Formation and evolution of carbonaceous asteroid Ryugu: Direct evidence from returned samples

Samples of the carbonaceous asteroid Ryugu were brought to Earth by the Hayabusa2 spacecraft. We analyzed 17 Ryugu samples measuring 1 to 8 millimeters. Carbon dioxide-bearing water inclusions are present within a pyrrhotite crystal, indicating that Ryugu's parent asteroid formed in the outer Solar System. The samples contain low abundances of materials that formed at high temperatures, such as chondrules and calcium- and aluminum-rich inclusions. The samples are rich in phyllosilicates and carbonates, which formed through aqueous alteration reactions at low temperature, high pH, and water/rock ratios of <1 (by mass). Less altered fragments contain olivine, pyroxene, amorphous silicates, calcite, and phosphide. Numerical simulations, based on the mineralogical and physical properties of the samples, indicate that Ryugu's parent body formed ~2 million years after the beginning of Solar System formation.

Two-dimensional Heisenberg behavior of J(eff)=1/2 isospins in the paramagnetic state of the spin-orbital Mott insulator Sr2IrO4

The dynamical correlations of J(eff)=1/2 isospins in the paramagnetic state of spin-orbital Mott insulator Sr2IrO4 were revealed by resonant magnetic x-ray diffuse scattering. We found a two-dimensional antiferromagnetic fluctuation with a large in-plane correlation length exceeding 100 lattice spacings at even 20 K above the magnetic ordering temperature. In marked contrast to the naive expectation of the strong magnetic anisotropy associated with an enhanced spin-orbit coupling, we discovered an isotropic isospin correlation that is well described by the two-dimensional S=1/2 quantum Heisenberg model. The estimated antiferromagnetic coupling constant as large as J∼0.1  eV that is comparable to the small Mott gap (<0.5  eV) points out the weak and marginal Mott character of this spin-orbital entangled system.

Anomalous metallic state in the vicinity of metal to valence-bond solid insulator transition in LiVS2

We investigate LiVS2 and LiVSe2 with a triangular lattice as itinerant analogues of LiVO2 known for the formation of a valence-bond solid (VBS) state out of an S=1 frustrated magnet. LiVS2, which is located at the border between a metal and a correlated insulator, shows a first order transition from a paramagnetic metal to a VBS insulator at Tc approximately 305 K upon cooling. The presence of a VBS state in the close vicinity of insulator-metal transition may suggest the importance of itinerancy in the formation of a VBS state. We argue that the high temperature metallic phase of LiVS2 has a pseudogap, likely originating from the VBS fluctuation. LiVSe2 was found to be a paramagnetic metal down to 2 K.

Design of a d(1)-analogue of cuprates: Sr(2)VO(4) and Ba(2)VO(4) under pressure

By means of a combination of the local density approximation and dynamical mean field theory (LDA + DMFT), we study the possibility of making a d(1) analogue of d(9) cuprates on the basis of Sr(2)VO(4). We calculate the electronic structure of Sr(2)VO(4) under pressure, and show that while the material is a 1/6-filled three-band system at ambient pressure with a small level splitting between the d(xy)- and d(yz/zx)-bands, an orbital polarization occurs under sufficiently high uniaxial pressure in the c-direction. While all energy scales are relatively small, the electronic structure of Sr(2)VO(4) under pressure is similar to that of La(2)CuO(4); it is a two-dimensional half-filled single-band system which has, relative to the nearest neighbour hopping, a similar Coulomb repulsion and next-nearest neighbour hopping. We also study the effect of substituting Sr by Ba, i.e., chemical pressure, and show that the pressure needed for the orbital polarization is considerably reduced.

Variation of the electronic structure in systematically synthesized Sr2MO4 (M = Ti, V, Cr, Mn, and Co)

We have systematically synthesized single-crystalline thin films of layered perovskites Sr2MO4 (M = Ti, V, Cr, Mn, and Co) which cannot be obtained in a form of bulk crystal apart from M = Mn. The two-dimensional electronic structure of these M4+ oxides, ranging from a correlated insulator to a ferromagnetic metal, has been investigated by using their optical conductivity spectra with polarizations E is perpendicular to c and E is parallel to c, which reveal systematic variation of the correlated charge gap, Mott-Hubbard gap, or charge-transfer gap. Temperature dependence of the gap-transition spectra is argued in the light of possible spin and/or orbital ordering.

