Evaluating Labor Market Reforms: A General Equilibrium Approach

fixed-term contracts, firing costs, general equilibrium, heterogeneous agents

Functional Characterization Of The Plant 15-Cis-Zeta-Carotene Isomerase Z-Iso

Vitamin A deficiency is a widespread health issue in the tropics. To solve this issue, efforts are underway to increase provitamin A carotenoids such as β-carotene in staple crops which can be achieved by breeding, metabolic engineering or a combination of both approaches. However, rational strategies to improve carotenoid content in crops require sufficient knowledge of pathway regulation. Therefore, to better understand how plants synthesize provitamin A and to guide metabolic engineering strategies in crops such as maize, the functional characterization of the new ζ-carotene isomerase (Z-ISO) is of significant importance. Z-ISO was recently discovered in maize and Arabidopsis (Chen et al., 2010). This new enzyme is a 15-cis-ζ-carotene isomerase present in all plants, diatoms and algae. Z-ISO is required in both green and non-green tissues including roots and the seed endosperm which is target for provitamin A biofortification. In this dissertation, to gain a better understanding of the role of Z-ISO in the isomerization of 15-cis-ζ-carotene, the Z-ISO polypeptide was biochemically characterized using extensive spectroscopy and its function was examined by developing an in vitro enzymatic assay and by an in vivo complementation system in Escherichia coli. Bioinformatic tools modeled Z-ISO as an integral membrane and heme or non-heme iron binding protein. Therefore, using the Z-ISO polypeptide sequence, we selected conserved putative residue ligands for heme and non-heme iron and mutagenized them to Alanine. These Z-ISO mutant versions were tested for enzymatic function using an E. coli complementation system. These experiments showed that from all the conserved histidines present in Z-ISO, two (H150 and H266) as well as one aspartic acid residue (D294), were essential for isomerase activity. The only cysteine (C263) residue present in Z-ISO was not required for activity. These results are in good agreement with the predicted Z-ISO model where the locations for H150 and H266 are consistent with the coordination of a common factor. Maize Z-ISO was also over-expressed and purified as a TEV protease-cleavable, maltose binding protein (MBP) fusion (MBP::Z-ISO). An in vitro assay utilizing substrate containing liposomes was developed to test Z-ISO activity. The conversion of the substrate 9,15,9\u27-tri-cis-ζ-carotene into the product 9,9\u27-di-cis-ζ-carotene by Z-ISO proceeded under reducing conditions but not under oxidizing conditions. MBP::Z-ISO purified protein was also tested for the presence of metals. Using inductively coupled plasma optical emission spectrometry (ICP-OES) iron was detected but there were no significant levels of Ca, Cu, Mg, Mn, Mo, or Zn. Therefore, we concluded that Z-ISO is a metalloprotein. E. coli culture pellets expressing MBP::Z-ISO are brown consistent with the presence of iron or heme. The presence of heme in purified MBP::Z-ISO was evaluated using heme staining and pyridine hemochrome assays. Heme staining of protein separated in SDS-PAGE gels revealed that MBP::Z-ISO and Z-ISO contain heme while MBP alone does not. Moreover, hemochrome assays showed the presence of heme b in Z-ISO. The UV-visible absorption spectrum of the intact, as isolated Z-ISO confirmed the presence of heme b in the oxidized, ferric Fe (III) state. Also, the spectrum of reduced Z-ISO is similar to that of cytochromes containing heme b with two axial histidine ligands and the reduced heme b binds carbon monoxide (CO). Z-ISO was also characterized using electron paramagnetic resonance (EPR). An X-band spectrum of the MBP::Z-ISO fusion detected high-spin ferric heme (e.g. heme b with a single histidine ligand), multiple low-spin heme species, and a non-heme iron center. EPR also indicated the presence of two low-spin heme species and that one of these hemes might have a bis-histidine coordination and the other might have a histidine-cysteine axial coordination. Reduced Z-ISO also binds NO which is consistent with the binding of CO. To gain more details on the heme cofactor, the same sample used in the EPR experiments was used to characterize the heme iron in MBP::Z-ISO using magnetic circular dichroism (MCD) which detects only heme iron. MCD characterization showed that Z-ISO has two ligand pairs (His/His and His/Cys), a result that is consistent with the EPR results. MCD experiments also showed a redox-dependent change in ligand (Cys↔His) coordination of a low-spin heme b. Our data also suggested the existence of a high-spin, 5-coordinate, His-ligated heme which was detected by EPR and as a minor species by MCD. We hypothesized that substrate could bind to this intermediate or that substrate displaces an axial ligand to coordinate with the heme iron. MCD, EPR, and UV-Vis analysis showed that exogenous ligands bind the Fe(II) state but it was not known whether an exogenous ligand can displace an axial ligand when the heme is in the Fe(III) state. To test this possibility we utilized addition of cyanide (CN-), which binds preferably to the ferric rather than the ferrous heme. CN- was added to both the as isolated (oxidized) Fe(III) ferric enzyme and to the Fe(II) (dithionite-reduced) ferrous enzyme. UV-Vis absorption spectra indicated CN- binding to the ferric Fe(III) heme which suggest that Z-ISO can also bind exogenous ligands in the oxidized form. Taken together our data suggest that heme is essential for Z-ISO activity. The presence of heme and its requirement for Z-ISO activity are surprising since heme is not often described in isomerization reactions. Based on our results, we propose a mechanism for Z-ISO function

