5,641 research outputs found
Iran, Sanctions, and Nuclear Proliferation: In Search of a Strategic Alternative
The complex political landscape of Iran is often excluded from political discourse, contributing to oversimplified, at times incoherent policy approaches that reflect fundamental misunderstandings and typically undermine rather than complement U.S. nonproliferation objectives. Domestic receptivity to international inducements is conditioned by specific characteristics of the domestic political environment. Economic sanctions have distributional effects that weaken moderate factions needed to pressure the hardline constituencies of the regime. Coercive instruments have strangled Iranian civil society, the private sector and the middle-class, severing crucial state-society networks, leaving reformist forces vulnerable to the new wave of hardline conservatism that has, in spite of U.S. pressure, gained control of the state apparatus since 2005. If external pressure solidifies the radical faction\u27s political hold on power while weakening their moderate competitors, the state will be less likely to embark on a course of denuclearization
Multiple mRNA isoforms of the transcription activator protein CREB
We have characterized cDNA clones representing mouse
CREB (cyclic AMP responsive element binding protein)
mRNA isoforms. These include CREBA and CREBa, of
which the rat and human homologues have been
previously identified. Both encode proteins with CREbinding
activity and identical transactivation potential.
The additional CREB mRNA isoforms potentially encode
CREB related proteins. From the structural organization
of the mouse CREB gene we conclude that the multiple
transcripts are generated by alternative splicing. Furthermore
we show that specific CREB mRNA isoforms are
expressed at a high level in the adult testis. Expression
of these isoforms is induced after commencement of
spermatogenesis. In situ hybridization suggests that this
expression occurs predominantly in the primary spermatocytes.
Comparison of the CREB gene with the recently
isolated CREM (cAMP responsive element modulator)
cDNAs illustrates that the two genes have arisen by gene
duplication and have diverged to encode transcriptional
activators and repressors of the cAMP signal transduction
pathway
Rational combinations of Betti diagrams of complete intersections
We investigate decompositions of Betti diagrams over a polynomial ring within
the framework of Boij-S\"oderberg theory. That is, given a Betti diagram, we
determine if it is possible to decompose it into the Betti diagrams of complete
intersections. To do so, we determine the extremal rays of the cone generated
by the diagrams of complete intersections and provide a rudimentary algorithm
for decomposition.Comment: This research was conducted at the Willamette Mathematics Consortium
RE
Using the Ca II Triplet to Trace Abundance Variations in Individual Red Giant Branch stars in Three Nearby Galaxies
Spectroscopic abundance determinations for stars spanning a Hubble time in
age are necessary in order to unambiguously determine the evolutionary
histories of galaxies. Using FORS1 in Multi-Object Spectroscopy mode on ANTU
(UT1) at the ESO-VLT on Paranal we obtained near infrared spectra from which we
measured the equivalent widths of the two strongest Ca II triplet lines to
determine metal abundances for a sample of Red Giant Branch stars, selected
from ESO-NTT optical (I, V-I) photometry of three nearby, Local Group,
galaxies: the Sculptor Dwarf Spheroidal, the Fornax Dwarf Spheroidal and the
Dwarf Irregular NGC 6822. The summed equivalent width of the two strongest
lines in the Ca II triplet absorption line feature, centered at 8500A, can be
readily converted into an [Fe/H] abundance using the previously established
calibrations by Armandroff & Da Costa (1991) and Rutledge, Hesser & Stetson
(1997). We measured metallicities for 37 stars in Sculptor, 32 stars in Fornax,
and 23 stars in NGC 6822, yielding more precise estimates of the metallicity
distribution functions for these galaxies than it is possible to obtain
photometrically. In the case of NGC 6822, this is the first direct measurement
of the abundances of the intermediate-age and old stellar populations. We find
metallicity spreads in each galaxy which are broadly consistent with the
photometric width of the Red Giant Branch, although the abundances of
individual stars do not always appear to correspond to their colour. This is
almost certainly predominantly due to a highly variable star formation rate
with time in these galaxies, which results in a non-uniform,
non-globular-cluster-like, evolution of the Ca/Fe ratio.Comment: Accepted for publication in MNRA
Formation and evolution of galaxy dark matter halos and their substructure
We use the ``Via Lactea'' simulation to study the co-evolution of a Milky
Way-size LambdaCDM halo and its subhalo population. While most of the host halo
mass is accreted over the first 6 Gyr in a series of major mergers, the
physical mass distribution [not M_vir(z)] remains practically constant since
z=1. The same is true in a large sample of LambdaCDM galaxy halos. Subhalo mass
loss peaks between the turnaround and virialization epochs of a given mass
shell, and declines afterwards. 97% of the z=1 subhalos have a surviving bound
remnant at the present epoch. The retained mass fraction is larger for
initially lighter subhalos: satellites with maximum circular velocities Vmax=10
