Sm Sites Research Articles

Sm core variation in spliceosomal small nuclear ribonucleoproteins from Trypanosoma brucei

Messenger RNA processing in trypanosomes by cis and trans splicing requires spliceosomal small nuclear ribonucleoproteins (snRNPs) U1, U2, U4/U6, and U5, as well as the spliced leader (SL) RNP. As in other eukaryotes, these RNPs share a core structure of seven Sm polypeptides. Here, we report that the identity of the Sm protein constituents varies between spliceosomal snRNPs: specifically, two of the canonical Sm proteins, SmB and SmD3, are replaced in the U2 snRNP by two novel, U2 snRNP-specific Sm proteins, Sm15K and Sm16.5K. We present a model for the variant Sm core in the U2 snRNP, based on tandem affinity purification-tagging and in vitro protein-protein interaction assays. Using in vitro reconstitutions with canonical and U2-specific Sm cores, we show that the exchange of two Sm subunits determines discrimination between individual Sm sites. In sum, we have demonstrated that the heteroheptameric Sm core structure varies between spliceosomal snRNPs, and that modulation of the Sm core composition mediates the recognition of small nuclear RNA-specific Sm sites.

An observation of electron phase transition in SmB 6 at low temperatures

In order to reveal the nature of the ground state of archetypal intermediate-valence compound SmB 6, a comprehensive study of its transport and magnetic properties was carried out on high-quality single crystals at temperatures of 1.8–300 K in magnetic fields up to 7 T. A drastic enhancement of negative magnetoresistance was observed below 14 K, with the maximum absolute value of Δ ρ/ ρB 2∼2.2×10 −3 T −2 at T≈5.2 K. This effect seems to be attributable to anomalous magnetic scattering of many-body (exciton–polaronic) complexes induced by fast valence fluctuations on Sm sites. The observed anomalies of magnetotransport, thermoelectric and magnetic characteristics are discussed in terms of electron phase transition to the coherent state of interacting many-body complexes occurring at T *∼5 K.

Atomic structure and the magnetic properties of Zr-doped Sm2Co17

Permanent magnets based on Sm2Co17 are of significant technological interest due to their high coercivities and elevated Curie temperatures. In this paper we examine the local environment of Zr-doped Sm2Co17. The presence of Zr in Sm2Co17-based magnets causes the formation of hexagonal Sm2Co17 lamellae in the microstructure in the commercial materials. These lamellae serve as diffusion paths, enabling the development of good magnetic properties. While it is presumed that the Zr enters the lattice into either the Sm or Co dumbbell sites, there is, to date, no direct evidence of this assumption. In this paper, we present evidence of how the Zr does substitutes into the Sm2Co17 lattice. We discuss the implication of this result on the magnetic properties and grain structure of the Zr-doped material.

A doughnut-shaped heteromer of human Sm-like proteins binds to the 3'-end of U6 snRNA, thereby facilitating U4/U6 duplex formation in vitro.

We describe the isolation and molecular characterization of seven distinct proteins present in human [U4/U6.U5] tri-snRNPs. These proteins exhibit clear homology to the Sm proteins and are thus denoted LSm (like Sm) proteins. Purified LSm proteins form a heteromer that is stable even in the absence of RNA and exhibits a doughnut shape under the electron microscope, with striking similarity to the Sm core RNP structure. The purified LSm heteromer binds specifically to U6 snRNA, requiring the 3'-terminal U-tract for complex formation. The 3'-end of U6 snRNA was also co-precipitated with LSm proteins after digestion of isolated tri-snRNPs with RNaseT(1). Importantly, the LSm proteins did not bind to the U-rich Sm sites of intact U1, U2, U4 or U5 snRNAs, indicating that they can only interact with a 3'-terminal U-tract. Finally, we show that the LSm proteins facilitate the formation of U4/U6 RNA duplices in vitro, suggesting that the LSm proteins may play a role in U4/U6 snRNP formation.

The Crystal Structure of Ba17Sm10Cl64

The structure of Ba17Sm10Cl64, prepared by solvolytic extraction of a program-cooled 1:1 BaCl2:SmCl3 molar mixture sealed in a quartz tube and heated to 750°C, was determined from single-crystal X-ray diffraction data. The compound exhibits cubic symmetry, space group Pa3 (No. 205) with a=21.366(2) Å and Z=4. Refinement effected with I>2σ(I) yielded R1= 0.0926 and wR2=0.216. One Ba atom is 12-coordinated by Cl atoms in a distorted icosahedral arrangement; the three other Ba atoms are 10-coordinated in a distorted bicapped cubic arrangement. There are two Sm atom sites. The coordination around one Sm atom is best described as square antiprismatic, but one Sm–Cl distance is too long for effective bonding. The other Sm atom site, occupied statistically by 13Ba and 23Sm atoms, is 9-coordinated by Cl atoms in a monocapped square antiprismatic arrangement. The two types of Sm sites combine to form an M6Cl37 cuboctahedral cluster of the composition BaSm5Cl37. It is shown that the cβ phase identified previously in the Yb–F and related fluoride systems is probably isostructural with Ba17Sm10Cl64.

