Conformational Changes of Escherichia coli σ54-RNA-Polymerase upon Closed–Promoter Complex Formation

RNA polymerase from the mesophile Escherichia coli exists in two forms, the core enzyme and the holoenzyme. Using cryo-electron microscopy and single-particle analysis, we have obtained the structure of the complete RNA polymerase from E. coli containing the σ 54 factor within the closed-promoter complex. Comparisons with earlier reconstructions of the core enzyme and the σ 54 holoenzyme reveal the behaviour of this major variant RNA polymerase in defined functional states. The binding of DNA leads to significant conformational changes in the enzyme's catalytic subunits, apparently a necessity for the initiation of enhancer-dependent promoter-specific transcription.