Observation of subpicosecond x-ray emission from laser-cluster interaction

We present the first experimental evidence of the subpicosecond duration of x-ray pulses emitted from laser-irradiated clusters, demonstrating the suitability of such a debris free target for ultrafast x-ray science applications. The K-shell emission (approximately 3 keV) from large Ar clusters (6 x 10(5) to 4 x 10(6) atoms) is time resolved, when irradiated by ultrashort (40 fs to 5 ps) and intense laser pulses (10(15-17) W/cm2). The observations are supported by hydrodynamical and collisional-radiative calculations, that reproduce the extremely short x-ray pulse duration.

[Expert opinion on the basic surgical technique for laparoscopic ventral hernia repair]

Laparoscopic ventral hernia repair is currently the subject of intense debate, even though it provides a series of advantages over open surgery and is feasible and safe. Various studies have shown this technique to be as effective as open repair with a lower recurrence rate. Despite the excellent results of laparoscopic repair of ventral hernias, there are numerous controversies associated with this procedure. These controversies concern the indications and contraindications of the procedure, and technical aspects such as how to create the pneumoperitoneum, perform adhesiolysis, manage the hernia sac, and insert and fix the mesh to the anterior abdominal wall. Also controversial are outcome, complications related to postoperative seroma, and which type and size of mesh should be used. The present article aims primarily to address many of these issues, based on the experience of distinct surgeons with expertise in this approach, in order to provide data to establish a consensus on how laparoscopic ventral hernia repair should be performed.

NarJ Chaperone Binds on Two Distinct Sites of the Aponitrate Reductase of Escherichia coli to Coordinate Molybdenum Cofactor Insertion and Assembly

Understanding when and how metal cofactor insertion occurs into a multisubunit metalloenzyme is of fundamental importance. Molybdenum cofactor insertion is a tightly controlled process that involves specific interactions between the proteins that promote cofactor delivery, enzyme-specific chaperones, and the apoenzyme. In the assembly pathway of the multisubunit molybdoenzyme, membrane-bound nitrate reductase A from Escherichia coli, a NarJ-assisted molybdenum cofactor (Moco) insertion step, must precede membrane anchoring of the apoenzyme. Here, we have shown that the NarJ chaperone interacts at two distinct binding sites of the apoenzyme, one interfering with its membrane anchoring and another one being involved in molybdenum cofactor insertion. The presence of the two NarJ-binding sites within NarG is required to ensure productive formation of active nitrate reductase. Our findings supported the view that enzyme-specific chaperones play a central role in the biogenesis of multisubunit molybdoenzymes by coordinating subunits assembly and molybdenum cofactor insertion.

Investigation of laser-irradiated Ar cluster dynamics from K-shell x-ray emission measurements

Intense (up to a few 10(17) W/ cm2) femtosecond (down to 40 fs) laser pulses are focused onto a partially clusterized argon gas jet. The target was previously characterized and optimized in order to get a homogeneous and dense jet of clusters with a well controlled size. The interaction leads to x-ray emission that is absolutely calibrated and spectrally resolved using a high resolution time-integrated spectrometer in the K-shell range (from 2.9 to 4.3 keV). X-ray spectra are investigated as a function of different laser temporal parameters such as the nanosecond prepulse contrast, the laser pulse duration, and the femtosecond delay between two different laser pulses. The cluster size ranges from 180 to 350 angstroms and irradiation by laser pulses with both linear and circular polarization is investigated. The experimental results are discussed in terms of the laser-cluster interaction dynamics. They are compared with the predictions of collision-dominated nanoplasma models. However, further interaction processes are required in order to explain the observed characteristic lines demonstrating highly charged ions up to Ar16+.

