Showing papers by "Polytechnic University of Valencia published in 1999"

A glimpse of the mechanisms of ion homeostasis during salt stress

Abstract: Cells respond to salt stress by osmolyte synthesis and by increase of K + uptake and Na + efflux at the plasma membrane and of Na + accumulation at the vacuole. This cellular level of response has been approached by the molecular genetics of Saccharomyces cerevisiae. Two plasma membrane proteins, Sln1 and Shol, operate as sensors for turgor loss under mild osmotic stress. They activate the Hog1 MAP kinase which induces several defence genes, including the ENA1 Na + -efflux pump. Hog1-mediated induction occurs by inactivation of the Skol repressor. High concentrations of Na + activate the calcium-dependent protein phosphatase calcineurin, which induces the ENA1 gene via the positive transcription factor Hal8/ Crz1/Tcn1. Vacuoles could operate as Na + sensors through a Na + -Ca 2+ exchange mechanism. The electrical potential of the plasma membrane is a major determinant of the uptake of toxic cations, including Na + , by non-specific leakage pathways. This biophysical parameter is modulated by the concerted activities of the proton pumping Pma1 ATPase and the Trk1,2 K + uptake system. Both calcineurin and the protein kinase Hal4,5 modulate Trk1,2 and, indirectly, the membrane potential and salt tolerance. The limiting role of vacuolar cation accumulation in salt tolerance has been demonstrated by expression in yeast vacuoles of the bacterial antiporter NhaA.

New and flexible fiber-optic delay-line filters using chirped Bragg gratings and laser arrays

Abstract: We propose a novel kind of radio-frequency filters that are composed by a linearly chirped fiber grating and a laser array. These structures are capable of providing full transfer-function reconfiguration and resonance tunability. A thorough theoretical modeling and operation features are provided along with the experimental demonstration of the proposed features.

A novel mechanism of ion homeostasis and salt tolerance in yeast: the Hal4 and Hal5 protein kinases modulate the Trk1-Trk2 potassium transporter.

Abstract: The regulation of intracellular ion concentrations is a fundamental property of living cells. Although many ion transporters have been identified, the systems that modulate their activity remain largely unknown. We have characterized two partially redundant genes from Saccharomyces cerevisiae, HAL4/SAT4 and HAL5, that encode homologous protein kinases implicated in the regulation of cation uptake. Overexpression of these genes increases the tolerance of yeast cells to sodium and lithium, whereas gene disruptions result in greater cation sensitivity. These phenotypic effects of the mutations correlate with changes in cation uptake and are dependent on a functional Trk1-Trk2 potassium transport system. In addition, hal4 hal5 and trk1 trk2 mutants exhibit similar phenotypes: (i) they are deficient in potassium uptake; (ii) their growth is sensitive to a variety of toxic cations, including lithium, sodium, calcium, tetramethylammonium, hygromycin B, and low pH; and (iii) they exhibit increased uptake of methylammonium, an indicator of membrane potential. These results suggest that the Hal4 and Hal5 protein kinases activate the Trk1-Trk2 potassium transporter, increasing the influx of potassium and decreasing the membrane potential. The resulting loss in electrical driving force reduces the uptake of toxic cations and improves salt tolerance. Our data support a role for regulation of membrane potential in adaptation to salt stress that is mediated by the Hal4 and Hal5 kinases.

Repressors and Upstream Repressing Sequences of the Stress- Regulated ENA1 Gene in Saccharomyces cerevisiae: bZIP Protein Sko1p Confers HOG-Dependent Osmotic Regulation

