584 research outputs found
Classification of a Haemophilus influenzae ABC Transporter HI1470/71 through Its Cognate Molybdate Periplasmic Binding Protein, MoIA
molA (HI1472) from H. influenzae encodes a periplasmic binding protein (PBP) that delivers substrate to the ABC transporter MolB_2C_2 (formerly HI1470/71). The structures of MolA with molybdate and tungstate in the binding pocket were solved to 1.6 and 1.7 Å resolution, respectively. The MolA-binding protein binds molybdate and tungstate, but not other oxyanions such as sulfate and phosphate, making it the first class III molybdate-binding protein structurally solved. The ~100 μM binding affinity for tungstate and molybdate is significantly lower than observed for the class II ModA molybdate-binding proteins that have nanomolar to low micromolar affinity for molybdate. The presence of two molybdate loci in H. influenzae suggests multiple transport systems for one substrate, with molABC constituting a low-affinity molybdate locus
An Overview of the Methylxanthines and their Regulation in the Horse
Caffiene, theophylline and theobromine are naturally occurring members of the methylxanthine family;pentoxfylline, dyphylline and enprofylline are structurally related synthetic pharmaceuticals. Caffiene has predominantly central nervous system effects, theophylline, dyphylline and enprofylline have predominantly bronchodilator effects, while theobromine is associated with diuretic responses. Pentoxfylline is thought to increase red cell deformability and facillitate blood flow through capillary beds. The methylxanthines are not highly potent agents; they are typically administered in gram doses and they tend to have relatively long half-lives. They remain detectable in plasma and urine for relatively long periods. Similarly, traces of the naturally occurring members of this family are not uncommonly identified in forensic samples. In this review we report on the detection, actions, uses and regulatory control of this group of agents in performance horses
Crystallization and X-ray Structure Determination of Cytochrome c_2 from Rhodobacter sphaeroides in Three Crystal Forms
Cytochrome c_2 serves as the secondary electron donor that reduces the photo-oxidized bacteriochlorophyll dimer in photosynthetic bacteria. Cytochrome c_2 from Rhodobacter sphaeroides has been crystallized in three different forms. At high ionic strength, crystals of a hexagonal space group (P6_122) were obtained, while at low ionic strength, triclinic (P1) and tetragonal (P4_12_12) crystals were formed. The three-dimensional structures of the cytochrome in all three crystal forms have been determined by X-ray diffraction at resolutions of 2.20 Å (hexagonal), 1.95 Å, (triclinic) and 1.53 Å (tetragonal). The most significant difference observed was the binding of an imidazole molecule to the iron atom of the heme group in the hexagonal structure. This binding displaces the sulfur atom of Met 100, which forms the axial ligand in the triclinic and tetragonal structures
Role of tyrosine M210 in the initial charge separation of reaction centers of Rhodobacter sphaeroides
Femtosecond spectroscopy was used in combination with site-directed mutagenesis to study the
influence of tyrosine M210 (YM210) on the primary electron transfer in the reaction center of Rhodobacter
sphaeroides. The exchange of YM210 to phenylalanine caused the time constant of primary electron transfer
to increase from 3.5 f 0.4 ps to 16 f 6 ps while the exchange to leucine increased the time constant even
more to 22 f 8 ps. The results suggest that tyrosine M210 is important for the fast rate of the primary
electron transfer
Cosmic Strings in an Open Universe with Baryonic and Non-Baryonic Dark Matter
We study the effects of cosmic strings on structure formation in open
universes. We calculate the power spectrum of density perturbations for two
class of models: one in which all the dark matter is non baryonic (CDM) and one
in which it is all baryonic (BDM). Our results are compared to the 1 in 6 IRAS
QDOT power spectrum. The best candidates are then used to estimate , the
energy per unit length of the string network. Some comments are made on
mechanisms by which structures are formed in the two theories.Comment: uu-encoded compressed tar of postscript files, Imperial/TP/94-95/0
Elevation change, mass balance, dynamics and surging of Langjökull, Iceland from 1997 to 2007
ABSTRACTGlaciers and ice caps around the world are changing quickly, with surge-type behaviour superimposed upon climatic forcing. Here, we study Iceland's second largest ice cap, Langjökull, which has both surge- and non-surge-type outlets. By differencing elevation change with surface mass balance, we estimate the contribution of ice dynamics to elevation change. We use DEMs, in situ stake measurements, regional reanalyses and a mass-balance model to calculate the vertical ice velocity. Thus, we not only compare the geodetic, modelled and glaciological mass balances, but also map spatial variations in glacier dynamics. Maps of emergence and submergence velocity successfully highlight the 1998 surge and subsequent quiescence of one of Langjökull's outlets by visualizing both source and sink areas. In addition to observing the extent of traditional surge behaviour (i.e. mass transfer from the accumulation area to the ablation area followed by recharge of the source area), we see peripheral areas where the surge impinged upon an adjacent ridge and subsequently retreated. While mass balances are largely in good agreement, discrepancies between modelled and geodetic mass balance may be explained by inaccurate estimates of precipitation, saturated adiabatic lapse rate or degree-day factors. Nevertheless, the study was ultimately able to investigate dynamic surge behaviour in the absence of in situ measurements during the surge.In situ mass balance survey is a joint effort of the Glaciology Group, Institute of Earth Sciences, University of Iceland and the National Power Company (Landsvirkjun). We thank Philippe Crochet and Tómas Jóhannesson from the Icelandic Meteorological Office for providing the gridded climate data and for useful discussions about the climatology of Langjökull. The 2007 lidar data were collected by the UK Natural Environment Research Council Airborne Research and Survey Facility (Grant IPY 07-08). Additional funding was provided by the United States National Science Foundation (Grant No. DGE-1038596), St Catharine’s, St John’s and Trinity Colleges and the University of Cambridge B.B. Roberts and Scandinavian Studies Funds. We thank Cameron Rye for initial help coding the mass balance model.This is the author accepted manuscript. The final version is available from Cambridge University Press via https://doi.org/10.1017/jog.2016.5
