10,818 research outputs found
The chloroplast of chlamydomonas reinhardtii as a testbed for engineering nitrogen fixation into plants
Eukaryotic organisms such as plants are unable to utilise nitrogen gas (N2) directly as a source of this essential element and are dependent either on its biological conversion to ammonium by diazotrophic prokaryotes, or its supply as chemically synthesised nitrate fertiliser. The idea of genetically engineering crops with the capacity to fix N2 by introduction of the bacterial nitrogenase enzyme has long been discussed. However, the expression of an active nitrogenase must overcome several major challenges: the coordinated expression of multiple genes to assemble an enzyme complex containing several different metal cluster co-factors; the supply of sufficient ATP and reductant to the enzyme; the enzyme’s sensitivity to oxygen; and the intracellular accumulation of ammonium. The chloroplast of plant cells represents an attractive location for nitrogenase expression, but engineering the organelle’s genome is not yet feasible in most crop species. However, the unicellular green alga Chlamydomonas reinhardtii represents a simple model for photosynthetic eukaryotes with a genetically tractable chloroplast. In this review, we discuss the main advantages, and limitations, of this microalga as a testbed for producing such a complex multi-subunit enzyme. Furthermore, we suggest that a minimal set of six transgenes are necessary for chloroplast-localised synthesis of an ‘Fe-only’ nitrogenase, and from this set we demonstrate the stable expression and accumulation of the homocitrate synthase, NifV, under aerobic conditions. Arguably, further studies in C. reinhardtii aimed at testing expression and function of the full gene set would provide the groundwork for a concerted future effort to create nitrogen-fixing crops
Optimal streaks in a Falkner-Skan boundary layer
This paper deals with the optimal streaky perturbations (which maximize the
perturbed energy growth) in a wedge flow boundary layer. These three
dimensional perturbations are governed by a system of linearized boundary layer
equations around the Falkner-Skan base flow. Based on an asymptotic analysis of
this system near the free stream and the leading edge singularity, we show that
for acute wedge semi-angle, all solutions converge after a streamwise transient
to a single streamwise-growing solution of the linearized equations, whose
initial condition near the leading edge is given by an eigenvalue problem first
formulated in this context by Tumin (2001). Such a solution may be regarded as
a streamwise evolving most unstable streaky mode, in analogy with the usual
eigenmodes in strictly parallel flows, and shows an approximate
self-similarity, which was partially known and is completed in this paper. An
important consequence of this result is that the optimization procedure based
on the adjoint equations heretofore used to define optimal streaks is not
necessary. Instead, a simple low-dimensional optimization process is proposed
and used to obtain optimal streaks. Comparison with previous results by Tumin
and Ashpis (2003) shows an excellent agreement. The unstable streaky mode
exhibits transient growth if the wedge semi-angle is smaller than a critical
value that is slightly larger than , and decays otherwise. Thus the
cases of right and obtuse wedge semi-angles exhibit less practical interest,
but they show a qualitatively different behavior, which is briefly described to
complete the analysis
NMR quantum simulation of localization effects induced by decoherence
The loss of coherence in quantum mechanical superposition states limits the
time for which quantum information remains useful. Similarly, it limits the
distance over which quantum information can be transmitted, resembling Anderson
localization, where disorder causes quantum mechanical states to become
localized. Here, we investigate in a nuclear spin-based quantum simulator, the
localization of the size of spin clusters that are generated by a Hamiltonian
driving the transmission of information, while a variable-strength perturbation
counteracts the spreading. We find that the system reaches a dynamic
equilibrium size, which decreases with the square of the perturbation strength.Comment: 5 pages, 5 figure
Amplificación sísmica: Una aproximación por Elementos Finitos
En las páginas que siguen se presenta el estudio de uno de los más típicos problemas de dinámica estructural, cual es la obtención de la respuesta de una estructura excitada por un movimiento de la base. Este es un caso muy frecuente en ingeniería sísmica, donde el objeto del estudio puede ser el edificio (sometido a un movimiento en la cimentación) o un estrato de terreno sobre fondo rígido. Al objeto de facilitar un soporte intuitivo a la exposicióri, ésta
se organiza en base al segundo de los casos citados (estrate en base rígida). La aproximación escogida, elementos finitos, pone de relieve una vez más la potencia y generalidad del método en lo que respecta a la formulación del sistema de equilibrio. La discusión se centra en un aspecto concreto del método: la elección de funciones de forma.The goal of this paper is to present the analysis of one of the most typical problems in dynamics: the response of a structure excited by a rigid-base motion. This is an usual case in seismic engineering, where the structure can be a building or a soil stratum lying on a rigid bed. In both cases the model will be the same but, in order to give a physical support to exposure, the later will be treated. The choosed approach, by finite elements, points up the power and
the generality of that method with respect to equilibrium formulation. Emphasis is done over the choosing of shape functions.Peer Reviewe
Time-dependent quantum scattering in 2+1 dimensional gravity
The propagation of a localized wave packet in the conical space-time created
by a pointlike massive source in 2+1 dimensional gravity is analyzed. The
scattering amplitude is determined and shown to be finite along the classical
scattering directions due to interference between the scattered and the
transmitted wave functions. The analogy with diffraction theory is emphasized.Comment: 15 pages in LaTeX with 3 PostScript figure
On balancedness and D-Completeness of the space of Semi-lipschitz functions
Let (X, d) be a quasi-metric space and (Y, q) be a quasi-normed
linear space. We show that the normed cone of semi-Lipschitz functions from
(X, d) to (Y, q) that vanish at a point x0 E X, is balanced. Moreover, it is complete
in the sense of D. Doitchinov whenever (Y, q) is a biBanach spac
Recurrence of the blue wing enhancements in the high ionization lines of SDSS 1004+4112 A
We present integral field spectroscopic observations of the quadruple-lensed
QSO SDSS 1004+4112 taken with the fiber system INTEGRAL at the William Herschel
Telescope on 2004 January 19. In May 2003 a blueward enhancement in the high
ionization lines of SDSS 1004+4112A was detected and then faded. Our
observations are the first to note a second event of similar characteristics
less than one year after. Although initially attributed to microlensing, the
resemblance among the spectra of both events and the absence of
microlensing-induced changes in the continuum of component A are puzzling. The
lack of a convincing explanation under the microlensing or intrinsic
variability hypotheses makes the observed enhancements particularly relevant,
calling for close monitoring of this object.Comment: 4 pages, 5 figure
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