km/s at z=1 have today about 40% of their mass back then. At the first
pericenter passage a larger average mass fraction is lost than during each
following orbit. Tides remove mass in substructure from the outside in, leading
to higher concentrations compared to field halos of the same mass. This effect,
combined with the earlier formation epoch of the inner satellites, results in
strongly increasing subhalo concentrations towards the Galactic center. We
present individual evolutionary tracks and present-day properties of the likely
hosts of the dwarf satellites around the Milky Way. The formation histories of
``field halos'' that lie today beyond the Via Lactea host are found to strongly
depend on the density of their environment. This is caused by tidal mass loss
that affects many field halos on eccentric orbits.Comment: 20 pages, 18 figures. Figures 6,7 and 8 corrected in this version,
for details see the erratum in ApJ 679, 1680 and
http://www.ucolick.org/~diemand/vl/publ/vlevolerr.pdf. Data, movies and
images are available at http://www.ucolick.org/~diemand/vl
In situ propellant production: Alternatives for Mars exploration
Current planning for the Space Exploration Initiative (SEI) recognizes the need for extraterrestrial resources to sustain long-term human presence and to attain some degree of self-sufficiency. As a practical matter, reducing the need to carry large supplies of propellant from Earth will make space exploration more economical. For nearly every round trip planned with conventional propulsion, the actual payload is only a small fraction - perhaps 10-15 percent - of the mass launched from Earth. The objective of this study was to analyze the potential application for SEI missions of propellants made exclusively from lunar or martian resources. Using such propellants could minimize or eliminate the cost of carrying propellant for surface excursion vehicles and return transfers through two high-energy maneuvers: Earth launch and trans-Mars injection. Certain chemical mono- and bipropellants are candidates for this approach; they could be recovered entirely from in situ resources on the Moon and Mars, without requiring a continuing Earth-based resupply of propellant constituents (e.g., fuel to mix with a locally obtained oxidizer) and, perhaps, with minimal need to resupply consumables (e.g., reagents or catalyst for process reactions). A complete assessment of the performance potential of these propellants must include the requirements for installation, operations, maintenance, and resupply of the chemical processing facility
Evolution of the holozoan ribosome biogenesis regulon
<p>Abstract</p> <p>Background</p> <p>The ribosome biogenesis (RiBi) genes encode a highly-conserved eukaryotic set of nucleolar proteins involved in rRNA transcription, assembly, processing, and export from the nucleus. While the mode of regulation of this suite of genes has been studied in the yeast, <it>Saccharomyces cerevisiae</it>, how this gene set is coordinately regulated in the larger and more complex metazoan genomes is not understood.</p> <p>Results</p> <p>Here we present genome-wide analyses indicating that a distinct mode of RiBi regulation co-evolved with the E(CG)-binding, Myc:Max bHLH heterodimer complex in a stem-holozoan, the ancestor of both Metazoa and Choanoflagellata, the protozoan group most closely related to animals. These results show that this mode of regulation, characterized by an E(CG)-bearing core-promoter, is specific to almost all of the known genes involved in ribosome biogenesis in these genomes. Interestingly, this holozoan RiBi promoter signature is absent in nematode genomes, which have not only secondarily lost Myc but are marked by invariant cell lineages typically producing small body plans of 1000 somatic cells. Furthermore, a detailed analysis of 10 fungal genomes shows that this holozoan signature in RiBi genes is not found in hemiascomycete fungi, which evolved their own unique regulatory signature for the RiBi regulon.</p> <p>Conclusion</p> <p>These results indicate that a Myc regulon, which is activated in proliferating cells during normal development as well as during tumor progression, has primordial roots in the evolution of an inducible growth regime in a protozoan ancestor of animals. Furthermore, by comparing divergent bHLH repertoires, we conclude that regulation by Myc but not by other bHLH genes is responsible for the evolutionary maintenance of E(CG) sites across the RiBi suite of genes.</p
Dual-probe decoherence microscopy: Probing pockets of coherence in a decohering environment
We study the use of a pair of qubits as a decoherence probe of a non-trivial
environment. This dual-probe configuration is modelled by three
two-level-systems which are coupled in a chain in which the middle system
represents an environmental two-level-system (TLS). This TLS resides within the
environment of the qubits and therefore its coupling to perturbing fluctuations
(i.e. its decoherence) is assumed much stronger than the decoherence acting on
the probe qubits. We study the evolution of such a tripartite system including
the appearance of a decoherence-free state (dark state) and non-Markovian
behaviour. We find that all parameters of this TLS can be obtained from
measurements of one of the probe qubits. Furthermore we show the advantages of
two qubits in probing environments and the new dynamics imposed by a TLS which
couples to two qubits at once.Comment: 29 pages, 10 figure
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