Abstract: The study of adaptation mechanisms during salinity stress in the yeast Saccharomyces cerevisiae has revealed several components of sensing and signal transduction pathways, as well as target genes whose expression is activated upon salt stress (for review see references 18, 45, and 46). Increased expression of the ENA1 gene has been found to represent a crucial cellular response after salt challenge. The ENA/PMR2 gene cluster of S. cerevisiae contains a tandem array of nearly identical genes encoding P-type ATPases involved in the extrusion of Na+ and Li+ ions from the cytoplasm (17, 59). Active export of these toxic ions is a crucial cellular process to avoid deleterious intracellular Na+ and Li+ concentrations. Mutants lacking the first ENA gene in the gene cluster (ena1) are hypersensitive to salt stress (17). The ENA1 gene is highly regulated at the transcriptional level, and its expression is increased strongly in response to salt stress (12) and glucose starvation (1, 41). Salt induction of ENA1 expression depends on both the calcineurin pathway and the high-osmolarity glycerol (HOG) mitogen-activated protein (MAP) kinase pathway (28). The first pathway is activated by high concentrations of either Na+ or Ca2+ and is dependent on the phosphoprotein phosphatase calcineurin (28, 33). A calcium signalling pathway composed of calmodulin (7), the calcineurin heterodimeric enzyme (6, 23), and the zinc finger transcriptional activator Crz1/Tcn1/Hal8p (31, 32, 50) has been reported to contribute to the resistance of yeast to elevated concentrations of several cations (Na+, Li+, and Mn2+). Therefore, Ca2+/calmodulin signalling may act, at least in part, through the transcriptional activation of ion transporter genes such as ENA1. The HOG pathway responds to moderate concentrations of osmotic agents and rapidly activates via a multistep phosphorelay mechanism the Hog1p MAP kinase by Tyr phosphorylation (2, 26, 39). Although a great number of genes have been found to need HOG signalling for their osmotic up-regulation, the mechanism of gene activation through phosphorylated Hog1p kinase is still unknown. In Schizosaccharomyces pombe, the basic leucine zipper (bZIP) transcriptional activator Atf1p has been identified as a direct phosphorylation target of the Hog1p homolog MAP kinase Sty1p (49, 51, 60). Activated Atf1p, in turn, can bind directly to UASs (upstream activating sequences) located in various stress-regulated promoters and then trigger gene expression (60). In S. cerevisiae, stress response promoter elements (STREs) represent UASs that respond to a great variety of stresses (22, 27, 43) and are bound by the zinc finger activators Msn2p and Msn4p (30, 42). Recent work, however, indicates that osmotic induction of several genes including ENA1 occurs by the release from transcriptional repression (29) and involves the general repressor complex Ssn6p-Tup1p. In the case of the HAL1 gene, an upstream repressing sequence (URS) regulated by osmotic stress has been identified (29). This mechanism based on regulated repressors bound to URSs is similar to the one operating in carbon source regulation. A great number of yeast genes, including ENA1, are derepressed under glucose starvation conditions, and for many of them the inactivation of the general repressor complex Mig1p-Ssn6p-Tup1p (21, 36, 54) through the protein kinase Snf1p (3, 53) has been reported as an important mechanism of glucose-regulated transcriptional control. In this work, we analyzed the promoter of the ENA1 gene and found that transcriptional regulation during osmotic stress as well as during glucose starvation occurs through a repression mechanism dependent on the Ssn6p-Tup1p general corepressor. Signalling through general glucose repression occurs through a Mig1/2p binding site (URSMIG-ENA1), whereas osmotic stress signalling through the HOG pathway is mediated through a cyclic AMP (cAMP) response element (CRE)-like sequence (URSCRE-ENA1) that is bound by the bZIP transcriptional factor Sko1p.

The dormancy-related peroxiredoxin anti-oxidant, PER1, is localized to the nucleus of barley embryo and aleurone cells.

Abstract: Protection against desiccation-induced injury, including damage by reactive oxygen species (ROS), is a necessary component of the genetic programmes active during late seed development. Likewise, protection against ROS respiration by-products is required during seed imbibition and germination. Late embryogenesis abundant (LEA) proteins are proposed to protect seed tissues against desiccation-induced damage. Specifically, the atypical Lea gene Per1 in barley (Hordeum vulgare L.) has been proposed to play a protective role in embryo and aleurone cells against free-radical damage during late seed development and early imbibition. PER1 represents a subgroup of the peroxiredoxin family of thiol-requiring anti-oxidants with one conserved cysteine residue (1-Cys), and displays in vitro anti-oxidant activity. In this work, we use antiserum generated against PER1 to study protein accumulation patterns as well as localization at the tissue, cellular and subcellular level. While previous studies have shown the Per1 transcript to be dormancy-related, we show here that the protein level is maintained in imbibed dormant seeds, but not in non-dormant seeds. Our data identify the location of this seed-specific peroxiredoxin as the nucleus of immature embryos and aleurone layers. Highest levels of protein are detected in nucleoli. In contrast, in mature imbibed dormant seeds, cytosolic levels are comparable to that of the nucleus. A putative nuclear localization signal (NLS) of bipartite nature was identified in the C-terminal end of the PER1 sequence. Protective roles for PER1 in seeds are discussed.

Porous silicon—mechanisms of growth and applications

Abstract: The present state-of-the-art in understanding the mechanisms of the formation of porous silicon (PS) and its physical properties is reviewed, with special emphasis on problems which were not much in the focus of existing review literature: mechanisms of the pore growth, stability of the PS properties in environment and electrical properties of PS layers. Emerging applications of porous silicon in different fields of technology are outlined.

Determination of Critical Transmission Range in Ad-Hoc Networks

Abstract: Ad-hoc networks are wireless mobile networks that do not rely on preexisting infrastructure. In such networks, multi-hop routing is used to achieve large degree of network connectivity and each node in the network acts as a router for its neighbors. The range of the radio transceivers is chosen as a trade-off between full network connectivity, the reuse of the available spectrum, and power consumption. This paper presents an algorithm to calculate the minimum transmission range of the transceivers that is required to achieve, with some probability, full network connectivity. Furthermore, we also study the effect of mobility on this value.

