In many bacterial viruses and in certain animal viruses, the double-stranded DNA genome enters and exits the capsid through a portal gatekeeper. We report a pseudoatomic structure of a complete ...portal system. The bacteriophage SPP1 gatekeeper is composed of dodecamers of the portal protein gp6, the adaptor gp15, and the stopper gp16. The solution structures of gp15 and gp16 were determined by NMR. They were then docked together with the X-ray structure of gp6 into the electron density of the almost equal to1-MDa SPP1 portal complex purified from DNA-filled capsids. The resulting structure reveals that gatekeeper assembly is accompanied by a large rearrangement of the gp15 structure and by folding of a flexible loop of gp16 to form an intersubunit parallel β-sheet that closes the portal channel. This stopper system prevents release of packaged DNA. Disulfide cross-linking between β-strands of the stopper blocks the key conformational changes that control genome ejection from the virus at the beginning of host infection.
We conducted an absolute paleointensity survey on 74 lava flows from the Okhotsk–Chukotka Volcanic Belt (NE Russia), emplaced 90–83 Ma toward the end of the Cretaceous Normal Superchron (CNS, ...121–84 Ma). Relying on preliminary results, we restricted our analysis to eight lava flows (140 Thellier–Coe experiments), two of which also yielded successful Wilson determinations. A detailed analysis of the magneto‐mineralogy—based on X‐ray structural analysis, reflected‐light microscopy, and thermomagnetic curves—indicates that the determinations from solely two lava flows can be fully trusted, with remanence carriers (a) dominated by low‐titanium titanomagnetite, (b) showing unambiguous traces of high‐temperature oxidation, and (c) yielding partial thermoremanent magnetization (pTRM) tails representative of pseudo‐single‐domain grains. Recalculated in terms of geomagnetic dipole strength, our two successful flow‐mean determinations yield virtual dipole moments of 4.76 ± 0.26 × 1022 Am2 (N = 7) and 9.07 ± 0.84 × 1022 Am2 (N = 8). Using an updated version of the paleointensity database for the Cretaceous epoch, we stress that determinations based on nonglassy whole rocks, submarine basaltic glasses, and single crystals are mutually inconsistent, suggesting a separate analysis of their distributions is more appropriate. Despite statistically indistinguishable estimates of dipole strength before and after the onset of the CNS—in accord with recent studies refuting the existence of a strict correlation between chron duration and dipole moment—we found that the distribution of dipole moments during the CNS is slightly bimodal with a leptokurtic dominant mode, thus more inclined to produce outliers and suggesting distinct geomagnetic field behavior during the CNS.
Plain Language Summary
The Cretaceous Normal Superchron (CNS, 121–84 Ma) is a salient feature of the geomagnetic field during the Phanerozoic, characterized by an anomalously long period of stable polarity whereas Earth’s magnetic field stochastically reversed its polarity approximately 4 times per Myr during the last 5 million years. Deciphering the geomagnetic behavior during the CNS, in terms of average dipole strength and variability, is thus essential to better understand the modus operandi of the geodynamo and better constrain numerical models. To this end, we conducted absolute paleointensity (API) experiments on lava flows from the Okhotsk–Chukotka Volcanic Belt (NE Russia), yielding two estimates of the geomagnetic dipole strength toward the end of the CNS. Using an updated version of the API database, we show that average dipole strength is statistically indistinguishable before and after the onset of the CNS, suggesting the absence of a strict correlation between duration of polarity intervals and average dipole strength. In contrast, we also show that the distribution of dipole strength estimates is slightly bimodal, with the dominant mode being more peaked than a normal distribution. This feature may point to a higher propensity of the geodynamo to produce outliers and thus distinct geomagnetic behavior during the CNS.
