Structural mechanism for rifampicin inhibition of bacterial rna polymerase.

Rifampicin (Rif) is one of the most potent and broad spectrum antibiotics against bacterial pathogens and is a key component of anti-tuberculosis therapy, stemming from its inhibition of the bacterial RNA polymerase (RNAP). We determined the crystal structure of Thermus aquaticus core RNAP complexed with Rif. The inhibitor binds in a pocket of the RNAP beta subunit deep within the DNA/RNA channel, but more than 12 A away from the active site. The structure, combined with biochemical results, explains the effects of Rif on RNAP function and indicates that the inhibitor acts by directly blocking the path of the elongating RNA when the transcript becomes 2 to 3 nt in length.

Bacterial RNA polymerase.

The recently determined crystal structure of a bacterial core RNA polymerase (RNAP) provides the first glimpse of this family of evolutionarily conserved cellular RNAPs. Using the structure as a framework, a consistent picture of protein-nucleic acid interactions in transcription complexes has been accumulated from cross-linking experiments. The molecule can be viewed as a molecular machine, with distinct structural features hypothesized to perform specific functions. Comparison with the alpha-carbon backbone of a eukaryotic RNAP reveals close structural similarity.

The beta ' subunit of Escherichia coli RNA polymerase is not required for interaction with initiating nucleotide but is necessary for interaction with rifampicin.

Using a modification of a highly selective affinity labeling protocol, we demonstrated that the alpha(2)beta subassembly of Escherichia coli RNA polymerase efficiently and specifically interacts with the initiating purine nucleotide. Isolated beta is also active in this reaction. In contrast, neither beta nor alpha(2)beta is able to interact with a chimeric molecule composed of rifampicin attached to an initiation substrate. Based on these results, we conclude that the RNA polymerase initiation site, specific for purine nucleotides, which ultimately become the 5'-end of the transcript, is essentially complete in the absence of the largest subunit, beta'. However, the rifampicin binding center is formed only in the alpha(2)betabeta' core enzyme. We interpret our results in light of the high resolution structure of core RNA polymerase from Thermus aquaticus.

Bacterial RNA polymerase subunit omega and eukaryotic RNA polymerase subunit RPB6 are sequence, structural, and functional homologs and promote RNA polymerase assembly.

Bacterial DNA-dependent RNA polymerase (RNAP) has subunit composition beta'betaalpha(I)alpha(II)omega. The role of omega has been unclear. We show that omega is homologous in sequence and structure to RPB6, an essential subunit shared in eukaryotic RNAP I, II, and III. In Escherichia coli, overproduction of omega suppresses the assembly defect caused by substitution of residue 1362 of the largest subunit of RNAP, beta'. In yeast, overproduction of RPB6 suppresses the assembly defect caused by the equivalent substitution in the largest subunit of RNAP II, RPB1. High-resolution structural analysis of the omega-beta' interface in bacterial RNAP, and comparison with the RPB6-RPB1 interface in yeast RNAP II, confirms the structural relationship and suggests a "latching" mechanism for the role of omega and RPB6 in promoting RNAP assembly.

A structural model of transcription elongation.

The path of the nucleic acids through a transcription elongation complex was tracked by mapping cross-links between bacterial RNA polymerase (RNAP) and transcript RNA or template DNA onto the x-ray crystal structure. In the resulting model, the downstream duplex DNA is nestled in a trough formed by the beta' subunit and enclosed on top by the beta subunit. In the RNAP channel, the RNA/DNA hybrid extends from the enzyme active site, along a region of the beta subunit harboring rifampicin resistance mutations, to the beta' subunit "rudder." The single-stranded RNA is then extruded through another channel formed by the beta-subunit flap domain. The model provides insight into the functional properties of the transcription complex.

The anti-sigma factor SpoIIAB forms a 2:1 complex with sigma(F), contacting multiple conserved regions of the sigma factor.

The developmental regulatory protein sigma(F) of Bacillus subtilis, a member of the sigma(70)-family of bacterial RNA polymerase sigma factors, is negatively regulated by the anti-sigma factor SpoIIAB, which binds to sigma(F), sequestering it in an inactive complex. SpoIIAB binding to sigma(F) is strongly stimulated by ATP. Here, we use a combination of gel filtration chromatography, dynamic light-scattering, analytical ultracentrifugation, limited proteolysis with N-terminal sequencing and electrospray mass spectrometry, and deletion analysis to probe the SpoIIAB-sigma(F) complex. The studies were facilitated by investigating the homologs from Bacillus stearothermophilus as well as co-expression of the proteins in Escherichia coli, allowing purification of large quantities of the in vivo assembled complex. We determined the stoichiometry of the complex to be SpoIIAB(2):sigma(F)(1). Alone, sigma(F) is rapidly degraded by the protease trypsin. In the complex with SpoIIAB, however, sigma(F) is remarkably resistant to proteolysis. Analysis of the protease cleavage data indicates the anti-sigma binds sigma(F) through contacts with mutliple conserved regions of the sigma factor, supporting previous findings based on genetic data.

