Transcription of polyoma DNA by Escherichia coli RNA polymerase: influence of ionic strength on promoter selection.

The influence of ionic strength on transcription of polyoma DNA by Escherichia coli RNA polymerase was investigated. At 0.15 M KCl, transcription was highly symmetrical and, due to the lack of reinitiation, a limited extent of RNA synthesis was observed. When the concentration of KCl was raised to 0.45 M, the affinity of the enzyme for its template, as well as its apparent affinity for ribonucleoside triphosphates, was reduced. Under optimal conditions, the rate and extent of RNA synthesis at 0.45 M KCl were greater than at 0.15 M KCl, and transcription was mostly asymmetric. Binding and initiation sites at both ionic strengths were identified; at 0.15 M KCl, transcription was initiated from two major sites, located at 0.99 and 0.06 map unit, whereas at 0.45 M KCl, a unique initiation site, at 0.99 map unit, was selected by RNA polymerase.