Key Points
We investigate geomagnetic field behavior during the Cretaceous Normal Superchron (CNS), a ∼40 Myr interval of stable polarity
We present new absolute paleointensity data from Chukotka (NE Russia) emplaced at high latitude toward the end of the CNS (∼87 Ma)
We show that geomagnetic dipole strength during the CNS may be bimodal, yet with a similar average than during the whole Cretaceous epoch
The spin dynamics in CsPbBr 3 lead halide perovskite nanocrystals are studied by picosecond pump−probe Faraday rotation in an external magnetic field. Coherent Larmor precession of electrons and ...holes with spin dephasing times of ∼600 ps is detected in a transversal magnetic field. The longitudinal spin relaxation time in weak magnetic fields reaches 80 ns at a temperature of 5 K. In this regime, the carrier spin dynamics is governed by nuclear spin fluctuations characterized by an effective hyperfine field strength of 25 mT. The Landéfactors determining the carrier Zeeman splittings are g e = +1.73 for electrons and g h = +0.83 for holes. A comparison with a CsPbBr 3 polycrystalline film and bulk single crystals evidences that the spatial confinement of electrons and holes in the nanocrystals only slightly affects their g factors and spin dynamics.
Coherent neutrino-nucleus scattering is a promising new tool in the toolbox of electroweak precision measurements at low
q
-transfer. It will enable precise measurements of standard model (SM) ...physics like the running of the Weinberg angle but also the search for new physics beyond the SM like sterile neutrinos. The
Nucleus
experiment aims at the first detection of fully coherent neutrino-nucleus scattering at the Chooz power plant in France, using its two 4GW
th
reactor cores as high-intensity source for anti-neutrinos. For this endeavour a new experimental site, the Very Near Site (VNS), with a shallow rock overburden of
≈
3
m w.e. is under development. To be competitive in this challenging environment,
Nucleus
developed the novel concept of fiducialised cryogenic bolometers based on CaWO
4
monocrystals operated at
O
(10 mK). The signature of a coherent neutrino-nucleus scattering is a nuclear recoil at the 10 eV-scale. Currently,
Nucleus
is preparing its first phase with 10 g of target mass at the VNS. In this contribution, we will first introduce
Nucleus
, report its current state and give an outlook to its future.
Abstract
The
Nucleus
experiment aims to measure coherent elastic neutrino nucleus scattering of reactor anti-neutrinos using cryogenic calorimeters. Operating at an overburden of 3 meters of water ...equivalent, muon-induced backgrounds are expected to be one of the dominant background contributions. Besides a high efficiency to identify muon events passing the experimental setup, the
Nucleus
muon veto has to fulfill tight spatial requirements to fit the constraints given by the experimental site and to minimize the induced detector dead-time. We developed highly efficient and compact muon veto modules based on plastic scintillators equipped with wavelength shifting fibers and silicon photo multipliers to collect and detect the scintillation light. In this paper, we present the full characterization of a prototype module with different light read-out configurations. We conclude that an efficient and compact muon veto system can be built for the
Nucleus
experiment from a cube assembly of the developed modules. Simulations show that an efficiency for muon identification of >99 % and an associated rate of 325 Hz is achievable, matching the requirements of the
Nucleus
experiment.
—A paleomagnetic study of the Okhotsk–Chukotka belt volcanics exposed in the region of the Kupol field (~66.9° N, 170.1° E) has been carried out. The studied rocks were formed between 85 and 95 Ma ...during the Cretaceous Normal Superchron (CNS). In most of the studied samples, the characteristic magnetization component is isolated. Its primary origin is supported by the positive fold test, magnetic mineralogy data, and similarity of the calculated paleomagnetic pole with that of coeval rocks from the same region. It is shown that the studied rocks were formed in high polar latitudes close to 80° N. The amplitude of paleosecular variation (PSV) of the geomagnetic field during CNS has been estimated. Depending on the calculation method, the estimates (and their confidence intervals) for ~81° N are
Sb
= 22.1° (19.5°; 24.7°) in case of using a fixed cut-off angle of 45° and
Sb
= 23.1° (20.3°; 25.7°) in case of the cut-off angle calculated by the Vandamme method. The obtained estimates generally support the existing models of PSV latitudinal dependence for CNS. At the same time, the estimates are less consistent with the model (McFadden et al., 1991) where PSV values are low at the equator and sharply increase with latitude than with the model (Biggin et al., 2008) predicting higher PSV values at the equator and their more gentle increase towards polar latitudes. Irrespective of the calculation method, the obtained estimates almost perfectly fit the PSV latitudinal dependences for the last 5 Ma. This means that the amplitude of secular geomagnetic variation during CNS, at least at high latitudes, was the same as it was during the last 5 Ma. The obtained result does not support the hypothesis that geodynamo regimes change at the transition from the periods of frequent geomagnetic reversals to superchrons (e.g., (Gallet and Pavlov, 2016)), which suggests, in particular, that the modern numerical models are not fully adequate to the real processes of generation of the geomagnetic field.
