Symmetric Sites Research Articles

Raman analysis of asymmetrical chains in YBa 2Cu 3O 7− δ films

YBa 2Cu 3O 7− δ /MgO(1 0 0) thin films with depletion of oxygen have been studied with Raman spectroscopy. The depletion changes the intensity of 233 and 596 cm −1 phonons induced from Cu(1)–O(1) asymmetrical chains. A simple model of a symmetry-broken chain row is introduced to explain this phenomenon. With depletion, the row is configured as asymmetrical [O(1)–Cu(1)–o(1)] chains, where O(1) and o(1) are full and deficient oxygen in the orthorhombic and tetragonal structures, respectively. The Raman peaks at 233 and 596 cm −1 increase with an increase in a population density of the asymmetrical chains in the row and decrease as the row has more symmetric sites of full or deficient oxygen. As a result, the intensity of the chain-induced modes reflects a degree of the opposite structure involved in the phase transition in a single-phase film, which is verified by both the relative intensity of the 232 cm −1 phonon and the full width at half maximum of a Cu(2) superposition band from the coexisting phases in the film. All the results support the asymmetrical-chain model.

Pathogenicity of diatraea saccharalis densovirus to host insets and characterization of its viral genome

Pathogenicity of the Diatraea saccharalis densovirus (DsDNV) was tested on its host larvae. The results showed that up to 4 days after inoculation, no larvae mortality was observed and the infected larvae started to exhibit the infection symptoms from the fourth day. After 5 days of infection, the cumulative mortality of infected larvae increased significantly and reached 60% after 12 days and 100% after 21 days of infection, whereas that of the control group was only 10% and 20%, respectively, after same periods of infection, suggesting that the high mortality of infected larvae groups was due to the high pathogenicity of DsDNV. The size of the DsDNA was determined by Electron microscopy visualization of viral DNA molecules and gel electrophoresis of both native and endonuclease digested DNA fragments. The total length of the native DsDNA was about 5.95 kb. The DsDNV DNA was digested with 16 restriction enzymes and a restriction map of those enzymes was constructed with 41 restriction sites. Comparison of the restriction map of the DsDNV genome with those of the genomes of Junonia coenia densovirus (JcDNV) and Galleria mellonella densovirus (GmDNV) indicated that the three densovirus genomes were found to share many identical restriction sites. Thus, most of the restriction sites of the following endonucleases Bam H I, Hha I, Xba I, Cla I, Asp 700, Spe I, Nco I and Bcl I, were found to be conserved among the three densovirus genomes. Symmetrical cleavage sites mapped at the both ends of the genome suggested the presence of inverted terminal repeats (ITRs) whose size was estimated to be about 500 bp. The similar genome size, almost identical restriction sites and presence of an ITR of about 500 bp for these three densoviruses suggested that they belong to the same group of ambisense densoviruses.

Influences of compositions and ligands on photoluminescent properties of Eu(III) ions in composite europium complex/PMMA systems

Three kinds of europium complexes; Eu(phen) 2Cl 3(H 2O) 2, Eu(DN-bpy)phenCl 3(H 2O) 2 and Eu(DB-bpy)phenCl 3(H 2O) 2 (phen: 1,10-phenanthroline, DN-bpy: 4,4′-Dinonyl-2,2′-dipyridyl, DB-bpy: 4,4′-Di- tert-butyl-2,2′-dipyridyl) were prepared and then incorporated into polymethyl methacrylate (PMMA) matrix with different molar ratios of C O groups/Eu 3+ ions. The final solid composites were formed by a self-assembly process among Eu 3+ ion, the ligands and PMMA during the solvent evaporation process, and then the ligands re-coordinate to Eu(III). It was found that the ligands affect not only the emission properties of the pure complexes, but also the miscibility of the complexes and PMMA. More than one kind of symmetric sites of Eu 3+ ions were formed in the composites due to the coordination of C O in PMMA to Eu 3+ ions. The micro-environments of Eu(III) in the composites were changed with the compositions and the ligands, leading to the change in the crystalline structure, and consequently, the emission characteristics.