A polarized neutron investigation of charge-ordering in mixed-valence Sm 4Bi 3

Cubic Sm 4Bi 3 undergoes a first-order transition into a rhombohedral phase at about 260 K. This change has been ascribed to a charge-ordering transition from the thermal valence fluctuating state into a static mixture of di- and trivalent sites. In order to determine the positions of the Sm 2+ and Sm 3+ ions below the transition temperature, we have investigated a Sm 4Bi 3 single crystal by polarized neutron diffraction. A uniaxial pressure was applied along the [ 1 1 1 ] direction to avoid the formation of domains. The rhombohedral nuclear structure was first refined using unpolarized neutron measurements. From the flipping ratios measured with polarized neutrons at T=6 K in a magnetic field of 4.6 T applied along the [1̄ 0 1] rhombohedral direction, the two Sm sites were found to have different magnetic characters: Sm I, along the compressed diagonal of the cube, carries a moment of 0.017(5) μ B with a Sm 3+ form factor, whereas Sm II, along the other diagonals, carries a moment of 0.063(5) μ B with a Sm 2+ form factor.

Crystal structures of new ternary rare earth silicides Sc 3Pr 2Si 4, Sc 1.26Pr 3.74Si 4 and Sc 3.96Nd 1.04Si 4 with ordered and partially ordered rare earth sites

The crystal structures of three new ternary silicides Sc 3Pr 2Si 4, Sc 1.26Pr 3.74Si 4 and Sc 3.96Nd 1.04Si 4 were determined by single crystal X-ray diffraction. The title compounds crystallize with three different substitution derivatives of the Sm 5Ge 4 structure type (orthorhombic space group Pnma) containing various distributions of rare earth atoms on the three Sm sites. Crystal chemistry analysis shows that these distributions are controlled by the atomic size factor.

Inhibitory effects of acupuncture manipulation and focal electrical stimulation of the nucleus submedius on a viscerosomatic reflex in anesthetized rats.

To examine the participation of nucleus submedius (Sm) in the medial thalamus of pain inhibitory systems, we investigated the effects of acupuncture and focal electrical stimulation of the Sm and adjacent brain sites (0.3 ms, 50 Hz, 50-100 microA, 10 s) on the EMG activity of the external oblique muscle evoked by colorectal distension in urethane-anesthetized Wistar rats. The viscerosomatic reflex (VSR) activity was suppressed after the administration of morphine (1.0 mg/kg, i.v.) and the effect was reversed by naloxone (0.5 mg/kg, i.v.). Transection of the spinal cord at the Th2 level also eliminated the VSR. Acupuncture manipulation applied to the cheek (manual rotation at 1 Hz) suppressed the VSR, and this inhibition was eliminated by microinjections of lidocaine into the bilateral Sm nuclei (0.5 microliter of 1.0% solution). Electrical stimulation in the ventral part but not the dorsal part of the Sm suppressed the VSR. The inhibition of the VSR induced by electrical stimulation of the Sm was not reversed by the administration of naloxone (1.0 mg/kg, i.v.). Electrical stimulation of the adjacent medial thalamic nuclei (mediodorsal nucleus (MD) or centromedial nucleus (CM)) and ventrobasal complex (VB) of the thalamus had very little effect on the VSR. These results suggests that the Sm is not only involved in the relay of nociceptive information to the cortex, but may also be involved in a non-opioid mediated pain inhibitory system and may participate, at least in part, in the suppressive effects of intense acupuncture manipulation on VSR activity.

Theory of sm1.5Ce0.5sr2Cu2Nbo10: Critical-temperature and doping effects

For Sm1.5Ce0.5Sr2Cu2NbO10, the charge-reservoir oxygen model of superconductivity predicts: (i) Tc≈30 K (vs. the observed ≈28K); (ii) Roughly 1 percent Ni on Cu sites should drive Tc to zero; (iii) The superconductivity originates in the SrO layers; and (iv) The conduction, as with all high-temperature oxide superconductors, is necessarily p-type. The model also explains why this material can superconduct in the presence of the magnetic rare-earth ion Sm and why Gd on Sm sites does not destroy superconductivity, despite structural similarities to Sm2-zCezCuO4 and Gd2-zCezCuO4, although the former superconducts and the latter does not.

Growth of rhodium dispersions in zeolite NaX: Identification of a precursor state by Rietveld refinement

The temperature-programmed decomposition and autoreduction of the rhodium chloropentaamine complex, ion exchanged into zeolite NaX, are monitored by mass spectrometry. An intermediate state of the decomposition process is analyzed by Rietveld refinement of X-ray diffractograms, revealing a location of partially deaminated compounds in Sm sites, i.e., in window positions between neighboring supercages. The final decomposition and autoreduction of this precursor state explain the preferential metal precipitation in window positions.