ITQ-7: A New Pure Silica Polymorph with a Three-Dimensional System of Large Pore Channels.

Abstract: A novel topology, consisting of a three-dimensional system of large pore channels, is found for the zeolite ITQ-7 (see picture). This material has been synthesized by combining the structure-directing effects of fluoride ions and a new organic cation purposefully designed to promote the crystallization of a low-density silica phase. ITQ-7 is the least dense crystalline silica phase known so far.

A proposal for nomenclature of aldehyde dehydrogenases in Saccharomyces cerevisiae and characterization of the stress-inducible ALD2 and ALD3 genes.

Abstract: The complete sequencing of the genome of Saccharomyces cerevisiae indicated that this organism contains five genes encoding aldehyde dehydrogenases. YOR374w and YER073w correspond to the mitochondrial isoforms and we propose as gene names ALD4 and ALD5, respectively. YPL061w has been described as the cytoplasmic constitutive isoform and named ALD6. We characterize here the tandem-repeated ORFs YMR170c and YMR169c as the cytoplasmic stress-inducible isoforms, with gene names ALD2 and ALD3, respectively. The expression of ALD2 and ALD3 is dependent on the general-stress transcription factors Msn2,4 but independent of the HOG MAP kinase pathway. ALD3 is induced by a variety of stresses, including osmotic shock, heat shock, glucose exhaustion, oxidative stress and drugs. ALD2 is only induced by osmotic stress and glucose exhaustion. A double null mutant, ald2 ald3, exhibited unchanged sensitivity to any of the above stresses. The only phenotype detected in this mutant was a reduced growth rate in ethanol medium as compared to the wild type.

Large two-dimensional sonic band gaps.

Abstract: We show that absolute sonic band gaps produced by two-dimensional square and triangular lattices of rigid cylinders in air can be increased by reducing the structure symmetry. In the case of square lattices, symmetry reduction is achieved by a smaller diameter cylinder placed at the center of each unit cell. For triangular lattices the reduction is achieved by decreasing the diameter of the cylinder at the center of the hexagons in the lattice. Theoretical predictions are also demonstrated experimentally: starting from a honeycomb lattice ~using cylinders of 4 cm of diameter size and 6.35 cm nearest-neighbor distance! we have studied the transition to a triangular symmetry by putting rods with increasing diameter ~in the range 0.6‐4 cm! at the center. The greatest enhancement of the attenuation strength observed in transmission experiments has been obtained in the high frequency region for diameter ratios in the range 0.1‐0.3. @S1063-651X~99!51212-9# PACS number~s!: 43.20.1g, 42.25.Bs, 52.35.Dm Since the seminal work of Yablonovitch @1# and John @2#, who open the research in photonic crystals ~PC!, we have witnessed the appearance of new devices based on the existence of photonic band gaps ~PBG! in these dielectric periodic structures. Also, because the underlying theory is applicable to other kind of waves, acoustic and elastic, a search for periodic structures having properties of sonic band gap ~SBG! or elastic band gap ~EBG!, respectively, is currently being performed. Therefore, the design, construction, and technological applications of a completely new type of crystals that could be called classical wave crystals is becoming a very promising project at the beginning of the new century. In the field of acoustics much theoretical work has been done proposing structures having SBG’s properties @3‐6#. The existence of SBG is due to a complex interplay between the sound velocity and density ratios of the composite materials, and their spatial arrangement. Experimentally, a few works have claimed to observe absolute band gaps @7,8#. Therefore, the actual possibility of building up composite material having SBG open a new technological research on environmental protection. One of the goals of PBG and SBG theory is the search of materials and/or topologies producing large gaps at the desired range of frequencies. In PC it has been shown that full PBG can be enlarged by decreasing the crystal symmetry through the introduction of a two-point basis set. Thus, for example, 3D PC based on a face-centered-cubic structure do not possess a full PBG between the first and second bands, but the ones having a diamond structure do because the additional point basis lifts the degeneracy of some bands @9# .I n the same manner, in two-dimensional ~2D! structures, Anderson and Giapis @10# obtained larger gaps when they add a different size rod at the center of each unit cell of square and honeycomb lattices. The latest conclusion awakened our interest about whether this sort of mechanism is effective in creating large SBG and/or enhancing the attenuation measured in some 2D sonic crystals ~SC! previously studied by us @8#. In this Rapid Communication we show that this mechanism is also effective in SC, being the cause for such SBG enlargement a combination of two mechanism: ~i! the achievement of higher filling fractions, and ~ii! the symmetry reduction. Also, here we demonstrate that mechanism ~ii! is much more effective than mechanism ~i!. The wave equation for the propagation of pressure waves p(r), with harmonic frequency, v, in a 2D-space r5(x, y) defined by a composite system having a sound velocity, v(r), and density, r(r), can be written as