Membrane-associated maturation of the heterotetrameric nitrate reductase of Thermus thermophilus

The nar operon, coding for the respiratory nitrate reductase of Thermus thermophilus (NRT), encodes a di-heme b-type (NarJ) and a di-heme c-type (NarC) cytochrome. The role of both cytochromes and that of a putative chaperone (NarJ) in the synthesis and maturation of NRT was studied. Mutants of T. thermophilus lacking either NarI or NarC synthesized a soluble form of NarG, suggesting that a putative NarCI complex constitutes the attachment site for the enzyme. Interestingly, the NarG protein synthesized by both mutants was inactive in nitrate reduction and misfolded, showing that membrane attachment was required for enzyme maturation. Consistent with its putative role as a specific chaperone, inactive and misfolded NarG was synthesized by narJ mutants, but in contrast to its Escherichia coli homologue, NarJ was also required for the attachment of the thermophilic enzyme to the membrane. A bacterial two-hybrid system was used to demonstrate the putative interactions between the NRT proteins suggested by the analysis of the mutants. Strong interactions were detected between NarC and NarI and between NarG and NarJ. Weaker interaction signals were detected between NarI, but not NarC, and both NarG and NarH. These results lead us to conclude that the NRT is a heterotetrameric (NarC/NarI/NarG/NarH) enzyme, and we propose a model for its synthesis and maturation that is distinct from that of E. coli. In the synthesis of NRT, a NarCI membrane complex and a soluble NarGJH complex are synthesized in a first step. In a second step, both complexes interact at the cytoplasmic face of the membrane, where the enzyme is subsequently activated with the concomitant conformational change and release of the NarJ chaperone from the mature enzyme.

Evidence for an EPR-Detectable Semiquinone Intermediate Stabilized in the Membrane-Bound Subunit NarI of Nitrate Reductase A (NarGHI) from Escherichia coli

Nitrate reductase A (NRA, NarGHI) is expressed in Escherichia coli by growing the bacterium in anaerobic conditions in the presence of nitrate. This enzyme reduces nitrate to nitrite and uses menaquinol (or ubiquinol) as the electron donor. The location of quinones in the enzyme, their number, and their role in the electron transfer mechanism are still controversial. In this work, we have investigated the spectroscopic and thermodynamic properties of a semiquinone (SQ) in membrane samples of overexpressed E. coli nitrate reductase poised in appropriate redox conditions. This semiquinone is highly stabilized with respect to free semiquinone. The g-values determined from the numerical simulation of its Q-band (35 GHz) EPR spectrum are equal to 2.0061, 2.0051, 2.0023. The midpoint potential of the Q/QH(2) couple is about -100 mV, and the SQ stability constant is about 100 at pH 7.5. The semiquinone EPR signal disappears completely upon addition of the quinol binding site inhibitor 2-n-nonyl-4-hydroxyquinoline N-oxide (NQNO). A semiquinone radical could also be stabilized in preparations where only the NarI membrane subunit is overexpressed in the absence of the NarGH catalytic dimer. Its thermodynamic and spectroscopic properties show only slight variations with those of the wild-type enzyme. The X-band continuous wave (cw) electron nuclear double resonance (ENDOR) spectra of the radicals display similar proton hyperfine coupling patterns in NarGHI and in NarI, showing that they arise from the same semiquinone species bound to a single site located in the NarI membrane subunit. These results are discussed with regard to the location and the potential function of quinones in the enzyme.

Structural and biochemical characterization of a quinol binding site of Escherichia coli nitrate reductase A

The crystal structure of Escherichia coli nitrate reductase A (NarGHI) in complex with pentachlorophenol has been determined to 2.0 A of resolution. We have shown that pentachlorophenol is a potent inhibitor of quinol:nitrate oxidoreductase activity and that it also perturbs the EPR spectrum of one of the hemes located in the membrane anchoring subunit (NarI). This new structural information together with site-directed mutagenesis data, biochemical analyses, and molecular modeling provide the first molecular characterization of a quinol binding and oxidation site (Q-site) in NarGHI. A possible proton conduction pathway linked to electron transfer reactions has also been defined, providing fundamental atomic details of ubiquinol oxidation by NarGHI at the bacterial membrane.

