2,774 research outputs found
A Mineralized Alga and Acritarch Dominated Microbiota from the Tully Formation (Givetian) of Pennsylvania, USA
Sphaeromorphic algal cysts, most probably of the prasinophyte Tasmanites, and acanthomorphic acritarch vesicles, most probably Solisphaeridium, occur in a single 20 cm thick bed of micritic limestone in the lower part of the Middle Devonian (Givetian) Tully Formation near Lock Haven, Pennsylvania. Specimens are composed of authigenic calcite and pyrite crystals about 5–10 µm in length. Some specimens are completely calcitic; some contain both pyrite and calcite; and many are composed totally of pyrite. The microfossils are about 80 to 150 µm in diameter. Many show signs of originally containing a flexible wall composed of at least two layers. Some appear to have been enclosed in a mucilaginous sheath or membrane when alive. The acanthomorphic forms have spines that are up to 20 µm in length, expand toward the base, and are circular in cross-section. The microflora occurs with microscopic molluscs, dacryoconarids, the enigmatic Jinonicella, and the oldest zooecia of ctenostome bryozoans known from North America. The microalgal horizon lacks macrofossils although small burrows are present. Microalgae and acritarchs have been preserved via a complex preservational process involving rapid, bacterially-mediated post-mortem mineralization of dead cells. The microfossil horizon, and possibly much of the Tully Formation at Lock Haven with similar lithology, formed in a relatively deep, off-shore basin with reduced oxygen availability in the substrate
THE EARLY ANTIPROTON WORK [Nobel Lecture]
Early work on the antiproton, particularly that part which led to the first paper on the subject, is described. Conclusions that can be drawn purely from the existence of the antiproton are discussed. (W.D.M.
Food of Bobcats and Coyotes from Cumberland Island, Camden County, Georgia
Fifty scats of bobcats and 105 scats of coyotes from Cumberland Island, Camden County, Georgia, were examined during this study. Major foods of bobcats were mammals (81.8% volume), followed by birds (13.8%) and vegetation (4.2%), with only 0.2% invertebrates. Major foods of coyotes were plant materials (46.6% volume), mammals (43.8%), and invertebrates (6.0%). This is apparently the first report of coyote foods from Cumberland Island
Constraints on the Intergalactic Transport of Cosmic Rays
Motivated by recent experimental proposals to search for extragalactic cosmic
rays (including anti-matter from distant galaxies), we study particle
propagation through the intergalactic medium (IGM). We first use estimates of
the magnetic field strength between galaxies to constrain the mean free path
for diffusion of particles through the IGM. We then develop a simple analytic
model to describe the diffusion of cosmic rays. Given the current age of
galaxies, our results indicate that, in reasonable models, a completely
negligible number of particles can enter our Galaxy from distances greater than
Mpc for relatively low energies ( GeV/n). We also find
that particle destruction in galaxies along the diffusion path produces an
exponential suppression of the possible flux of extragalactic cosmic rays.
Finally, we use gamma ray constraints to argue that the distance to any
hypothetical domains of anti-matter must be roughly comparable to the horizon
scale.Comment: 24 pages, AAS LaTex, 1 figure, accepted to Ap
The linker domain of the SNARE protein SNAP25 acts as a flexible molecular spacer that ensures efficient S-acylation
S-Acylation of the SNARE protein SNAP25 (synaptosomeassociated protein of 25 kDa) is mediated by a subset of Golgi zinc finger DHHC-type palmitoyltransferase (zDHHC) enzymes, particularly zDHHC17. The ankyrin repeat domain of zDHHC17 interacts with a short linear motif known as the zDHHC ankyrin repeat- binding motif (zDABM) in SNAP25 ( 112VVASQP 117), which is downstream of its S-acylated, cysteine-rich domain ( 85CGLCVCPC 92). Here, we investigated the importance of a flexible linker region (amino acids 93-111, referred to hereafter as the “mini-linker” region) that separates the zDABM and S-acylated cysteines in SNAP25. Shortening the mini-linker did not affect the SNAP25-zDHHC17 interaction but blocked S-acylation. Insertion of additional flexible glycine-serine repeats had no effect on S-acylation, but extended and rigid alanine-proline repeats perturbed it. A SNAP25 mutant in which the mini-linker region was substituted with a flexible glycine-serine linker of the same length underwent efficient S-acylation. Furthermore, this mutant displayed the same intracellular localization as WT SNAP25, indicating that the amino acid composition of the mini-linker is not important for SNAP25 localization. Using the results of previous peptide array experiments, we generated a SNAP25 mutant predicted to have a higher-affinity zDABM. This mutant interacted with zDHHC17 more strongly but was S-acylated with reduced efficiency in HEK293T cells, implying that a lower-affinity interaction of the SNAP25 zDABM with zDHHC17 is optimal for S-acylation efficiency. These results show that amino acids 93-111 in SNAP25 act as a flexible molecular spacer that ensures efficient coupling of the SNAP25-zDHHC17 interaction and S-acylation of SNAP25
Nulling interferometry: performance comparison between Antarctica and other ground-based sites
Detecting the presence of circumstellar dust around nearby solar-type main
sequence stars is an important pre-requisite for the design of future
life-finding space missions such as ESA's Darwin or NASA's Terrestrial Planet
Finder (TPF). The high Antarctic plateau may provide appropriate conditions to
perform such a survey from the ground. We investigate the performance of a
nulling interferometer optimised for the detection of exozodiacal discs at Dome
C, on the high Antarctic plateau, and compare it to the expected performance of
similar instruments at temperate sites. Based on the currently available
measurements of the turbulence characteristics at Dome C, we adapt the GENIEsim
software (Absil et al. 2006, A&A 448) to simulate the performance of a nulling
interferometer on the high Antarctic plateau. To feed a realistic instrumental
configuration into the simulator, we propose a conceptual design for ALADDIN,
the Antarctic L-band Astrophysics Discovery Demonstrator for Interferometric
Nulling. We assume that this instrument can be placed above the 30-m high
boundary layer, where most of the atmospheric turbulence originates. We show
that an optimised nulling interferometer operating on a pair of 1-m class
telescopes located 30 m above the ground could achieve a better sensitivity
than a similar instrument working with two 8-m class telescopes at a temperate
site such as Cerro Paranal. The detection of circumstellar discs about 20 times
as dense as our local zodiacal cloud seems within reach for typical Darwin/TPF
targets in a integration time of a few hours. Moreover, the exceptional
turbulence conditions significantly relax the requirements on real-time control
loops, which has favourable consequences on the feasibility of the nulling
instrument.Comment: 10 pages, accepted for publication in A&
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