The dimeric repressor SoxR binds cooperatively to the promoter(s) regulating expression of the sulfur oxidation (sox) operon of Pseudaminobacter salicylatoxidans KCT001

Sulfur oxidation in Pseudaminobacter salicylatoxidans KCT001 is rendered by the combined action of several enzymes encoded by a thiosulfate-inducible sox operon. In this study it has been conclusively demonstrated by insertional mutagenesis that the regulatory gene of this operon is soxR, which encodes a DNA-binding protein belonging to the ArsR-SmtB family. SoxR was found to bind to two promoter-operator segments within the sox cluster, of which the one (wx) located between soxW and soxX controls the expression of sulfur-oxidation genes soxX through soxD while the other, a bi-directional element (sv) located between soxS and soxV, controls the expression of soxVW in one direction and the putative regulatory cluster soxSRT in the other. In the case of the wx promoter the repressor was found to bind in a cooperative manner to two distinct binding sites having different affinities, while in the case of the sv promoter binding occurred at a symmetric dimeric site and involved a higher degree of cooperativity. The high degree of cooperativity observed in the binding of SoxR to its target sites seemed to be due to the propensity of SoxR monomers to form dimers. The apparent dissociation constants of the SoxR-operator complexes were in the nanomolar range, indicating relatively strong interactions. It was demonstrated using a reporter system in Escherichia coli that this high-affinity binding of SoxR led to efficient repression in trans. Thus the role of SoxR as a repressor of the sox operon has not only been conclusively established but it has also been shown that this repression is brought about through cooperative interactions of SoxR with dimeric binding sites that occlude the operon promoters.

Multiple drugbinding sites on the R482G isoform of the ABCG2 transporter

Drug-resistant cancer cells frequently display efflux pumps such as P-glycoprotein (P-gp), the multidrug resistance associated protein (MRP1) or the transporter ABCG2. These transporters are each capable of mediating the active efflux of numerous anticancer drugs and display relatively distinct substrate preferences. The last, most recently discovered member, ABCG2, plays a major role in resistance in several types of cancer and the precise pharmacology of this multidrug transporter remain unresolved as does the nature of substrate binding. Plasma membranes from insect cells expressing ABCG2 were used to characterise binding of [3H]daunomycin to the multidrug transporter. The kinetics of association and dissociation for this substrate and several other compounds were also determined in this experimental system. The dissociation constant for [3H]daunomycin binding was 564 +/- 57 nM and a Hill slope of 1.4 suggested cooperative binding. Doxorubicin, prazosin and daunomycin completely displaced the binding of radioligand, while mitoxantrone and Hoechst 33342 produced only a partial displacement. Analysis of the dissociation rates revealed that [3H]daunomycin and doxorubicin bind to multiple sites on the transporter. Both kinetic and equilibrium data support the presence of at least two symmetric drug binding sites on ABCG2, which is distinct from the asymmetry observed for P-gp. The data provide the first molecular details underlying the mechanism by which this transporter is capable of interacting with multiple substrates.

Non-canonical CRP sites control competence regulons in Escherichia coli and many other gamma-proteobacteria.

Escherichia coli's cAMP receptor protein (CRP), the archetypal bacterial transcription factor, regulates over a hundred promoters by binding 22 bp symmetrical sites with the consensus core half-site TGTGA. However, Haemophilus influenzae has two types of CRP sites, one like E.coli's and one with the core sequence TGCGA that regulates genes required for DNA uptake (natural competence). Only the latter ‘CRP-S’ sites require both CRP and the coregulator Sxy for activation. To our knowledge, the TGTGA and TGCGA motifs are the first example of one transcription factor having two distinct binding-site motifs. Here we show that CRP-S promoters are widespread in the γ-proteobacteria and demonstrate their Sxy-dependence in E.coli. Orthologs of most H.influenzae CRP-S-regulated genes are ubiquitous in the five best-studied γ-proteobacteria families, Enterobacteriaceae, Pasteurellaceae, Pseudomonadaceae, Vibrionaceae and Xanthomonadaceae. Phylogenetic footprinting identified CRP-S sites in the promoter regions of the Enterobacteriaceae, Pasteurellaceae and Vibrionaceae orthologs, and canonical CRP sites in orthologs of genes known to be Sxy-independent in H.influenzae. Bandshift experiments confirmed that E.coli CRP-S sequences are low affinity binding sites for CRP, and mRNA analysis showed that they require CRP, cAMP (CRP's allosteric effector) and Sxy for gene induction. This work suggests not only that the γ-proteobacteria share a common DNA uptake mechanism, but also that, in the three best studied families, their competence regulons share both CRP-S specificity and Sxy dependence.