Expression profiles of a human pancreatic cancer cell line upon induction of apoptosis search for modulators in cancer therapy

We analyzed the differential gene expression in the pancreatic cancer cell line NP-18 upon induction of apoptosis caused by cyclin-dependent kinase inhibition triggered by either overexpression of the tumor suppressor gene p16(INK4A)using an adenoviral construction or incubation with the chemical inhibitors, roscovitine or olomoucine. Screening was performed using cDNA arrays from Clontech that allowed the determination of the expression of 1,176 genes specifically related with cancer. The analysis was carried out using the Atlas Image 2.01 (Clontech) and GeneSpring 4.2 (Silicon Genetics) softwares. Among the differentially expressed genes, we chose for further validation histone deacetylase 1 (HDAC1), von Hippel Lindau and decorin as upregulated genes, and Sp1, hypoxia-inducible factor-1 alpha and DNA primase as downregulated genes. The changes in the expression of these genes to mRNA were validated by quantitative RT-PCR and the final translation into protein by Western blot analysis. Inhibition of HDAC activity, Sp1 binding and DNA primase expression led to an increase in the level of apoptosis, both in parental cells and in doxorubicin-resistant cells. Therefore, these proteins could constitute possible targets to develop modulators in cancer chemotherapy that would increase or restore apoptosis.

Involvement of the Molybdenum Cofactor Biosynthetic Machinery in the Maturation of the Escherichia coli Nitrate Reductase A

The maturation of Escherichia coli nitrate reductase A requires the incorporation of the Mo-(bis-MGD) cofactor to the apoprotein. For this process, the NarJ chaperone is strictly required. We report the first description of protein interactions between molybdenum cofactor biosynthetic proteins (MogA, MoeA, MobA, and MobB) and the aponitrate reductase (NarG) using a bacterial two-hybrid approach. Two conditions have to be satisfied to allow the visualization of the interactions, (i) the presence of an active and mature molybdenum cofactor and (ii) the presence of the NarJ chaperone and of the NarG structural partner subunit, NarH. Formation of tungsten-substituted cofactor prevents the interaction between NarG and the four biosynthetic proteins. Our results suggested that the final stages of molybdenum cofactor biosynthesis occur on a complex made up by MogA, MoeA, MobA, and MobB, which is also in charge with the delivery of the mature cofactor onto the aponitrate reductase A in a NarJ-assisted process.

A New Type of NADH Dehydrogenase Specific for Nitrate Respiration in the Extreme Thermophile Thermus thermophilus

A four-gene operon (nrcDEFN) was identified within a conjugative element that allows Thermus thermophilus to use nitrate as an electron acceptor. Three of them encode homologues to components of bacterial respiratory chains: NrcD to ferredoxins; NrcF to iron-sulfur-containing subunits of succinate-quinone oxidoreductase (SQR); and NrcN to type-II NADH dehydrogenases (NDHs). The fourth gene, nrcE, encodes a membrane protein with no homologues in the protein data bank. Nitrate reduction with NADH was catalyzed by membrane fractions of the wild type strain, but was severely impaired in nrc::kat insertion mutants. A fusion to a thermophilic reporter gene was used for the first time in Thermus spp. to show that expression of nrc required the presence of nitrate and anoxic conditions. Therefore, a role for the nrc products as a new type of membrane NDH specific for nitrate respiration was deduced. Consistent with this, nrc::kat mutants grew more slowly than the wild type strain under anaerobic conditions, but not in the presence of oxygen. The oligomeric structure of this Nrc-NDH was deduced from the analysis of insertion mutants and a two-hybrid bacterial system. Attachment to the membrane of NrcD, NrcF, and NrcN was dependent on NrcE, whose cytoplasmic C terminus interacts with the three proteins. Interactions were also detected between NrcN and NrcF. Inactivation of nrcF produced solubilization of NrcN, but not of NrcD. These data lead us to conclude that the Nrc proteins form a distinct third type of bacterial respiratory NDH.