Elevation Change, Mass Balance, Dynamics, and Surging of Langjökull, Iceland from 1997 to 2007
Glaciers and ice caps around the world are changing quickly, with surge-type behaviour superimposed upon climatic forcing. Here, we study Iceland’s second largest ice cap, Langjökull, which has both surge- and non-surge-type outlets. By differencing elevation change with surface mass balance, we estimate the contribution of ice dynamics to elevation change. We use DEMs, in situ stake measurements, regional reanalyses, and a mass balance model to calculate the vertical ice velocity. Thus, we not only compare the geodetic, modelled, and glaciological mass balances, but also map spatial variations in glacier dynamics. Maps of emergence and submergence velocity successfully highlight the 1998 surge and subsequent quiescence of one of Langjökull’s outlets by visualizing both source and sink areas. In addition to observing the extent of traditional surge behaviour (i.e., mass transfer from the accumulation area to the ablation area followed by recharge of the source area), we see peripheral areas where the surge impinged upon an adjacent ridge and subsequently retreated. While mass balances are largely in good agreement, discrepancies between modelled and geodetic mass balance may be explained by inaccurate estimates of precipitation, saturated adiabatic lapse rate, or degree day factors. Nevertheless, the study was ultimately able to investigate dynamic surge behaviour in the absence of in situ measurements during the surge.In situ mass balance survey is a joint effort of the Glaciology Group, Institute of Earth Sciences, University of Iceland and the National Power Company (Landsvirkjun). We thank Philippe Crochet and Tómas Jóhannesson from the Icelandic Meteorological Office for providing the gridded climate data and for useful discussions about the climatology of Langjökull. The 2007 lidar data were collected by the UK Natural Environment Research Council Airborne Research and Survey Facility (Grant IPY 07-08). Additional funding was provided by the United States National Science Foundation (Grant No. DGE-1038596), St Catharine’s, St John’s and Trinity Colleges and the University of Cambridge B.B. Roberts and Scandinavian Studies Funds. We thank Cameron Rye for initial help coding the mass balance model.This is the author accepted manuscript. The final version is available from Cambridge University Press via https://doi.org/10.1017/jog.2016.5
Milagro Constraints on Very High Energy Emission from Short Duration Gamma-Ray Bursts
Recent rapid localizations of short, hard gamma-ray bursts (GRBs) by the
Swift and HETE satellites have led to the observation of the first afterglows
and the measurement of the first redshifts from this type of burst. Detection
of >100 GeV counterparts would place powerful constraints on GRB mechanisms.
Seventeen short duration (< 5 s) GRBs detected by satellites occurred within
the field of view of the Milagro gamma-ray observatory between 2000 January and
2006 December. We have searched the Milagro data for >100 GeV counterparts to
these GRBs and find no significant emission correlated with these bursts. Due
to the absorption of high-energy gamma rays by the extragalactic background
light (EBL), detections are only expected for redshifts less than ~0.5. While
most long duration GRBs occur at redshifts higher than 0.5, the opposite is
thought to be true of short GRBs. Lack of a detected VHE signal thus allows
setting meaningful fluence limits. One GRB in the sample (050509b) has a likely
association with a galaxy at a redshift of 0.225, while another (051103) has
been tentatively linked to the nearby galaxy M81. Fluence limits are corrected
for EBL absorption, either using the known measured redshift, or computing the
corresponding absorption for a redshift of 0.1 and 0.5, as well as for the case
of z=0.Comment: Accepted for publication in the Astrophysical Journa
The role of tyrosine M210 in the initial charge separation in the reaction center of Rhodobacter sphaeroides
The distribution of satellites around massive galaxies at 1<z<3 in ZFOURGE/CANDELS: dependence on star formation activity
We study the statistical distribution of satellites around star-forming and
quiescent central galaxies at 1<z<3 using imaging from the FourStar Galaxy
Evolution Survey (ZFOURGE) and the Cosmic Assembly Near-IR Deep Extragalactic
Legacy Survey (CANDELS). The deep near-IR data select satellites down to
at z<3. The radial satellite distribution around centrals
is consistent with a projected NFW profile. Massive quiescent centrals,
, have 2 times the number of satellites compared
to star-forming centrals with a significance of 2.7 even after
accounting for differences in the centrals' stellar-mass distributions. We find
no statistical difference in the satellite distributions of intermediate-mass
quiescent and star-forming centrals, . Comparing
to the Guo2011 semi-analytic model, the excess number of satellites indicates
that quiescent centrals have halo masses 0.3 dex larger than star-forming
centrals, even when the stellar-mass distributions are fixed. We use a simple
toy model that relates halo mass and quenching, which roughly reproduces the
observed quenched fractions and the differences in halo mass between
star-forming and quenched galaxies only if galaxies have a quenching
probability that increases with halo mass from 0 for
11 to 1 for 13.5. A single
halo-mass quenching threshold is unable to reproduce the quiescent fraction and
satellite distribution of centrals. Therefore, while halo quenching may be an
important mechanism, it is unlikely to be the only factor driving quenching. It
remains unclear why a high fraction of centrals remain star-forming even in
relatively massive halos.Comment: 19 pages, 17 figures, accepted by ApJ. Information on ZFOURGE can be
found at http://zfourge.tamu.ed
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