The crystal structure ofThermus aquaticusRNA polymerase holoenzyme (α₂ββ′ωσ^A) complexed with a fork-junction promoter DNA fragment has been determined by fitting high-resolution x-ray structures of individual components into a 6.5-angstrom resolution map. The DNA lies across one face of the holoenzyme, completely outside the RNA polymerase active site channel. All sequence-specific contacts with core promoter elements are mediated by the σ subunit. A universally conserved tryptophan is ideally positioned to stack on the exposed face of the base pair at the upstream edge of the transcription bubble. Universally conserved basic residues of the σ subunit provide critical contacts with the DNA phosphate backbone and play a role in directing the melted DNA template strand into the RNA polymerase active site. The structure explains how holoenzyme recognizes promoters containing variably spaced –10 and –35 elements and provides the basis for models of the closed and open promoter complexes.

The crystal structure of the initiating form of Thermus aquaticus RNA polymerase, containing core RNA polymerase (α ₂ ββ′ω) and the promoter specificity σ subunit, has been determined at 4 angstrom resolution. Important structural features of the RNA polymerase and their roles in positioning σ within the initiation complex are delineated, as well as the role played by σ in modulating the opening of the RNA polymerase active-site channel. The two carboxyl-terminal domains of σ are separated by 45 angstroms on the surface of the RNA polymerase, but are linked by an extended loop. The loop winds near the RNA polymerase active site, where it may play a role in initiating nucleotide substrate binding, and out through the RNA exit channel. The advancing RNA transcript must displace the loop, leading to abortive initiation and ultimately to σ release.

Escherichia coli RNA polymerase (RNAP) is the most studied bacterial RNAP and has been used as the model RNAP for screening and evaluating potential RNAP-targeting antibiotics. However, the x-ray crystal structure of E. coli RNAP has been limited to individual domains. Here, I report the x-ray structure of the E. coli RNAP σ(70) holoenzyme, which shows σ region 1.1 (σ1.1) and the α subunit C-terminal domain for the first time in the context of an intact RNAP. σ1.1 is positioned at the RNAP DNA-binding channel and completely blocks DNA entry to the RNAP active site. The structure reveals that σ1.1 contains a basic patch on its surface, which may play an important role in DNA interaction to facilitate open promoter complex formation. The α subunit C-terminal domain is positioned next to σ domain 4 with a fully stretched linker between the N- and C-terminal domains. E. coli RNAP crystals can be prepared from a convenient overexpression system, allowing further structural studies of bacterial RNAP mutants, including functionally deficient and antibiotic-resistant RNAPs.