RNA polymerase and an activator form discrete subcomplexes in a transcription initiation complex

Abstract

Using high‐resolution atomic force microscopy (AFM) we show that in a ternary complex of an activator protein, FIS, and RNA polymerase containing the σ70 specificity factor at the Escherichia coli tyrT promoter the polymerase and the activator form discrete, but connected, subcomplexes in close proximity. This is the first time that a ternary complex between an activator, a σ70 polymerase holoenzyme and promoter DNA has been visualised. Individually FIS and RNA polymerase wrap ∼80 and 150 bp of promoter DNA, respectively. We suggest that the architecture of the ternary complex provides a general paradigm for the facilitation of direct, but weak, interactions between polymerase and an activator.