Abstract
We present the experimentally determined 3D structure of an intact activator-dependent transcription initiation complex comprising the
Escherichia coli
catabolite activator protein (CAP), RNA polymerase holoenzyme (RNAP), and a DNA fragment containing positions −78 to +20 of a Class I CAP-dependent promoter with a CAP site at position −61.5 and a premelted transcription bubble. A 20-Å electron microscopy reconstruction was obtained by iterative projection-based matching of single particles visualized in carbon-sandwich negative stain and was fitted using atomic coordinate sets for CAP, RNAP, and DNA. The structure defines the organization of a Class I CAP-RNAP-promoter complex and supports previously proposed interactions of CAP with RNAP α subunit C-terminal domain (αCTD), interactions of αCTD with σ
70
region 4, interactions of CAP and RNAP with promoter DNA, and phased-DNA-bend-dependent partial wrapping of DNA around the complex. The structure also reveals the positions and shapes of species-specific domains within the RNAP β′, β, and σ
70
subunits.