ToxR activates the Vibrio cholerae virulence genes by tethering DNA to the membrane through versatile binding to multiple sites.

ToxR, a Vibrio cholerae transmembrane one-component signal transduction factor, lies within a regulatory cascade that results in the expression of ToxT, toxin coregulated pilus, and cholera toxin. While ToxR has been extensively studied for its ability to activate or repress various genes in V. cholerae, here we present the crystal structures of the ToxR cytoplasmic domain bound to DNA at the toxT and ompU promoters. The structures confirm some predicted interactions, yet reveal other unexpected promoter interactions with implications for other potential regulatory roles for ToxR. We show that ToxR is a versatile virulence regulator that recognizes diverse and extensive, eukaryotic-like regulatory DNA sequences, that relies more on DNA structural elements than specific sequences for binding. Using this topological DNA recognition mechanism, ToxR can bind both in tandem and in a twofold inverted-repeat-driven manner. Its regulatory action is based on coordinated multiple binding to promoter regions near the transcription start site, which can remove the repressing H-NS proteins and prepares the DNA for optimal interaction with the RNA polymerase.

tRNA deamination by ADAT requires substrate-specific recognition mechanisms and can be inhibited by tRFs.

Adenosine deaminase acting on transfer RNA (ADAT) is an essential eukaryotic enzyme that catalyzes the deamination of adenosine to inosine at the first position of tRNA anticodons. Mammalian ADATs modify eight different tRNAs, having increased their substrate range from a bacterial ancestor that likely deaminated exclusively tRNAArg Here we investigate the recognition mechanisms of tRNAArg and tRNAAla by human ADAT to shed light on the process of substrate expansion that took place during the evolution of the enzyme. We show that tRNA recognition by human ADAT does not depend on conserved identity elements, but on the overall structural features of tRNA. We find that ancestral-like interactions are conserved for tRNAArg, while eukaryote-specific substrates use alternative mechanisms. These recognition studies show that human ADAT can be inhibited by tRNA fragments in vitro, including naturally occurring fragments involved in important regulatory pathways.

The structure of the complex between α-tubulin, TBCE and TBCB reveals a tubulin dimer dissociation mechanism.

Tubulin proteostasis is regulated by a group of molecular chaperones termed tubulin cofactors (TBC). Whereas tubulin heterodimer formation is well-characterized biochemically, its dissociation pathway is not clearly understood. Here, we carried out biochemical assays to dissect the role of the human TBCE and TBCB chaperones in α-tubulin-ß-tubulin dissociation. We used electron microscopy and image processing to determine the three-dimensional structure of the human TBCE, TBCB and α-tubulin (αEB) complex, which is formed upon α-tubulin-ß-tubulin heterodimer dissociation by the two chaperones. Docking the atomic structures of domains of these proteins, including the TBCE UBL domain, as we determined by X-ray crystallography, allowed description of the molecular architecture of the αEB complex. We found that heterodimer dissociation is an energy-independent process that takes place through a disruption of the α-tubulin-ß-tubulin interface that is caused by a steric interaction between ß-tubulin and the TBCE cytoskeleton-associated protein glycine-rich (CAP-Gly) and leucine-rich repeat (LRR) domains. The protruding arrangement of chaperone ubiquitin-like (UBL) domains in the αEB complex suggests that there is a direct interaction of this complex with the proteasome, thus mediating α-tubulin degradation.

The structure of a transcription activation subcomplex reveals how σ(70) is recruited to PhoB promoters.

PhoB is a two-component response regulator that activates transcription by interacting with the σ(70) subunit of the E. coli RNA polymerase in promoters in which the -35 σ(70)-recognition element is replaced by the pho box. The crystal structure of a transcription initiation subcomplex that includes the σ(4) domain of σ(70) fused with the RNA polymerase ß subunit flap tip helix, the PhoB effector domain and the pho box DNA reveals how σ(4) recognizes the upstream pho box repeat. As with the -35 element, σ(4) achieves this recognition through the N-terminal portion of its DNA recognition helix, but contact with the DNA major groove is less extensive. Unexpectedly, the same recognition helix contacts the transactivation loop and helices α2 and α3 of PhoB. This result shows a simple and elegant mechanism for polymerase recruitment to pho box promoters in which the lost -35 element contacts are compensated by new ones with the activator. In addition, σ(4) is reoriented, thereby suggesting a remodelling mechanism for transcription initiation.

A novel family of soluble minimal scaffolds provides structural insight into the catalytic domains of integral membrane metallopeptidases.

