea that transcription initiation is sensitive to DNA twist.