Synthesis and luminescent properties of Ln 3+ (Eu 3+, Sm 3+, Dy 3+)-doped lanthanum aluminum germanate LaAlGe 2O 7 phosphors

The novel phosphor of LaAlGe 2O 7 activated with the trivalent rare-earth Ln 3+ (Ln = Eu, Sm, Dy) ions were synthesized by solid-state method, and their characterization and luminescent properties were investigated. The absorption, emission and excitation spectra, and decay curves were employed to study the luminescence properties. The calcined powders of the Eu 3+, Sm 3+ and Dy 3+ ions doped in the LaAlGe 2O 7 emit bright red, reddish orange and yellowish white light, respectively. In the photoluminescence investigations, there is a single and highly symmetric site for activator ions in the LaAlGe 2O 7 host lattice. The sharp emission properties show that the LaAlGe 2O 7 is a suitable host for rare-earth doped laser crystal and phosphor material.

Structural Basis for Rab11-mediated Recruitment of FIP3 to Recycling Endosomes

The Rab11 GTPase regulates recycling of internalized plasma membrane receptors and is essential for completion of cytokinesis. A family of Rab11 interacting proteins (FIPs) that conserve a C-terminal Rab-binding domain (RBD) selectively recognize the active form of Rab11. Normal completion of cytokinesis requires a complex between Rab11 and FIP3. Here, we report the crystal structure and mutational analysis of a heterotetrameric complex between constitutively active Rab11 and a FIP3 construct that includes the RBD. Two Rab11 molecules bind to dyad symmetric sites at the C terminus of FIP3, which forms a non-canonical coiled-coiled dimer with a flared C terminus and hook region. The RBD overlaps with the coiled coil and extends through the C-terminal hook. Although FIP3 engages the switch and interswitch regions of Rab11, the mode of interaction differs significantly from that of other Rab-effector complexes. In particular, the switch II region undergoes a large structural rearrangement from an ordered but non-complementary active conformation to a remodeled conformation that facilitates the interaction with FIP3. Finally, we provide evidence that FIP3 can form homo-oligomers in cells, and that a critical determinant of Rab11 binding in vitro is necessary for FIP3 recruitment to recycling endosomes during cytokinesis.

Lidocaine inactivation of the ventral pallidum affects responding for brain stimulation reward more than it affects the stimulation's reward value

The ventral pallidum (VP) supports self-stimulation and has anatomical connections that suggest it could be linked to medial forebrain bundle (MFB) self-stimulation. Dorsal VP appears to be more related to dorsal striatopallidum and thus to cognitive control of movement, while ventral VP appears to be more related to linking motivation to action. In this study we challenged MFB self-stimulation by temporarily inactivating dorsal and ventral VP. We assessed changes in performance capacity and stimulation reward value using the rate–frequency curve shift paradigm. VP inactivation, especially in the ventral aspect of the VP ipsilateral to the stimulation site, was more likely to substantially impair maximum response rate than to affect the frequency required to maintain half-maximal responding. These effects were transient, typically disappearing within 20 min following inactivation. Contralateral inactivation was relatively ineffective and bilateral inactivation was surprisingly less effective than ipsilateral inactivation alone, although bilaterally symmetric injection sites were largely confined to the dorsal VP. The fact that inactivation-induced changes in maximum response rate were more prominent than changes in the frequency required to maintain half-maximal responding suggests a role for the ventral VP in linking reward to responding rather than detecting or computing reward value.

Asymmetry of Shufflon-specific Recombination Sites in Plasmid R64 Inhibits Recombination between Direct sfx Sequences

The shufflon of plasmid R64 consists of four DNA segments separated and flanked by seven sfx recombination sites. Rci-mediated recombination between any inverted sfx sequences causes inversion of the DNA segments independently or in groups. The R64 shufflon selects one of seven pilV genes encoding type IV pilus adhesins, in which the N-terminal region is constant, while the C-terminal regions are variable. The R64 sfx sequences are asymmetric. The sfx central region and right arm sequences are conserved, but left arm sequences are not. Here we constructed a symmetric sfx sequence, in which the sfx left arm sequence was changed to the inverted repeat of the right arm sequence and made artificial shufflon segments carrying symmetric sfx sequences in inverted or direct orientations. The symmetric sfx sequence exhibited the highest inversion frequency in a shufflon segment flanked by two inverted sfx sequences. Rci-dependent deletion of a shufflon segment flanked by two direct symmetric sfx sequences was observed, suggesting that asymmetry of R64 sfx sequences inhibits recombination between direct sfx sequences. In addition, intermolecular recombination between symmetric sfx sequences was also observed. The extra C-terminal domain of Rci was shown to be essential for inversion of the R64 shufflon using asymmetric sfx sequences but not essential for recombination using symmetric sfx sequences, suggesting that the Rci C-terminal segment helps the binding of Rci to asymmetric sfx sequences. Rci protein lacking the C-terminal domain bound to both arms of symmetric sfx sequence but only to the right arm of asymmetric sfx sequence.