In the search for structural models of integral-membrane metallopeptidases (MPs), we discovered three related proteins from thermophilic prokaryotes, which we grouped into a novel family called "minigluzincins." We determined the crystal structures of the zymogens of two of these (Pyrococcus abyssi proabylysin and Methanocaldococcus jannaschii projannalysin), which are soluble and, with ∼100 residues, constitute the shortest structurally characterized MPs to date. Despite relevant sequence and structural similarity, the structures revealed two unique mechanisms of latency maintenance through the C-terminal segments previously unseen in MPs as follows: intramolecular, through an extended tail, in proabylysin, and crosswise intermolecular, through a helix swap, in projannalysin. In addition, structural and sequence comparisons revealed large similarity with MPs of the gluzincin tribe such as thermolysin, leukotriene A4 hydrolase relatives, and cowrins. Noteworthy, gluzincins mostly contain a glutamate as third characteristic zinc ligand, whereas minigluzincins have a histidine. Sequence and structural similarity further allowed us to ascertain that minigluzincins are very similar to the catalytic domains of integral membrane MPs of the MEROPS database families M48 and M56, such as FACE1, HtpX, Oma1, and BlaR1/MecR1, which are provided with trans-membrane helices flanking or inserted into a minigluzincin-like catalytic domain. In a time where structural biochemistry of integral-membrane proteins in general still faces formidable challenges, the minigluzincin soluble minimal scaffold may contribute to our understanding of the working mechanisms of these membrane MPs and to the design of novel inhibitors through structure-aided rational drug design approaches.

PhoB transcriptional activator binds hierarchically to pho box promoters.

The PhoR-PhoB phosphorelay is a bacterial two-component system that activates the transcription of several genes involved in phosphate uptake and assimilation. The response begins with the autophosphorylation of the sensor kinase PhoR, which activates the response regulator PhoB. Upon binding to the pho box DNA sequence, PhoB recruits the RNA polymerase and thereby activates the transcription of specific genes. To unveil hitherto unknown molecular mechanisms along the activation pathway, we report biochemical data characterizing the PhoB binding to promoters containing multiple pho boxes and describe the crystal structure of two PhoB DNA-binding domains bound in tandem to a 26-mer DNA.

Cloning, expression, purification and crystallization of the Rho transcription termination factor from Thermotoga maritima.

Rho is an essential ATP-dependent homohexameric helicase that is found in the vast majority of bacterial species. It is responsible for transcription termination at factor-dependent terminators. Rho binds to a specific region of the newly-synthesised mRNA and translocates along the chain until it reaches and disassembles the transcription complex. Basically, two crystallographic structures of Rho hexamer from Escherichia coli have been reported: an open ring with RNA (or ssDNA) bound to the RNA-binding domain, and a closed ring with the RNA bound to both the RNA-binding domain and the ATP-ase domain. The structure of the protein free from RNA is still unknown, but thermophilic bacteria enable an alternative approach to its characterization as their proteins often crystallize more easily than those of their mesophilic homologs. We report here the heterologous expression in E. coli of full-length Rho from the thermophile Thermotoga maritima, a simple protocol for the purification of its hexameric nucleic acid-free form, and the obtainment of 2.4 A-diffracting crystals.

Intercalative DNA binding of the marine anticancer drug variolin B.

Variolin B is a rare marine alkaloid that showed promising anti-cancer activity soon after its isolation. It acts as a cyclin-dependent kinase inhibitor, although the precise mechanism through which it exerts the cytotoxic effects is still unknown. The crystal structure of a variolin B bound to a DNA forming a pseudo-Holliday junction shows that this compound can also contribute, through intercalative binding, to either the formation or stabilization of multi-stranded DNA forms.

σ70 and PhoB activator: getting a better grip.

Transcription factors modulate gene expression by distinct, barely understood mechanisms. The crystal structure of a bacterial transcription subcomplex comprising the effector domain of factor PhoB, its target DNA and the σ4 domain of the RNA polymerase σ70 subunit supports the notion that a stronger grip on the promoter-factor complex results in an enhanced RNAP architecture.

Site-directed mutagenesis of mouse glutathione transferase P1-1 unlocks masked cooperativity, introduces a novel mechanism for 'ping pong' kinetic behaviour, and provides further structural evidence for participation of a water molecule in proton abstraction from glutathione.