Arabidopsis thaliana AtHAL3: a flavoprotein related to salt and osmotic tolerance and plant growth

Abstract: We have isolated two Arabidopsis thaliana genes, AtHAL3a and AtHAL3b, showing homology with HAL3, a yeast protein which regulates the cell cycle and tolerance to salt stress through inhibition of the PPZ1 type-1 protein phosphatase. Expression of AtHAL3a in yeast hal3 mutants partially complements their LiCl sensitivity, suggesting possible conserved functions between both proteins. AtHAL3a and AtHAL3b are induced by salt stress and AtHAL3a is the most expressed in non-stressed plants, particularly in seeds. In situ hybridization demonstrates enrichment of AtHAL3a mRNA in seed embryos and in the vascular phloem of different plant tissues. AtHAL3 proteins show striking homology with a group of proteins found in fungi, plants and animals and some homology with a large family of prokaryotic flavoproteins. Recombinant AtHAL3a protein purified from Escherichia coli was yellow because it contained a non-covalently bound chromophore revealed as flavin mononucleotide. Trans- genic Arabidopsis plants, with gain of AtHAL3a function, show altered growth rates and improved tolerance to salt and osmotic stress.

Reactivity in the removal of SO2 and NOx on Co/Mg/Al mixed oxides derived from hydrotalcites

Abstract: Metal containing hydrotalcites, where metal oxides present redox properties and hydrotalcite shows a basic character, appear to be new important environmental catalysts for the removal of SOx and NOx. Redox and basic properties of a mixed Co/Mg/Al oxide derived from hydrotalcites are tuned in order to achieve the optimal catalytic behavior required. This sample has been characterized showing that cobalt is present in two forms, as isolated and well dispersed paramagnetic ions, and as very small Co-containing particles (in the nanometric range), with an internal antiferromagnetic ordering at low temperature. The redox properties of cobalt allow the reduction of NO with propane at high temperatures and in presence of oxygen. The reduced cobalt species are proposed as the active sites. Nevertheless, for the removal of SO2 and contrary to the case of Cu/Mg/Al samples, the addition of an oxidant as cerium oxide on Co/Mg/Al is necessary in order to oxidize SO2 to SO3. In this case, similar results than those obtained with previously reported catalyst, i.e. cerium or copper–cerium hydrotalcite, are obtained. These results indicate that this catalyst could be an adequate material for the simultaneous removal of SO2 and NOx in a FCC unit.

Pipeline start-up with entrapped air

Abstract: A mathematical model for the assessment of the pressure head maxima that air pockets within a pipeline can originate on start-up is presented. This model is based on a general model addressing the ...

Cu2+-phthalocyanine and Co2+-perfluorophthalocyanine incorporated inside Y faujasite and mesoporous MCM-41 as heterogeneous catalysts for the oxidation of cyclohexane

Abstract: A series of Cu 2+ -phthalocyanine and Co 2+ -perfluorophthalocyanine has been prepared inside the pores of MCM-41 (MPcMCM-41) by synthesis from o -dicyanobenzene. The complexes were characterized by comparison of the UV–Vis diffuse reflectance and IR spectra with that of the same unsupported complexes. Transition metal ion exchange levels higher than 40% were found necessary for the success of the synthesis. o -Cyanobenzoic acid and phthalic anhydride were isolated as byproducts. Differential scanning calorimetry indicates that the complexes are more stabilized inside MCM-41 than within the supercages of Y faujasite. This has been interpreted as a reflection of the different conformations (planar for MCM-41 and distorted for Y faujasite) depending on the pore size of the host. The catalytic performance of novel MPcMCM-41 for cyclohexane oxidation was compared with that of the same unsupported complexes or when encaged within Y faujasite. tert -Butyl hydroperoxide was found as a more convenient oxidizing reagent than hydrogen peroxide. No leaching of the MPc or uncomplexed metal ions was observed. It was found that CoF 16 PcMCM-41 was more active and selective than CoF 16 PcY or unsupported CoF 16 Pc. However, in the case of the CuPc complex, the highest activity and selectivity was achieved using CuPcY. Our work shows the influence that the void dimensions of the porous host can play in the performance of metallophthalocyanines as oxidation catalysts.

Bragg diffraction from indium phosphide infilled fcc silica colloidal crystals

Abstract: Here we present results on Bragg diffraction in the optical range from indium phosphide infilled solid face-centered-cubic silica colloidal crystals. Scanning electron microscopy and optical properties indicate that the semiconductor is homogeneously grown within the three-dimensional lattice of voids in the colloidal crystal. The photonic-crystal behavior of the periodic dielectric is enhanced as a result of the high contrast of dielectric constants when InP is introduced.