Abstract
Coherent elastic neutrino-nucleus scattering (CE
ν
NS) offers a unique way to study neutrino properties and to search for new physics beyond the Standard Model. The NUCLEUS experiment aims ...to measure CEνNS of reactor anti-neutrinos down to unprecedented low nuclear recoil energies. The novel gram-scale cryogenic detectors feature an ultra-low energy threshold of ≤20eV
nr
and a rise time of a few 100
μ
s which allows the operation above ground. The fiducialization of the detectors provides an effective discrimination of ambient
γ
- and surface backgrounds. Furthermore, the use of multiple targets promises a high physics potential. The NUCLEUS experiment will be located at a new experimental site at the Chooz nuclear power plant in France, providing a high anti-neutrino flux of
1.7
⋅
10
12
ν
¯
e
/
(
s
⋅
cm
2
)
. The commissioning of the experimental setup with a comprehensive background measurement is planned for 2022.
The majority of known bacteriophages have long noncontractile tails (Siphoviridae) that serve as a pipeline for genome delivery into the host cytoplasm. The tail extremity distal from the phage head ...is an adsorption device that recognises the bacterial receptor at the host cell surface. This interaction generates a signal transmitted to the head that leads to DNA release. We have determined structures of the bacteriophage SPP1 tail before and after DNA ejection. The results reveal extensive structural rearrangements in the internal wall of the tail tube. We propose that the adsorption device–receptor interaction triggers a conformational switch that is propagated as a domino‐like cascade along the 1600 Å‐long helical tail structure to reach the head‐to‐tail connector. This leads to opening of the connector culminating in DNA exit from the head into the host cell through the tail tube.
Abstract
We investigated the acquisition of thermochemical remanent magnetization (TCRM) on basaltic rocks from the volcanic island of São Tomé (Gulf of Guinea) and from the southern part of the Red ...Sea Rift, both containing homogeneous titanomagnetite grains with Curie temperatures of 100–200 °C. The TCRM was created in a rotating thermomagnetometer by cooling the samples from 570 to 200 °C at a rate of 1 °C/hr in the presence of a laboratory magnetic field of 50 μT. The TCRM acquisition occurred at high temperature
T
> 520 °C through the nucleation of ilmenite lamellae dividing the titanium‐magnetite cells. Mutual Fe‐Ti diffusion moved the composition of the cells closer to that of magnetite, leading to an increase in the Curie temperature
T
с
. The TCRM was formed at practically fixed volume of the titanomagnetite cells when
T
с
exceeded
T
. Theory indicates that the TCRM should be very close to the value of a pure thermoremanent magnetization acquired in the same field. The Thellier‐style experiments conducted on the samples bearing a laboratory induced TCRM confirmed these predictions, with palaeointensity estimates in agreement to within 5% with expected value. This conclusion radically differs from previous results obtained in the case of a pure chemical remanent magnetization and gives hope that a TCRM could be a robust source of palaeomagnetic information, yielding unbiased palaeointensity determinations.
Key Points
Thermochemical remanences in titanomagnetites are reproduced in laboratory experiments
Thermochemical remanent magnetizations yield similar intensities and close blocking temperatures spectra
Thermochemical remanent magnetization created in titanomagnetites can provide reliable absolute palaeointensity estimates
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