Evidence for symmetric cationic sites in zirconium-bearing oxide glasses

The local environment of Zr in a borosilicate glass has been investigated by combining molecular dynamics (MD) simulations with Zr $K$-edge x-ray absorption spectroscopy (XAS) measurements. Two- and three-body distribution functions in agreement with the experimental data were obtained. The oxygen first neighbors around Zr are found to be narrowly distributed, revealing almost perfectly regular $\mathrm{Zr}{\mathrm{O}}_{6}$ octahedra. Significant three-body contributions (Zr-O-O) to the XAS signal are observed, probing the existence of highly symmetric cationic sites in a glassy network. The ${g}_{\mathrm{Zr}\mathrm{O}}(r)$ first peak derived from the experimental data is narrower than the one given by the MD model, thus providing elements for further improvements of the MD potentials.

High-resolution structure of a plasmid-encoded dihydrofolate reductase: pentagonal network of water molecules in theD2-symmetric active site

R67 plasmid-encoded dihydrofolate reductase (R67 DHFR) is an NADPH-dependent homotetrameric enzyme that catalyzes the reduction of dihydrofolate to tetrahydrofolate. The amino-acid sequence and molecular architecture of R67 DHFR and its inhibitory properties toward folate analogues are different from those of chromosomal DHFR. Here, the crystal structure of R67 DHFR refined using 1.1 A resolution data is presented. Blocked full-matrix least-squares refinement without restraints resulted in a final R factor of 11.4%. The anisotropic atomic displacement parameters analyzed by Rosenfield matrices and translation-libration-screw validation suggested four quasi-rigid domains. A total of ten Calpha-H...O hydrogen bonds were identified between the beta-strands. There is reasonable structural evidence that His62 is not protonated in the tetramer, which is in accord with previous pH-profile studies. The side chain of Gln67 that protrudes into the active site exhibits dual conformation, a feature noticed for the first time owing to the availability of atomic resolution data. The R67 DHFR active site is unique: it has D2 symmetry and is a large active site with a pentagonal network of water molecules and exposure of backbone atoms to solvent; the central pore is favorable for planar ring-stacking interactions. The geometrical shape, overall symmetry, local asymmetry and waters appear to dominate the binding of ligands, catalysis and inhibition.

Evaluation of the epidermal refractive index measured by optical coherence tomography

It has recently been proposed that the refractive index (RI) measured by means of optical coherence tomography (OCT) may be a valid measure for hydration of skin. In this pilot study, using OCT in vivo, we aimed to investigate the interday variability of RI measurements and acute changes of RI following the application of a moisturizer. Twenty healthy Caucasian volunteers were investigated on their forearms using a commercially available OCT system (SkinDex 300, ISIS optronics GmbH, Mannheim, Germany) fitted with an integrated algorithm for the evaluation of the RI. The interday repeatability of the OCT method was determined performing symmetrical measurements on both forearms on day 1, 5, 9, and 13. In order to investigate the acute effect of a moisturizer on RI, OCT assessments were performed before and 10 min after the application of an aqueous lotion with a lipophilic phase. As a control, the contralateral site was investigated in the same way, except for the use of distilled water instead of the lotion. Assessments of interday variability revealed insignificant (P>0.05) variances between the four measurement times as expressed in very small repeatability coefficients (right arm: 0.039; left arm 0.053) and small coefficients of variance (right arm: 1.02%; left arm: 1.38%). With regard to the RIs measured over time, we could not observe significant (P>0.05) differences between the two symmetrical anatomic sites (mean+/-SD of RI: 1.3893+/-0.0142 (right arm); 1.3875+/-0.0192 (left arm)). The acute effect of the moisturizer was indicated by a significant decrease of the RI 5 min after the application of the lotion (1.399+/-0.01 vs. 1.387+/-0.02; difference between means: 0.012; P=0.033; 95% confidence interval: 0.001-0.0023). However the control site treated with distilled water did not show significant differences between the two measurement times (1.387+/-0.013 vs. 1.391+/-0.023; difference between means: -0.004; P=0.57; 95% confidence interval: -0.019-0.011). In this pilot study, we have demonstrated that RI evaluation via OCT is a promising technique that may be used for the assessment of skin hydration in vivo. However, the direct comparison of OCT with standard methods, ideally such as nuclear magnetic resonance spectroscopy, is necessary.