Seasonal dynamic factor analysis and bootstrap inference : application to electricity market forecasting

Year-ahead forecasting of electricity prices is an important issue in the current context of electricity markets. Nevertheless, only one-day-ahead forecasting is commonly tackled up in previous published works. Moreover, methodology developed for the short-term does not work properly for long-term forecasting. In this paper we provide a seasonal extension of the Non-Stationary Dynamic Factor Analysis, to deal with the interesting problem (both from the economic and engineering point of view) of long term forecasting of electricity prices. Seasonal Dynamic Factor Analysis (SeaDFA) allows to deal with dimensionality reduction in vectors of time series, in such a way that extracts common and specific components. Furthermore, common factors are able to capture not only regular dynamics (stationary or not) but also seasonal one, by means of common factors following a multiplicative seasonal VARIMA(p,d,q)×(P,D,Q)s model. Besides, a bootstrap procedure is proposed to be able to make inference on all the parameters involved in the model. A bootstrap scheme developed for forecasting includes uncertainty due to parameter estimation, allowing to enhance the coverage of forecast confidence intervals. Concerning the innovative and challenging application provided, bootstrap procedure developed allows to calculate not only point forecasts but also forecasting intervals for electricity prices.Dynamic factor analysis, Bootstrap, Forecasting, Confidence intervals

Chapter Hybrid Access Techniques for Densely Populated Wireless Local Area Networks

Probability & statistic

Cooperative ARQ: A Medium Access Control (MAC) Layer Perspective

Postprint (published version

Adult Education and The Scientific Community

In this paper, we analyzed the recent developments of the AE Scientific Community in Spain. Particularly, we focus in which factors have made possible to overcome its isolated initial situation towards a high quality one. Finally, we identified current works and future trends in order to characterized its main aspects such as its dialogic approach as a result of their interdisciplinary and interinstitutional efforts

On the Impact of Shadowing on the Performance of Cooperative Medium Access Control Protocols

International audienceAccurate representation of the physical layer is instrumental for a sound design and optimization of Medium Access Control (MAC) protocols for cooperative wireless networks. However, the vast majority of MAC protocols are designed and analyzed by considering simplified physical layer and channel models, which often lead to too optimistic performance predictions. In particular, even though many experimental activities have showcased the important role played by shadow-fading, most protocols are designed and evaluated by taking into account only the transmission distance (circular coverage model) or only the fast-fading. Motivated by the proved unsuitability of these models, the contribution of this paper is threefold: i) we provide important considerations on how to adequately include the effect of shadowing into the design of MAC protocols for cooperative networks; ii) we provide an analytical framework to determine the subset of active relays in order to meet a given Quality-of-Service (QoS) requirement; and iii) we study, through analysis and simulation, the performance of a promising MAC protocol for cooperative networks, which is called Persistent Relay Carrier Sensing Multiple Access (PRCSMA), by explicitly taking into account the effect of shadowing. Our study shows that shadowing can dramatically change system and protocol performance