168 research outputs found
A simple model based on known plant defence reactions is sufficient to explain most aspects of nodulation
We present the following hypothesis; that lipo-oligochitin Nod-factors can act in an elicitor-like fashion inducing, amongst other effects, a plant chitolytic enzyme, capable of hydrolysing the oligochitin chain of the Nod-factor. Decorative groups on the oligochitin chain, e.g. sulphate, may confer partial resistance to hydrolysis upon particular Nod-factors. After entry into the plant, Nod-factor synthesis must be down-regulated in order to avoid further, unwanted, eli-citation and the consequent abortion of the symbiosis. The plant-derived compounds inhibiting the synthesis of bacterial Nod-factors are limiting in root tissue, leading to residual elicitation and the abortion of infection thread formation. Nod-gene anti-induction is, furthermore, inactivated by both light and nitrate, thus contributing to the inhibition of nodulation under these conditions. In nitrogen-fixing nodules, the bacteroids are exposed to both nod-gene inducing and repressing compounds. The slow accumulation of Nod-factors within the peribacteroid space eventually results in the elicitation of phytoalexin synthesis and nodule senescenc
Structural features distinguishing infectious ex vivo mammalian prions from non-infectious fibrillar assemblies generated in vitro
Seeded polymerisation of proteins forming amyloid fibres and their spread in tissues has been implicated in the pathogenesis of multiple neurodegenerative diseases: so called "prion-like" mechanisms. While ex vivo mammalian prions, composed of multichain assemblies of misfolded host-encoded prion protein (PrP), act as lethal infectious agents, PrP amyloid fibrils produced in vitro generally do not. The high-resolution structure of authentic infectious prions and the structural basis of prion strain diversity remain unknown. Here we use cryo-electron microscopy and atomic force microscopy to examine the structure of highly infectious PrP rods isolated from mouse brain in comparison to non-infectious recombinant PrP fibrils generated in vitro. Non-infectious recombinant PrP fibrils are 10 nm wide single fibres, with a double helical repeating substructure displaying small variations in adhesive force interactions across their width. In contrast, infectious PrP rods are 20 nm wide and contain two fibres, each with a double helical repeating substructure, separated by a central gap of 8-10 nm in width. This gap contains an irregularly structured material whose adhesive force properties are strikingly different to that of the fibres, suggestive of a distinct composition. The structure of the infectious PrP rods, which cause lethal neurodegeneration, readily differentiates them from all other protein assemblies so far characterised in other neurodegenerative diseases
A Survey of z>5.7 Quasars in the Sloan Digital Sky Survey II: Discovery of Three Additional Quasars at z>6
We present the discovery of three new quasars at z>6 in 1300 deg^2 of SDSS
imaging data, J114816.64+525150.3 (z=6.43), J104845.05+463718.3 (z=6.23) and
J163033.90+401209.6 (z=6.05). The first two objects have weak Ly alpha emission
lines; their redshifts are determined from the positions of the Lyman break.
They are only accurate to 0.05 and could be affected by the presence of broad
absorption line systems. The last object has a Ly alpha strength more typical
of lower redshift quasars. Based on a sample of six quasars at z>5.7 that cover
2870 deg^2 presented in this paper and in Paper I, we estimate the comoving
density of luminous quasars at z 6 and M_{1450} < -26.8 to be (8 +/-
3)x10^{-10} Mpc^{-3} (for H_0 = 50 km/s/Mpc, Omega = 1). HST imaging of two
z>5.7 quasars and high-resolution ground-based images (seeing 0.4'') of three
additional z>5.7 quasars show that none of them is gravitationally lensed. The
luminosity distribution of the high-redshfit quasar sample suggests the bright
end slope of the quasar luminosity function at z 6 is shallower than Psi
L^{-3.5} (2-sigma), consistent with the absence of strongly lensed objects.Comment: AJ in press (Apr 2003), 26 pages, 9 figure
Probing the Origins of the CIV and Fe Kalpha Baldwin Effect
We use UV/optical and X-ray observations of 272 radio-quiet Type 1 AGNs and
quasars to investigate the CIV Baldwin Effect (BEff). The UV/optical spectra
are drawn from the Hubble Space Telescope, International Ultraviolet Explorer
and Sloan Digital Sky Survey archives. The X-ray spectra are from the Chandra
and XMM-Newton archives. We apply correlation and partial-correlation analyses
to the equivalent widths, continuum monochromatic luminosities, and alpha_ox,
which characterizes the relative X-ray to UV brightness. The equivalent width
of the CIV 1549 emission line is correlated with both alpha_ox and luminosity.
We find that by regressing l_UV with EW(CIV) and alpha_ox, we can obtain
tighter correlations than by regressing l_UV with only EW(CIV). Both
correlation and regression analyses imply that l_UV is not the only factor
controlling the changes of EW(CIV); alpha_ox (or, equivalently, the soft X-ray
emission) plays a fundamental role in the formation and variation of CIV.
Variability contributes at least 60% of the scatter of the EW(CIV)-l_UV
relation and at least 75% of the scatter of the of the EW(CIV)-alpha_ox
relation.