High-Pressure Mössbauer Measurements on Nanocrystalline Perovskite (La,Sr)(Mn,Fe)O3

We report here the Mossbauer measurements on nanocrystalline perovskite structured manganite La0.8Sr0.2Mn0.8Fe0.1957Fe0.01O3 as a function of pressure up to 10 GPa at room temperature. The nanocrystalline sample, prepared by sol–gel technique found to have crystallite sizes of ∼138 ± 10 A. Zero-field electrical resistivity measurements with temperature support the nanocrystalline nature. At ambient pressure, Fe3+ as well as Fe4+ ions are distributed in two different environments – Fe3+ in low symmetric site surrounded by Mn3+ ions only while Fe4+ in high symmetric site with at least one Mn3+ ion. Pressure seems to affect the higher symmetric site. A sudden increase in isomer shift at 0.52 GPa indicates the first order phase transition representing the transformation of Fe4+ to Fe3+. Another transition at 3.7 GPa, represents the presence of Fe3+ in single kind of environment. Pressure dependence of electrical resistivity measurements verifies the transitions attributing the first order transition to the cross over of localized-electron to band magnetism.

Local Tunneling Spectroscopy as a Signature of the Fulde-Ferrell-Larkin-Ovchinnikov State ins- andd-Wave Superconductors

The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states for two-dimensional s- and d-wave superconductors (s- and d-SCs) are self-consistently studied under an in-plane magnetic field. While the stripe solution of the order parameter is found to have lower free energy in s-SCs, a square lattice solution appears to be energetically more favorable in the case of d-SCs. At certain symmetric sites, we find that the features in the local density of states (LDOS) can be ascribed to two types of bound states. We also show that the LDOS maps for d-SCs exhibit bias-energy-dependent checkerboard patterns. These characteristics can serve as signatures of the FFLO states.

Redox-Dependent Structural Changes in the Azotobacter vinelandii Bacterioferritin: New Insights into the Ferroxidase and Iron Transport Mechanism,

In this work, we report the X-ray crystal structure of the aerobically isolated (oxidized) and the anaerobic dithionite-reduced (at pH 8.0) forms of the native Azotobacter vinelandii bacterioferritin to 2.7 and 2.0 A resolution, respectively. Iron K-edge multiple anomalous dispersion (MAD) experiments unequivocally identified the presence of three independent iron-containing sites within the protein structure. Specifically, a dinuclear (ferroxidase) site, a b-type heme site, and the binding of a single iron atom at the four-fold molecular axis of the protein shell were observed. In addition to the novel observation of iron at the four-fold pore, these data also reveal that the oxidized form of the protein has a symmetrical ferroxidase site containing two five-coordinate iron atoms. Each iron atom is ligated by four carboxylate oxygen atoms and a single histidyl nitrogen atom. A single water molecule is found within hydrogen bonding distance of the ferroxidase site that bridges the two iron atoms on the side opposite the histidine ligands. Chemical reduction of the protein under anaerobic conditions results in an increase in the average Fe-Fe distance in the ferroxidase site from approximately 3.5 to approximately 4.0 A and the loss of one of the ligands, H130. In addition, there is significant movement of the bridging water molecule and several other amino acid side chains in the vicinity of the ferroxidase site and along the D helix to the three-fold symmetry axis. In contrast to previous work, the higher-resolution data for the dithionite-reduced structure suggest that the heme may be bound in multiple conformations. Taken together, these data allow a molecular movie of the ferroxidase gating mechanism to be developed and provide further insight into the iron uptake and/or release and mineralization mechanism of bacterioferritins in general.

