698 research outputs found
Lorentz Violation for Photons and Ultra-High Energy Cosmic Rays
Lorentz symmetry breaking at very high energies may lead to photon dispersion
relations of the form omega^2=k^2+xi_n k^2(k/M_Pl)^n with new terms suppressed
by a power n of the Planck mass M_Pl. We show that first and second order terms
of size xi_1 > 10^(-14) and xi_2 < -10^(-6), respectively, would lead to a
photon component in cosmic rays above 10^(19) eV that should already have been
detected, if corresponding terms for electrons and positrons are significantly
smaller. This suggests that Lorentz invariance breakings suppressed up to
second order in the Planck scale are unlikely to be phenomenologically viable
for photons.Comment: 4 revtex pages, 3 postscript figures included, version published in
PR
ApoFnr binds as a monomer to promoters regulating expression of enterotoxin genes of Bacillus cereus.
International audienceBacillus cereus Fnr is a member of the Crp/Fnr (cAMP-binding protein/fumarate nitrate reduction regulatory protein) family of helix-turn-helix transcriptional regulators. It is essential for the expression of Hbl and Nhe enterotoxin genes independently of the oxygen tension in the environment. We studied aerobic Fnr binding to target sites in promoters regulating the expression of enterotoxin genes. B. cereus Fnr was overexpressed and purified as either a C-terminal His-tagged (FnrHis) fusion protein or an N-terminal fusion protein tagged with the Strep-tag (IBA BioTAGnology) (StrepFnr). Both recombinant Fnr proteins were produced as apoforms (clusterless) and occured as mixtures of monomers and oligomers in solution. However, apoFnrHis was mainly monomeric, while apoStrepFnr was mainly oligomeric, suggesting that the His-tagged C-terminal extremity may interfere with oligomerization. The oligomeric state of apoStrepFnr was dithiothreitol sensitive, underlining the importance of a disulphide bridge for apoFnr oligomerization. Electrophoretic mobility shift assays showed that monomeric apoFnr, but not oligomeric apoFnr, bound to specific sequences located in the promoter regions of the enterotoxin regulators fnr, resDE and plcR and the structural genes hbl and nhe. The question of whether apoFnr binding is regulated in vivo by redox-dependent oligomerization is discussed
Nutrient recycling facilitates long-term stability of marine microbial phototroph–heterotroph interactions
Biological interactions underpin the functioning of marine ecosystems, be it via competition, predation, mutualism or symbiosis processes. Microbial phototroph–heterotroph interactions propel the engine that results in the biogeochemical cycling of individual elements, and they are critical for understanding and modelling global ocean processes. Unfortunately, studies thus far have focused on exponentially growing cultures in nutrient-rich media, meaning knowledge of such interactions under in situ conditions is rudimentary at best. Here, we have performed long-term phototroph–heterotroph co-culture experiments under nutrient-amended and natural seawater conditions, and show that it is not the concentration of nutrients but rather their circulation that maintains a stable interaction and a dynamic system. Using the Synechococcus–Roseobacter interaction as a model phototroph–heterotroph case study, we show that although Synechococcus is highly specialized for carrying out photosynthesis and carbon fixation, it relies on the heterotroph to remineralize the inevitably leaked organic matter, making nutrients circulate in a mutualistic system. In this sense we challenge the general belief that marine phototrophs and heterotrophs compete for the same scarce nutrients and niche space, and instead suggest that these organisms more probably benefit from each other because of their different levels of specialization and complementarity within long-term stable-state systems
Recombinant expression of the fdxD gene of Rhodobacter capsulatus and characterization of its product, a [2Fe-2S] ferredoxin
Insights into the secondary fraction of the organic aerosol in a Mediterranean urban area: Marseille
A comprehensive aerosol characterization was conducted at Marseille during
summer, including organic (OC) and elemental carbon (EC), major ionic
species, radiocarbon (<sup>14</sup>C), water-soluble OC and HULIS (HUmic LIke
Substances), elemental composition and primary and secondary organic
markers. This paper is the second paper of a two-part series that uses this
dataset to investigate the sources of Organic Aerosol (OA). While the first
paper investigates the primary sources (El Haddad et al., 2010), this second
paper focuses on the secondary fraction of the organic aerosol.