Mouse liver glutathione transferase P1-1 has three cysteine residues at positions 14, 47 and 169. We have constructed the single, double and triple cysteine to alanine mutants to define the behaviour of all three thiols. We confirm that C47 is the 'fast' thiol (pK 7.4), and define C169 as the alkaline reactive residue with a pK(a) of 8.6. Only a small proportion of C14 is reactive with 5,5'-dithiobis-(2-nitrobenoic acid) (DTNB) at pH 9 in the C47A/C169A double mutant. The native enzyme and the C169A mutant exhibited Michaelis-Menten kinetics, but all other thiol to alanine mutants exhibited sigmoidal kinetics to varying degrees. The C169A mutant exhibited 'ping pong' kinetics, consistent with a mechanism whereby liberation of a proton from a reduced enzyme-glutathione (GSH) complex to form an enzyme-GS(-) (unprotonated) complex is essentially irreversible. Intriguingly, similar behaviour has recently been reported for a mutant of the yeast prion Ure2p. This cooperative behaviour is 'mirrored' in the crystal structure of the C47A mutant, which binds the p-nitrobenzyl moiety of p-nitrobenzyglutathione in distinct orientations in the two crystallographic subunits. The asymmetry seen in this structure for product binding is associated with absence of a water molecule W0 in the standard wild-type conformation of product binding that is clearly identifiable in the new structure, which may represent a structural model for binding of incoming GSH prior to displacement of W0. Elimination of W0 as a hydroxonium ion may be the mechanism for the initial proton extrusion from the active site.

The structure of RNA-free Rho termination factor indicates a dynamic mechanism of transcript capture.

The Rho factor is a ring-shaped ATP-dependent helicase that mediates transcription termination in most prokaryotic cells by disengaging the transcription elongation complex formed by the RNA polymerase, DNA, and the nascent RNA transcript. The crystal structures of key intermediates along the kinetic pathway of RNA binding to Rho unveiled an unprecedented mode of helicase loading and provided a model for the ATP turnover coupled to coordinated strand movement. Here we report the structure of the early RNA-free state of Rho, which had eluded crystallization for many years but now completes the series. The structure allows the characterization of the apo-form Rho from Thermotoga maritima to 2.3 A resolution, reveals an RNA-recruiting site that becomes hidden after occupancy of the adjacent specific primary RNA-binding site, and suggests an enriched model for mRNA capture that is consistent with previous data.

DNA-binding drugs caught in action: the latest 3D pictures of drug-DNA complexes.

In this paper, we review recent DNA-binding agents that are expected to influence the field of DNA-targeting. We restrict ourselves to binders for which the three-dimensional structure in complex with DNA or RNA has been determined by X-ray crystallography or NMR. Furthermore, we primarily focus on unprecedented ways of targeting peculiar DNA structures, such as junctions, quadruplexes, and duplex DNAs different from the B-form. Classical binding modes of small molecular weight compounds to DNA, i.e. groove binding, intercalation and covalent addition are discussed in those cases where the structures represent a novelty. In addition, we review 3D structures of triple-stranded DNA, of the so-called Peptide Nucleic Acids (PNAs), which are oligonucleotide bases linked by a polypeptide backbone, and of aptamers, which are DNA or RNA receptors that are designed combinatorially. A discussion on perspectives in the field of DNA-targeting and on sequence recognition is also provided.

The anticancer agent ellipticine unwinds DNA by intercalative binding in an orientation parallel to base pairs.

Ellipticine is a natural plant product that has been found to be a powerful anticancer drug. Although still unclear, its mechanism of action is considered to be mainly based on DNA intercalation and/or the inhibition of topoisomerase II. Many experimental data suggest an intercalation based on stacking interactions along the major base-pair axis, but alternative binding modes have been proposed, in particular for ellipticine derivatives. The 1.5 A resolution structure of ellipticine complexed to a 6 bp oligonucleotide unveils its mode of binding and enables a detailed analysis of the distorting effects of the drug on the DNA.

Structure of xylanase Xys1delta from Streptomyces halstedii.

Xylanases hydrolyze the beta-1,4-linked xylose backbone of xylans. They are of increasing interest in the paper and food industries for their pre-bleaching and bio-pulping applications. Such industries demand new xylanases to cover a wider range of cleavage specificity, activity and stability. The catalytic domain of xylanase Xys1 from Streptomyces halstedii JM8 was expressed, purified and crystallized and native data were collected to 1.78 A resolution with an R(merge) of 4.4%. The crystals belong to space group P2(1)2(1)2(1), with unit-cell parameters a = 34.05, b = 79.60, c = 87.80 A. The structure was solved by the molecular-replacement method using the structure of the homologue Xyl10A from Streptomyces lividans. In a similar manner to other members of its family, Xys1 folds to form a standard (beta/alpha)(8) barrel with the two catalytic functions, the acid/base and the nucleophile, at its C-terminal side. The overall structure is described and compared with those of related xylanases.