Local structure of gold impurities in silicon determined by EXAFS

The incorporation of Au atoms within the silicon lattice was determined from X-ray absorption fine structure (XAFS). A detection limit of about 10 14 cm - 2 doses equivalent Au atoms in silicon was achieved by grazing incidence of the X-rays and fluorescence detection. Our results are (i) after Au implantation (as-implanted state) single Au atoms occupy regular high symmetric substitutional lattice sites in silicon, (ii) after thermal treatments some of the Au-atoms remain substitutional other diffuse to the sample surface. For the Au atoms near the surface very similar short range parameters as for metallic gold are detected and X-ray reflectrometry gives evidence for a near-surface segregation of gold atoms.

Lower Levels of Transgene Silencing in Roots is Associated with Reduced DNA Methylation Levels at Non-Symmetrical Sites but not at Symmetrical Sites

Transgene transcripts were recently shown to accumulate at higher levels in roots, relative to leaves, of silenced-transgenic Nicotiana benthamiana plants and to be inversely related with the accumulation of small interfering RNAs (siRNAs), suggesting that RNA silencing is less active in roots than in leaves (Andika et al., 2005. Mol. Plant-Microbe Interact. 18: 194). Here we show that the lower transgene RNA silencing activity in roots was associated with lower transgene methylation levels at non-symmetrical CpNpN context but not at symmetrical CpG or CpNpG context in three sets of transformant plants with different exogenous genes. In contrast, such a difference between roots and leaves was not observed for the Tnt1 retrotransposon: no Tnt1 transcript was detected in roots or in leaves of N. benthamiana, while equal levels of Tnt1-derived siRNA accumulation and Tnt1 methylation were found. From our data and previously reported information, we suggest that roots have less of an activity that acts at the step of generation of siRNAs.

Effect of Incorporated Alkaline and Alkaline Earth Elements on the Structural and Electronic Properties of Cordierite by First Principles Calculations

Cordierite has structural channels and alkaline elements can be incorporated into the channel. In order to study the incorporation effect of the alkaline and alkaline earth elements into the channel on structural and electronic properties, density functional theory calculations were performed to analyze the structural changes associated with the incorporation of alkaline and alkaline earth elements. It was found that alkaline and alkaline earth metals can be stable incorporated in the channel. Good correlation between the calculated ionic radius and the first ionization energy was observed. It was also found that the incorporated elements occupy a specific site in the channel. Incorporated elements cause the expansion of the lattice in c-axis because of the repulsive and attractive forces, interaction of the incorporated elements with the framework cations and oxygen atoms, respectively. The incorporated element at the less symmetrical sites in the channel was found to cause the more remarkable distortion.

Characterization of Dual Substrate Binding Sites in the Homodimeric Structure of Escherichia coli mRNA Interferase MazF

MazF and MazE constitute a so-called addiction module that is critical for bacterial growth arrest and eventual cell death in response to stress. The MazF toxin was recently shown to possess mRNA interferase (MIase) activity, and acts as a protein synthesis inhibitor by cleaving cellular mRNA. As a cognate regulator, the short-lived antitoxin, MazE, inhibits MazF MIase activity and hence maintains the delicate homeostasis between these two components. In the present study, we have shown that the MazF homodimer contains two symmetric binding sites, each of which is capable of interacting with a MazE C-terminal peptide, MazEp(54-77). The slow exchange phenomenon between free and peptide-bound MazF on the NMR timescale indicates relatively high affinities for MazEp(54-77) at both sites (Kd,K'd < 10(-7) M). However, the observed sequential binding behavior suggests a negative cooperativity between the two sites (Kd < K'd). A 13 base single-stranded DNA, employed as an uncleavable RNA substrate analog, can also bind to both sites on the MazF homodimer with moderate affinity (Kd approximately 10(-5) -10(-6) M). Chemical shift perturbation data deduced from NMR experiments indicates that the two binding sites for the MazEp peptide coincided with those for the single-stranded DNA competitive inhibitor. These dual substrate-binding sites are located on the concave interface of the MazF homodimer, consisting of a highly basic region underneath the S1-S2 loop and two hydrophobic regions containing the H1 helix of one subunit and the S3-S4 loop of the opposing subunit. We show that the MazF homodimer is a bidentate endoribonuclease equipped with two identical binding sites for mRNA processing and that a single MazE molecule occupying one of the binding sites can affect the conformation of both sites, hence efficiently hindering the activity of MazF.