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In the context of overall OC mass balance, primary OC (POC) contributes on
average for only 22% and was dominated by vehicular emissions accounting
on average for 17% of OC. As a result, 78% of OC mass cannot be
attributed to the major primary sources and remains un-apportioned.
Radiocarbon measurements suggest that more than 70% of this fraction is
of non-fossil origin, assigned predominantly to biogenic secondary organic
carbon (BSOC). Therefore, contributions from three traditional BSOC
precursors, isoprene, -pinene and β-caryophyllene, were
considered. These were estimated using the ambient concentrations of
Secondary Organic Aerosol (SOA) markers from each precursor and
laboratory-derived marker mass fraction factors.
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Secondary organic markers derived from isoprene photo-oxidation (ie:
2-methylglyceric acid and 2-methyltetrols) do not exhibit the same temporal
trends. This variability was assigned to the influence of NO<sub>x</sub>
concentration on their formation pathways and to their potential decay by
further processing in the atmosphere. The influence of changes in isoprene
chemistry on assessment of isoprene SOC contribution was evaluated
explicitly. The results suggest a 60-fold variation between the different
estimates computed using different isoprene SOC markers, implying that the
available profiles do not reflect the actual isoprene SOC composition
observed in Marseille.
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Using the marker-based approach, the aggregate contribution from traditional
BSOC was estimated at only 4.2% of total OC and was dominated by α-pinene
SOC accounting on average for 3.4% of OC. As a result, these
estimates underpredict the inexplicably high loadings of OC. This
underestimation can be associated with (1) uncertainties underlying the
marker-based approach, (2) presence of other SOC precursors and (3) further
processing of fresh SOC, as indicated by organosulfates (RSO<sub>4</sub>H) and
HUmic LIke Substances (HULIS) measurements
Final results of the EDELWEISS-II WIMP search using a 4-kg array of cryogenic germanium detectors with interleaved electrodes
The EDELWEISS-II collaboration has completed a direct search for WIMP dark
matter with an array of ten 400-g cryogenic germanium detectors in operation at
the Laboratoire Souterrain de Modane. The combined use of thermal phonon
sensors and charge collection electrodes with an interleaved geometry enables
the efficient rejection of gamma-induced radioactivity as well as near-surface
interactions. A total effective exposure of 384 kg.d has been achieved, mostly
coming from fourteen months of continuous operation. Five nuclear recoil
candidates are observed above 20 keV, while the estimated background is 3.0
events. The result is interpreted in terms of limits on the cross-section of
spin-independent interactions of WIMPs and nucleons. A cross-section of
4.4x10^-8 pb is excluded at 90%CL for a WIMP mass of 85 GeV. New constraints
are also set on models where the WIMP-nucleon scattering is inelastic.Comment: 23 pages, 5 figures; matches published versio
A search for low-mass WIMPs with EDELWEISS-II heat-and-ionization detectors
We report on a search for low-energy (E < 20 keV) WIMP-induced nuclear
recoils using data collected in 2009 - 2010 by EDELWEISS from four germanium
detectors equipped with thermal sensors and an electrode design (ID) which
allows to efficiently reject several sources of background. The data indicate
no evidence for an exponential distribution of low-energy nuclear recoils that
could be attributed to WIMP elastic scattering after an exposure of 113 kg.d.
For WIMPs of mass 10 GeV, the observation of one event in the WIMP search
region results in a 90% CL limit of 1.0x10^-5 pb on the spin-independent
WIMP-nucleon scattering cross-section, which constrains the parameter space
associated with the findings reported by the CoGeNT, DAMA and CRESST
experiments.Comment: PRD rapid communication accepte
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