The Effects of Ionic Strength on Termination of Transcription of DNAs from Bacteriophages T4, T5 and T7 by DNA‐Dependent RNA Polymerase from Escherichia coli, and the Nature of Termination by Factor ρ

Ionic strength influences (a) the equilibrium between the monomeric and the dimeric form of DNA‐dependent RNA polymerase from Escherichia coli , in the free state, (b) the primary binding of the enzyme to sites on the template, (c) the velocity of a state transition involved in initiation and (d) termination (or block) of transcription. On T7‐ and T5‐DNA transcription is terminated, with release of the transcript, at both high and low ionic strength at the same sites but the enzyme is released only at high ionic strength allowing reinitiation whereas at low salt concentration the complex of enzyme with terminator and σ does not dissociate. On T4‐DNA transcription at low ionic strength is arrested at a class of low‐ionic‐strength blocking sites without release of the transcript. In the presence of “termination factor”ϱ and ϰ at low ionic strength transcription of T4, T5 and T7‐DNA is arrested at another class of blocking sites again without release of the transcript. The average relative positions of the three different types of blocking sites between the initiation site and the terminator have been determined for T 4 ‐DNA. Implications of these results are discussed.