Direct localization of a beta-subunit domain on the three-dimensional structure of Escherichia coli RNA polymerase.

To identify the location of a domain of the beta-subunit of Escherichia coli RNA polymerase (RNAP) on the three-dimensional structure, we developed a method to tag a nonessential surface of the multisubunit enzyme with a protein density easily detectable by electron microscopy and image processing. Four repeats of the IgG-binding domain of Staphylococcus aureus protein A were inserted at position 998 of the E. coli RNAP beta-subunit. The mutant RNAP supported E. coli growth and showed no apparent functional defects in vitro. The structure of the mutant RNAP was determined by cryoelectron microscopy and image processing of frozen-hydrated helical crystals. Comparison of the mutant RNAP structure with the previously determined wild-type RNAP structure by Fourier difference analysis at 20-A resolution directly revealed the location of the inserted protein domain, thereby locating the region around position 998 of the beta-subunit within the RNAP three-dimensional structure and refining a model for the subunit locations within the enzyme.

Crystal structure of Thermus aquaticus core RNA polymerase at 3.3 A resolution.

The X-ray crystal structure of Thermus aquaticus core RNA polymerase reveals a "crab claw"-shaped molecule with a 27 A wide internal channel. Located on the back wall of the channel is a Mg2+ ion required for catalytic activity, which is chelated by an absolutely conserved motif from all bacterial and eukaryotic cellular RNA polymerases. The structure places key functional sites, defined by mutational and cross-linking analysis, on the inner walls of the channel in close proximity to the active center Mg2+. Further out from the catalytic center, structural features are found that may be involved in maintaining the melted transcription bubble, clamping onto the RNA product and/or DNA template to assure processivity, and delivering nucleotide substrates to the active center.

Mapping interactions of Escherichia coli GreB with RNA polymerase and ternary elongation complexes.

Escherichia coli GreA and GreB modulate transcription elongation by interacting with the ternary elongation complex (containing RNA polymerase, DNA template, and RNA transcript) to induce hydrolytic cleavage of the transcript and release of the 3'-terminal fragment. Hydroxyl radical protein footprinting and alanine-scanning mutagenesis were used to investigate the interactions of GreB with RNA polymerase alone and in a ternary elongation complex. A major determinant for binding GreB to both RNA polymerase and the ternary elongation complex was identified. In addition, the hydroxyl radical footprinting indicated major conformational changes of GreB, in terms of reorientations of the N- and C-terminal domains with respect to each other, particularly upon interactions with the ternary elongation complex.

Averaging data derived from images of helical structures with different symmetries.

There are many examples of macromolecules that form helical tubes or crystals, which are useful for structure determination by electron microscopy and image processing. Helical crystals can be thought of as two-dimensional crystals that have been rolled into a cylinder such that two lattice points are superimposed. In many real cases, helical crystals of a particular macromolecule derive from an identical two-dimensional lattice but have different lattice points superimposed, thus producing different helical symmetries which cannot be simply averaged in Fourier-space. When confronted with this situation, one can select images corresponding to one of the observed symmetries at the expense of reducing the number of images that can be used for data collection and averaging, or one can calculate separate density maps from each symmetry, then align and average them together in real-space. Here, we present a third alternative, which is based on averaging of the Fourier-Bessel coefficients, gn,l(r), and which allows the inclusion of data from all symmetry groups derived from a common two-dimensional lattice. The method is straightforward and simple in practice and is shown, through a specific example with real data, to give results comparable to real-space averaging.

Visualization of the binding site for the transcript cleavage factor GreB on Escherichia coli RNA polymerase.

The structure of Escherichia coli core RNA polymerase (RNAP) complexed with the transcript cleavage factor GreB was determined from electron micrographs of negatively stained, flattened helical crystals. A binding assay was developed to establish that GreB was incorporated into the RNA polymerase crystals with high occupancy through interactions between the globular C-terminal domain and the RNA polymerase. Comparison of the core RNAP:GreB structure with the previously determined structure of core RNAP located the GreB binding site on one face of the RNA polymerase, next to but not in the 25 A-diameter channel of RNA polymerase.

Structure of the Escherichia coli RNA polymerase alpha subunit amino-terminal domain.

