600 research outputs found
Optical properties of metal nanoparticles with no center of inversion symmetry: observation of volume plasmons
We present theoretical and experimental studies of the optical response of
L-shaped silver nanoparticles. The scattering spectrum exhibits several plasmon
resonances that depend sensitively on the polarization of the incident
electromagnetic field. The physical origin of the resonances is traced to
different plasmon phenomena. In particular, a high energy band with unusual
properties is interpreted in terms of volume plasmon oscillations arising from
the asymmetry of a nanoparticle.Comment: 14 pages, 5 figures. Physical Review B, 2007, accepte
Radio Continuum Emission at 1.4 GHz from KISS Emission-Line Galaxies
We have searched the Faint Images of the Radio Sky at Twenty centimeters
(FIRST) and the NRAO VLA Sky Survey (NVSS) 1.4 GHz radio surveys for sources
that are coincident with emission-line galaxy (ELG) candidates from the KPNO
International Spectroscopic Survey (KISS). A total of 207 of the 2157 KISS ELGs
(~10%) in the first two H-alpha-selected survey lists were found to possess
radio detections in FIRST and/or NVSS. Follow-up spectra exist for all of the
radio detections, allowing us to determine the activity type (star-forming vs.
AGN) for the entire sample. We explore the properties of the radio-detected
KISS galaxies in order to gain a better insight into the nature of
radio-emitting galaxies in the local universe (z < 0.1). No dwarf galaxies were
detected, despite the large numbers of low-luminosity galaxies present in KISS,
suggesting that lower mass, lower luminosity objects do not possess strong
galaxian-scale magnetic fields. Due to the selection technique used for KISS,
our radio ELGs represent a quasi-volume-limited sample, which allows us to
develop a clearer picture of the radio galaxy population at low redshift.
Nearly 2/3rds of the KISS radio galaxies are starburst/star-forming galaxies,
which is in stark contrast to the results of flux-limited radio surveys that
are dominated by AGNs and elliptical galaxies (i.e., classic radio galaxies).
While there are many AGNs among the KISS radio galaxies, there are no objects
with large radio powers in our local volume. We derive a radio luminosity
function (RLF) for the KISS ELGs that agrees very well with previous RLFs that
adequately sample the lower-luminosity radio population.Comment: Accepted for publication in the Astronomical Journal (April 2004); 23
pages, 16 figure
SMA-Causing Missense Mutations in \u3cem\u3eSurvival motor neuron (Smn)\u3c/em\u3e Display a Wide Range of Phenotypes When Modeled in \u3cem\u3eDrosophila\u3c/em\u3e
Mutations in the human survival motor neuron 1 (SMN) gene are the primary cause of spinal muscular atrophy (SMA), a devastating neuromuscular disorder. SMN protein has a well-characterized role in the biogenesis of small nuclear ribonucleoproteins (snRNPs), core components of the spliceosome. Additional tissue-specific and global functions have been ascribed to SMN; however, their relevance to SMA pathology is poorly understood and controversial. Using Drosophila as a model system, we created an allelic series of twelve Smn missense mutations, originally identified in human SMA patients. We show that animals expressing these SMA-causing mutations display a broad range of phenotypic severities, similar to the human disease. Furthermore, specific interactions with other proteins known to be important for SMN\u27s role in RNP assembly are conserved. Intragenic complementation analyses revealed that the three most severe mutations, all of which map to the YG box self-oligomerization domain of SMN, display a stronger phenotype than the null allele and behave in a dominant fashion. In support of this finding, the severe YG box mutants are defective in self-interaction assays, yet maintain their ability to heterodimerize with wild-type SMN. When expressed at high levels, wild-type SMN is able to suppress the activity of the mutant protein. These results suggest that certain SMN mutants can sequester the wild-type protein into inactive complexes. Molecular modeling of the SMN YG box dimer provides a structural basis for this dominant phenotype. These data demonstrate that important structural and functional features of the SMN YG box are conserved between vertebrates and invertebrates, emphasizing the importance of self-interaction to the proper functioning of SMN. Author Summary
Spinal Muscular Atrophy (SMA) is a prevalent childhood neuromuscular disease, which in its most common form causes death by the age of two. One in fifty Americans is a carrier for SMA, making this genetic disease a serious health concern. SMA is caused by loss of function mutations in the survival motor neuron 1 (SMN1) gene. SMN is an essential protein and has a well-characterized function in the assembly of small nuclear ribonucleoproteins (snRNPs), which are core components of the spliceosome. To elucidate the phenotypic consequences of disrupting specific SMN protein interactions, we have generated a series of SMA-causing point mutations, modeled in Drosophila melanogaster. Using this system, we have shown that key aspects of SMN structure and function are conserved between humans and flies. Intragenic complementation analyses reveal the potential for dominant negative interactions between wild-type and mutant SMN subunits, highlighting the essential nature of the YG box in formation of higher-order SMN multimers. These results provide a basis for future studies investigating therapy targeted at restoration of functional SMN oligomers
Halo Star Streams in the Solar Neighborhood
We have assembled a sample of halo stars in the solar neighborhood to look
for halo substructure in velocity and angular momentum space. Our sample
includes red giants, RR Lyrae, and red horizontal branch stars within 2.5 kpc
of the Sun with [Fe/H] less than -1.0. It was chosen to include stars with
accurate distances, space velocities, and metallicities as well as