The catalytic subunit of Escherichia coli nitrate reductase A contains a novel [4Fe-4S] cluster with a high-spin ground state

We have used EPR spectroscopy, redox potentiometry, and protein crystallography to characterize the [4Fe-4S] cluster (FS0) of the Escherichia coli nitrate reductase A (NarGHI) catalytic subunit (NarG). FS0 is clearly visible in the crystal structure of NarGHI [Bertero, M. G., et al. (2003) Nat. Struct. Biol. 10, 681-687] but has novel coordination comprising one His residue and three Cys residues. At low temperatures (<15 K), reduced NarGHI exhibits a previously unobserved EPR signal comprising peaks at g = 5.023 and g = 5.556. We have assigned these features to a [4Fe-4S](+) cluster with an S = (3)/(2) ground state, with the g = 5.023 and g = 5.556 peaks corresponding to subpopulations exhibiting DeltaS = (1)/(2) and DeltaS = (3)/(2) transitions, respectively. Both peaks exhibit midpoint potentials of approximately -55 mV at pH 8.0 and are eliminated in the EPR spectrum of apomolybdo-NarGHI. The structure of apomolybdo-NarGHI reveals that FS0 is still present but that there is significant conformational disorder in a segment of residues that includes one of the Cys ligands. On the basis of these observations, we have assigned the high-spin EPR features of reduced NarGHI to FS0.

CYP175A1 from Thermus thermophilus HB27, the first �-carotene hydroxylase of the P450 superfamily

The biological function of thermostable P450 monooxygenase CYP175A1 from Thermus thermophilus HB27 was studied by functional complementation in Escherichia coli. The gene product of CYP175A1 added hydroxyl groups to both beta rings of beta-carotene to form zeaxanthin (beta,beta-carotene-3,3'-diol) in E. coli, which produces beta-carotene due to the Erwinia uredovora carotenoid biosynthesis genes. In addition, spectroscopic methods revealed that E. coli carrying CYP175A1 and the cDNA of the Haematococcus pluvialis carotene ketolase was able to synthesise hydroxyechinenone. The predicted amino acid sequence of the enzyme from T. thermophilus does not show substantial similarity with other known beta-carotene hydroxylases, but 41% with the cytochrome P450 monooxygenase from Bacillus megaterium (CYP102A1, P450 BM3). It is concluded that CYP175 A1 represents a new type of beta-carotene hydroxylase of the P450 superfamily.

Insights into the respiratory electron transfer pathway from the structure of nitrate reductase A

The facultative anaerobe Escherichia coli is able to assemble specific respiratory chains by synthesis of appropriate dehydrogenases and reductases in response to the availability of specific substrates. Under anaerobic conditions in the presence of nitrate, E. coli synthesizes the cytoplasmic membrane-bound quinol-nitrate oxidoreductase (nitrate reductase A; NarGHI), which reduces nitrate to nitrite and forms part of a redox loop generating a proton-motive force. We present here the crystal structure of NarGHI at a resolution of 1.9 A. The NarGHI structure identifies the number, coordination scheme and environment of the redox-active prosthetic groups, a unique coordination of the molybdenum atom, the first structural evidence for the role of an open bicyclic form of the molybdo-bis(molybdopterin guanine dinucleotide) (Mo-bisMGD) cofactor in the catalytic mechanism and a novel fold of the membrane anchor subunit. Our findings provide fundamental molecular details for understanding the mechanism of proton-motive force generation by a redox loop.

In vivo interactions between gene products involved in the final stages of molybdenum cofactor biosynthesis in Escherichia coli

The final stages of bacterial molybdenum cofactor (Moco) biosynthesis correspond to molybdenum chelation and nucleotide attachment onto an unique and ubiquitous structure, the molybdopterin. Using a bacterial two-hybrid approach, here we report on the in vivo interactions between MogA, MoeA, MobA, and MobB implicated in several distinct although linked steps in Escherichia coli. Numerous interactions among these proteins have been identified. Somewhat surprisingly, MobB, a GTPase with a yet unclear function, interacts with MogA, MoeA, and MobA. Probing the effects of various mo. mutations on the interaction map allowed us (i) to distinguish Moco-sensitive interactants from insensitive ones involving MobB and (ii) to demonstrate that molybdopterin is a key molecule triggering or facilitating MogA-MoeA and MoeA-MobA interactions. These results suggest that, in vivo, molybdenum cofactor biosynthesis occurs on protein complexes rather than by the separate action of molybdenum cofactor biosynthetic proteins.