In our sample, narrow Fe Kalpha 6.4 keV emission lines are detected in 50
objects. Although narrow Fe Kalpha exhibits a BEff similar to that of CIV, its
equivalent width has almost no dependence on either alpha_ox or EW(CIV). This
suggests that the majority of narrow Fe Kalpha emission is unlikely to be
produced in the broad emission-line region. We do find suggestive correlations
between the emission-line luminosities of CIV and Fe Kalpha, which could be
potentially used to estimate the detectability of the Fe Kalpha line of quasars
from rest-frame UV spectroscopic observations
An Initial Survey of White Dwarfs in the Sloan Digital Sky Survey
An initial assessment is made of white dwarf and hot subdwarf stars observed
in the Sloan Digital Sky Survey. In a small area of sky (190 square degrees),
observed much like the full survey will be, 269 white dwarfs and 56 hot
subdwarfs are identified spectroscopically where only 44 white dwarfs and 5 hot
subdwarfs were known previously. Most are ordinary DA (hydrogen atmosphere) and
DB (helium) types. In addition, in the full survey to date, a number of WDs
have been found with uncommon spectral types. Among these are blue DQ stars
displaying lines of atomic carbon; red DQ stars showing molecular bands of C_2
with a wide variety of strengths; DZ stars where Ca and occasionally Mg, Na,
and/or Fe lines are detected; and magnetic WDs with a wide range of magnetic
field strengths in DA, DB, DQ, and (probably) DZ spectral types. Photometry
alone allows identification of stars hotter than 12000 K, and the density of
these stars for 15<g<20 is found to be ~2.2 deg^{-2} at Galactic latitudes
29-62 deg. Spectra are obtained for roughly half of these hot stars. The
spectra show that, for 15<g<17, 40% of hot stars are WDs and the fraction of
WDs rises to ~90% at g=20. The remainder are hot sdB and sdO stars.Comment: Accepted for AJ; 43 pages, including 12 figures and 5 table
High-Redshift SDSS Quasars with Weak Emission Lines
We identify a sample of 74 high-redshift quasars (z>3) with weak emission
lines from the Fifth Data Release of the Sloan Digital Sky Survey and present
infrared, optical, and radio observations of a subsample of four objects at
z>4. These weak emission-line quasars (WLQs) constitute a prominent tail of the
Lya+NV equivalent width distribution, and we compare them to quasars with more
typical emission-line properties and to low-redshift active galactic nuclei
with weak/absent emission lines, namely BL Lac objects. We find that WLQs
exhibit hot (T~1000 K) thermal dust emission and have rest-frame 0.1-5 micron
spectral energy distributions that are quite similar to those of normal
quasars. The variability, polarization, and radio properties of WLQs are also
different from those of BL Lacs, making continuum boosting by a relativistic
jet an unlikely physical interpretation. The most probable scenario for WLQs
involves broad-line region properties that are physically distinct from those
of normal quasars.Comment: Updated to match version published in ApJ. 20 pages, 12 figure
Iatrogenic cerebral amyloid angiopathy: an emerging clinical phenomenon
In the last 6 years, following the first pathological description of presumed amyloid-beta (Aβ) transmission in humans (in 2015) and subsequent experimental confirmation (in 2018), clinical cases of iatrogenic cerebral amyloid angiopathy (CAA)—attributed to the transmission of Aβ seeds—have been increasingly recognised and reported. This newly described form of CAA is associated with early disease onset (typically in the third to fifth decade), and often presents with intracerebral haemorrhage, but also seizures and cognitive impairment. Although assumed to be rare, it is important that clinicians remain vigilant for potential cases, particularly as the optimal management, prognosis, true incidence and public health implications remain unknown. This review summarises our current understanding of the clinical spectrum of iatrogenic CAA and provides a diagnostic framework for clinicians. We provide clinical details for three patients with pathological evidence of iatrogenic CAA and present a summary of the published cases to date (n=20), identified following a systematic review. Our aims are: (1) To describe the clinical features of iatrogenic CAA, highlighting important similarities and differences between iatrogenic and sporadic CAA; and (2) To discuss potential approaches for investigation and diagnosis, including suggested diagnostic criteria for iatrogenic CAA
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BioTIME: A database of biodiversity time series for the Anthropocene.
MotivationThe BioTIME database contains raw data on species identities and abundances in ecological assemblages through time. These data enable users to calculate temporal trends in biodiversity within and amongst assemblages using a broad range of metrics. BioTIME is being developed as a community-led open-source database of biodiversity time series. Our goal is to accelerate and facilitate quantitative analysis of temporal patterns of biodiversity in the Anthropocene.Main types of variables includedThe database contains 8,777,413 species abundance records, from assemblages consistently sampled for a minimum of 2 years, which need not necessarily be consecutive. In addition, the database contains metadata relating to sampling methodology and contextual information about each record.Spatial location and grainBioTIME is a global database of 547,161 unique sampling locations spanning the marine, freshwater and terrestrial realms. Grain size varies across datasets from 0.0000000158 km2 (158 cm2) to 100 km2 (1,000,000,000,000 cm2).Time period and grainBioTIME records span from 1874 to 2016. The minimal temporal grain across all datasets in BioTIME is a year.Major taxa and level of measurementBioTIME includes data from 44,440 species across the plant and animal kingdoms, ranging from plants, plankton and terrestrial invertebrates to small and large vertebrates.Software format.csv and .SQL
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