Metallic ferromagnet with square-lattice CoO2 sheets

A single-crystalline film of Sr2CoO4 with square-lattice CoO2 sheets (K2NiF4-type structure) was synthesized, and its electronic properties were characterized. The compound exhibited both ferromagnetic and metallic behaviors, with a fairly high Curie temperature T(C) of approximately 250 K, in contrast to the superconductivity recently found in a triangular-lattice CoO2-sheet compound. The film's large magnetoresistance with current perpendicular to the CoO2 plane showed field-hysteretic behavior analogous to tunneling magnetoresistance. The electronic structure of Sr2CoO4 was also investigated to characterize the quasi-two-dimensional ferromagnetic metal state in terms of optical spectroscopy and first-principles calculation.

Requirement of transfer-messenger RNA for the growth of Bacillus subtilis under stresses

BACKGROUND

Bacterial transfer-messenger RNA (tmRNA, 10Sa RNA) is involved in a trans-translation reaction which contributes to the degradation of incompletely synthesized peptides and to the recycling of stalled ribosomes. However, its physiological role in the cell remains elusive. In this study, an efficient system for controlling the expression of the gene for tmRNA (ssrA), as well as a tmRNA gene-defective strain (ssrA:cat), were constructed in Bacillus subtilis. The effects of tmRNA on the growth of the cells were investigated under various physiological culture conditions using these strains.

RESULTS

The cells were viable in the absence of ssrA expression under the usual culture conditions. However, the growth rate of cells without tmRNA expression, relative to that of the expressed cells, decreased with elevating temperature (> 45 degrees C), and at 52 degrees C, the highest temperature for growth of the wild-type, cells grew depending on the expression level of tmRNA. Furthermore, the transcription level of the ssrA from the authentic promoter at a high temperature (51 degrees C) was about 10-fold higher than that at a lower temperature (37 degrees C). tmRNA-dependent growth and an increase in tmRNA amount were also observed in cells under other stresses, such as high concentrations of ethanol or cadmium chloride. It is also shown that alanylated tmRNA rather than tmRNA-mediated proteolysis is required for growth at high temperature.

CONCLUSION

The expression of tmRNA gene (ssrA) is required for the efficient growth of B. subtilis under several strong stresses. The transcription of ssrA increases under several stressful conditions, suggesting that it is a stress-response gene. Alanyl-tmRNA, probably via its ability of recycling stalled ribosomes via trans-translation, is involved in the stress tolerance of bacteria.

Transient nuclear factor kappaB (NF-kappaB) activation stimulated by interleukin-1beta may be partly dependent on proteasome activity, but not phosphorylation and ubiquitination of the IkappaBalpha molecule, in C6 glioma cells. Regulation of NF-kappaB linked to chemokine production

We previously reported that several stresses can induce cytokine-induced neutrophil chemoattractant expression in a nuclear factor kappaB (NF-kappaB)-dependent manner. In this study, we focused further on the regulation of NF-kappaB. The activation of NF-kappaB and the subsequent cytokine-induced neutrophil chemoattractant induction in response to interleukin-1beta (IL-1beta) were inhibited by proteasome inhibitors, MG132 and proteasome inhibitor I. Translocation of NF-kappaB into nuclei occurs by the phosphorylation, multi-ubiquitination, and degradation of IkappaBalpha, a regulatory protein of NF-kappaB. Nascent IkappaBalpha began to degrade 5 min after treatment with IL-1beta and disappeared completely after 15 min. However, IkappaBalpha returned to basal levels after 45-60 min. Interestingly, resynthesized IkappaBalpha was already phosphorylated at Ser-32. These results suggest that 1) the upstream signals are still activated, although the translocation of NF-kappaB peaks at 15 min; and 2) the regulated protein(s) acts downstream of IkappaBalpha phosphorylation. Western blotting showed that the resynthesized and phosphorylated IkappaB molecules were also upward-shifted by multi-ubiquitination in response to IL-1beta treatment. On the other hand, ATP-dependent Leu-Leu-Val-Tyr cleaving activity transiently increased, peaked at 15 min, and then decreased to basal levels at 60 min. Furthermore, the cytosolic fraction that was stimulated by IL-1beta for 15 min, but not for 0 and 60 min, could degrade phosphorylated and multi-ubiquitinated IkappaBalpha. These results indicate that the transient translocation of NF-kappaB in response to IL-1beta may be partly dependent on transient proteasome activation.