Photon acceleration of ultrashort laser pulses by relativistic ionization fronts

We present experimental results from the collision of weak ultrashort pulses with relativistic ionization fronts in copropagation and counterpropagation. The observed frequency upshifts of the probe pulses provide not only information about the electron density of the ionization front but also reveal the fine structure of the front. The connection between the correlation lengths for copropagation and counterpropagation and the longitudinal and transverse dimensions of the ionization front is also demonstrated thus showing the feasibility of using the frequency upshift experienced by short probe pulses to fully characterize relativistic ionization fronts and other relativistic coherent structures in laser-produced plasmas.

Hot-electron influence on L-shell spectra of multicharged Kr ions generated in clusters irradiated by femtosecond laser pulses

Strong L-shell x-ray emission has been obtained from Kr clusters formed in gas jets and irradiated by 60-500-fs laser pulses. Spectral lines from the F-, Ne- Na-, and Mg-like charge states of Kr have been identified from highly resolved x-ray spectra. Spectral line intensities are used in conjunction with a detailed time-dependent collisional-radiative model to diagnose the electron distribution functions of plasmas formed in various gas jet nozzles with various laser pulse durations. It is shown that L-shell spectra formed by relatively long nanosecond-laser pulses can be well described by a steady-state model without hot electrons when opacity effects are included. In contrast, adequate modeling of L-shell spectra from highly transient and inhomogeneous femtosecond-laser plasmas requires including the influence of hot electrons. It is shown that femtosecond-laser interaction with gas jets from conical nozzles produces plasmas with higher ionization balances than plasmas formed by gas jets from Laval nozzles, in agreement with previous work for femtosecond laser interaction with Ar clusters.

Detection and interpretation of redox potential optima in the catalytic activity of enzymes

It is no surprise that the catalytic activity of electron-transport enzymes may be optimised at certain electrochemical potentials in ways that are analogous to observations of pH-rate optima. This property is observed clearly in experiments in which an enzyme is adsorbed on an electrode surface which can supply or receive electrons rapidly and in a highly controlled manner. In such a way, the rate of catalysis can be measured accurately as a function of the potential (driving force) that is applied. In this paper, we draw attention to a few examples in which this property has been observed in enzymes that are associated with membrane-bound respiratory chains, and we discuss its possible origins and implications for in vivo regulation.

Electron transfer from heme bL to the [3Fe-4S] cluster of Escherichia coli nitrate reductase A (NarGHI)

We have investigated the functional relationship between three of the prosthetic groups of Escherichia coli nitrate reductase A (NarGHI): the two hemes of the membrane anchor subunit (NarI) and the [3Fe-4S] cluster of the electron-transfer subunit (NarH). In two site-directed mutants (NarGHI(H56R) and NarGHI(H205Y)) that lack the highest potential heme of NarI (heme b(H)), a large negative DeltaE(m,7) is elicited on the NarH [3Fe-4S] cluster, suggesting a close juxtaposition of these two centers in the holoenzyme. In a mutant retaining heme b(H), but lacking heme b(L) (NarGHI(H66Y)), there is no effect on the NarH [3Fe-4S] cluster redox properties. These results suggest a role for heme b(H) in electron transfer to the [3Fe-4S] cluster. Studies of the pH dependence of the [3Fe-4S] cluster, heme b(H), and heme b(L) E(m) values suggest that significant deprotonation is only observed during oxidation of the latter heme (a pH dependence of -36 mV pH(-1)). In NarI expressed in the absence of NarGH [NarI(DeltaGH)], apparent exposure of heme b(H) to the aqueous milieu results in both it and heme b(L) having E(m) values with pH dependencies of approximately -30 mV pH(-1). These results are consistent with heme b(H) being isolated from the aqueous milieu and pH effects in the holoenzyme. Optical spectroscopy indicates that inhibitors such as HOQNO and stigmatellin bind and inhibit oxidation of heme b(L) but do not inhibit oxidation of heme b(H). Fluorescence quench titrations indicate that HOQNO binds with higher affinity to the reduced form of NarGHI than to the oxidized form. Overall, the data support the following model for electron transfer through the NarI region of NarGHI: Q(P) site --> heme b(L) --> heme b(H) --> [3Fe-4S] cluster.