well-quantified errors. We confirm the existence of the streams found by Helmi
and coworkers, which we refer to as the H99 streams. These streams have a
double-peaked velocity distribution in the z direction. We use the results of
modeling of the H99 streams by Helmi and collaborators to test how one might
use v_z velocity information and radial velocity information to detect
kinematic substructure in the halo. We find that detecting the H99 streams with
radial velocities alone would require a large sample. We use the velocity
distribution of the H99 streams to estimate their age. From our model of the
progenitor of the H99 streams, we determine that it was accreted between 6 and
9 Gyr ago. The H99 streams have [alpha/Fe] abundances similar to other halo
stars in the solar neighborhood, suggesting that the gas that formed these
stars were enriched mostly by Type II SNe. We have also discovered in angular
momentum space two other possible substructures, which we refer to as the
retrograde and prograde outliers. The retrograde outliers are likely to be halo
substructure, but the prograde outliers are most likely part of the smooth
halo. The retrograde outliers have significant structure in the v_phi direction
and show a range of [alpha/Fe]. The methods presented in this paper can be used
to exploit the kinematic information present in future large databases like
RAVE, SDSSII/SEGUE, and Gaia.Comment: 46 pages, 13 figures, and 9 tables. Minor changes to text to match
proofed version of the paper. Low resolution figures. High resolution version
at http://www.astro.wisc.edu/~kepley/solar_streams.p
Shedding New Light on the 3C 273 Jet with the Spitzer Space Telescope
We have performed infrared imaging of the jet of the quasar 3C 273 at
wavelengths 3.6 and 5.8 microns with the Infrared Array Camera (IRAC) on the
Spitzer Space Telescope. When combined with the radio, optical and X-ray
measurements, the IRAC photometry clearly shows that the optical emission is
dominated by the high-energy component of the jet, not by the radio synchrotron
component, as had been assumed to date. The high-energy component may be due to
a second synchrotron component or to IC scattering of ambient photons. In the
former case, we argue that the acceleration of protons exceeding 10^16 eV or
possibly even to 10^19 eV would be taking place in the jet. In contrast, the IC
model, into which highly relativistic Doppler beaming has to be incorporated,
requires very low-energy electrons (~ 1 MeV). The present polarization data in
the radio and optical would favor the former interpretation in the case of the
3C 273 jet. Sensitive and detailed measurements of optical polarization are
important to establish the radiation mechanism responsible for the high-energy
emission. The present study offers new clues as to the controversial origin of
the X-ray emission seen in many quasar jets.Comment: 12 pages, 8 figures (2 color figures), accepted for publication in
ApJ, color images are also available at
http://www.astro.isas.jaxa.jp/~uchiyama/Site2/Spitzer_3C273.htm
New and novel intrinsic host repressive factors against HIV-1: PAF1 complex, HERC5 and others
Raman spectra and calculated vibrational frequencies of size-selected C16, C18, and C20 clusters
The surface plasmon polariton-enhanced Raman spectra of size-selected C16, C18, and C20 clusters isolated in nitrogen matrices are presented along with the calculated vibrational frequencies for the ring and linear chain isomers. The Raman spectra, recorded at a range of excitation wavelengths from 457.9 to 670 nm, show strong resonance enhancement for the three clusters. The calculated vibrational frequencies for ring and linear chain isomers and the cage and bowl structures for C20 are compared to the experimental frequencies. Systematic shifts in the series of peaks in the 200 cm-1 region for C16, C18, and C20 suggest that the observed isomers have the same geometry, thereby ruling out the bowl and cage isomers for C20. The measured spectra appear to be most consistent with the linear chain isomer. This high-energy isomer may be produced during neutralization of the deposited cluster ions
SMA-Causing Missense Mutations in Survival motor neuron (Smn) Display a Wide Range of Phenotypes When Modeled in Drosophila
Mutations in the human survival motor neuron 1 (SMN) gene are the primary cause of spinal muscular atrophy (SMA), a devastating neuromuscular disorder. SMN protein has a well-characterized role in the biogenesis of small nuclear ribonucleoproteins (snRNPs), core components of the spliceosome. Additional tissue-specific and global functions have been ascribed to SMN; however, their relevance to SMA pathology is poorly understood and controversial. Using Drosophila as a model system, we created an allelic series of twelve Smn missense mutations, originally identified in human SMA patients. We show that animals expressing these SMA-causing mutations display a broad range of phenotypic severities, similar to the human disease. Furthermore, specific interactions with other proteins known to be important for SMN's role in RNP assembly are conserved. Intragenic complementation analyses revealed that the three most severe mutations, all of which map to the YG box self-oligomerization domain of SMN, display a stronger phenotype than the null allele and behave in a dominant fashion. In support of this finding, the severe YG box mutants are defective in self-interaction assays, yet maintain their ability to heterodimerize with wild-type SMN. When expressed at high levels, wild-type SMN is able to suppress the activity of the mutant protein. These results suggest that certain SMN mutants can sequester the wild-type protein into inactive complexes. Molecular modeling of the SMN YG box dimer provides a structural basis for this dominant phenotype. These data demonstrate that important structural and functional features of the SMN YG box are conserved between vertebrates and invertebrates, emphasizing the importance of self-interaction to the proper functioning of SMN
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