The 2.5 angstrom resolution x-ray crystal structure of the Escherichia coli RNA polymerase (RNAP) alpha subunit amino-terminal domain (alphaNTD), which is necessary and sufficient to dimerize and assemble the other RNAP subunits into a transcriptionally active enzyme and contains all of the sequence elements conserved among eukaryotic alpha homologs, has been determined. The alphaNTD monomer comprises two distinct, flexibly linked domains, only one of which participates in the dimer interface. In the alphaNTD dimer, a pair of helices from one monomer interact with the cognate helices of the other to form an extensive hydrophobic core. All of the determinants for interactions with the other RNAP subunits lie on one face of the alphaNTD dimer. Sequence alignments, combined with secondary-structure predictions, support proposals that a heterodimer of the eukaryotic RNAP subunits related to Saccharomyces cerevisiae Rpb3 and Rpb11 plays the role of the alphaNTD dimer in prokaryotic RNAP.

Inhibition of Escherichia coli RNA polymerase by bacteriophage T4 AsiA.

The 10 kDa bacteriophage T4 antisigma protein AsiA binds the Escherichia coli RNA polymerase promoter specificity subunit, sigma 70, with high affinity and inhibits its transcription activity. AsiA binds to sigma 70 primarily through an interaction with sigma 70 conserved region 4.2, which has also been implicated in sequence-specific recognition of the -35 consensus promoter element. Here we show that AsiA forms a stable ternary complex with core RNA polymerase (RNAP) and sigma 70 and thus does not inhibit sigma 70 activity by preventing its binding to core RNAP. We investigated the effect of AsiA on open promoter complex formation and abortive initiation at two -10/-35 type promoters and two "extended -10" promoters. Our results indicate that the binding of AsiA to sigma 70 and the interaction of sigma 70 region 4.2 with the -35 consensus promoter element of -10/-35 promoters is mutually exclusive. In contrast, AsiA has much less effect on open promoter complex formation and abortive initiation from extended -10 promoters, which lack a -35 consensus element and do not require sigma 70 conserved region 4.2. From these results we conclude that T4 AsiA inhibits E. coli RNAP sigma 70 holoenzyme transcription at -10/-35 promoters by interfering with the required interaction between sigma 70 conserved region 4.2 and the -35 consensus promoter element.

Disruption of Escherichia coli hepA, an RNA polymerase-associated protein, causes UV sensitivity.

During the development of purification procedures for Escherichia coli RNA polymerase (RNAP), we noticed the consistent co-purification of a 110-kDa polypeptide. Here, we report the identification of the 110-kDa protein as the product of the hepA gene, a member of the SNF2 family of putative helicases. We have cloned the hepA gene and overexpressed and purified the HepA protein. We show in vitro that RNAP preparations have an ATPase activity only in the presence of HepA and that HepA binds core RNAP competitively with the promoter specificity sigma70 subunit with a 1:1 stoichiometry and a dissociation constant (Kd) of 75 nM. An E. coli strain with a disruption in the hepA gene shows sensitivity to ultraviolet light.

Insights into Escherichia coli RNA polymerase structure from a combination of x-ray and electron crystallography.

Our goal is to understand the mechanism of transcription and its regulation. Determining structures of RNA polymerase and transcription complexes is an essential step. Because of their large size and complexity, determination of these structures will require a combination of electron microscopy, biophysical methods, and biochemical methods to identify functionally and structurally relevant subassemblies and domains and x-ray crystallography to determine high-resolution structures of RNA polymerase components and accessory factors. We recently solved the 2.5-A crystal structure of the Escherichia coli RNA polymerase alpha subunit N-terminal domain, which is the first high-resolution structure of a core component required for RNA polymerase assembly and basal transcription. This structure, combined with a new 19-A resolution structure determined by cryo-electron microscopy of helical crystals of E. coli core RNAP embedded in vitreous ice, leads to a model for the organization of the RNAP subunits.

Tethering of the large subunits of Escherichia coli RNA polymerase.

The rpoB and rpoC genes of eubacteria and archaea, coding, respectively, for the beta and beta'-like subunits of DNA-dependent RNA polymerase, are organized in an operon with rpoB always preceding rpoC. Here, we show that in Escherichia coli the two genes can be fused and that the resulting 2751-amino acid beta::beta' fusion polypeptide assembles into functional RNA polymerase in vivo and in vitro. The results establish that the C terminus of the beta subunit and the N terminus of the beta' subunit are in close proximity to each other on the surface of the assembled RNA polymerase during all phases of the transcription cycle and also suggest that RNA polymerase assembly in vivo may occur co-translationally.

Determinants for Escherichia coli RNA polymerase assembly within the beta subunit.

We used binding assays and other approaches to identify fragments of the Escherichia coli RNAP beta subunit involved in the obligatory interaction with the alpha subunit to form the stable assembly intermediate alpha2beta as well as in the interaction to recruit the beta' subunit into the alpha2beta sub-assembly. We show that two regions of evolutionarily conserved sequence near the C terminus of beta (conserved regions H and I) are central to the assembly of RNAP and likely make subunit-subunit contacts with both alpha and beta'.