Nuclear magnetic resonance study of expanded lattice permanent magnet nitrides and carbides at high pressure

The nuclear magnetic resonance spin-echo spectra of 89Y and 147Sm for Y2Fe17, Y2Fe17C0.4, Y2Fe17C1.5, Y2Fe17N2.5, Y2Fe17CN1.5, Y2Fe17H1.6, and Sm2Fe17 have been measured at pressures up to 10 kbar at 4.2 K. A reduction of the magnitude of the valence electron polarization contribution to the effective field Be at Y and Sm sites with pressure is observed. The effect is strongly related to the nearest neighbor environment of the site. A significant reduction of ∂B/∂p is observed for the sites with nitrogen neighbors. Carbon increases (decreases) ∂B/∂p in the rhombohedral (hexagonal) structure of the compound. A decrease of the lattice electric field gradient at the Sm site in Sm2Fe17 with pressure is observed. A corresponding decrease of the crystal electric field coefficient A02 at the Sm site with pressure of 0.7% per kbar is derived, indicating an anisotropic compressibility of the material.

Transport, magnetic, and 119Sn Mössbauer studies on magnetically ordered valence fluctuating compound SmRuSn3

SmRuSn3 is a unique compound among the known Sm-based valence fluctuation (VF) systems. Its crystallographic structure has two inequivalent Sm sites and Sm ions at only one of them are in VF state while the Sm ions in the other site orders magnetically. Our 119Mössbauer studies show a quadrupolar splitting at the Sn site, consistent with the noncubic symmetry of the Sn site. A broadening of the Mössbauer spectrum is seen due to magnetic ordering of the material. The transferred hyperfine field at Sn site at 4.2 K is small.

Nuclear magnetic resonance study of the Sm 2Fe 17 carbides

The 147Sm and 149Sm NMR spin echo spectra and the spin echo decays of Sm 2Fe 17C x for x = 0, 0.5, 1 and about 3 at 4.2 K are reported. The quadrupole septets in the spectra are assigned to the samarium 6c site with different numbers of carbon atoms occupying the neighbouring 9e interstitial sites. A significant influence of carbon neighbours on the Sm quadrupole splitting and hyperfine field is observed and is attributed to a strong change of the Sm valence electron distribution and polarisation caused by carbon neighbours. From the quadrupole splittings the values of the lattice electric field gradient and the corresponding crystal electric field coefficient A 2 0 are derived. For the Sm sites with 0 (3) carbon nearest neighbours the A 2 0 value of −66 K a 0 −2 (−350 K a 0 −2 is obtained.

Magnetic excitation spectrum of mixed-valence SmB6 studied by neutron scattering on a single crystal

The paramagnetic response of mixed-valence samarium hexaboride has been studied by inelastic neutron scattering using a low-absorption double-isotope single crystal of 154Sm11 B6. Measurements were performed for energy transfers 0<or=h(cross) omega <or=50 meV at temperatures ranging from 2 to 100 K. Two main contributions were observed in the spectra: a broad intermultiplet transition of single-ion type at h(cross) omega approximately=36 meV, and a narrow low-energy excitation centred at h(cross) omega approximately=14 meV, which is strongly anisotropic and temperature dependent and exhibits a weak but visible dispersion. The latter excitation appears to be an inherent feature of the mixed-valence state due to f-electron hybridization effects, which denotes the formation of a local bound state around the Sm sites.

Valence state of Sm in SmRuSn3.

T. Fukuhara, I. Sakamoto, and H. Sato [J. Phys.: Condens. Matter 3, 8917 (1991)] reported valence fluctuations of Sm ions in ${\mathrm{SmRuSn}}_{3}$, which orders antiferromagnetically at 6 K. We have carried out x-ray, microprobe, heat-capacity, susceptibility, and resistivity measurements on a sample of nearly the same stoichiometry. There are two inequivalent Sm sites in ${\mathrm{SmRuSn}}_{3}$ and from the analysis of our data we conclude that Sm ions at one site are in the trivalent state and those at the second site are either divalent or weakly valence fluctuating.

The influence of II–VI lattice damage on rare earth sites as determined by site selective spectroscopy and cathodoluminescence: argon implanted ZnS:Sm 3+

Using site selective and time resolved spectroscopy more than 10 different Sm sites have been discriminated. Beside the direct 4f-intra-shell excitation of the rare earth (RE) impurities a second indirect excitation channel via the host lattice is active with increasing laser power due to two-photon processes. The RE sites differ mainly in their ability to gain energy from this second channel. This knowledge is used to study the influence of host lattice damage — caused by Ar-implantation — on the Sm 3+ luminescence. Using cathodoluminescence (CL) and photoluminescence (PL) it is shown that the RE emission is less affected by the damage than the near-band-edge (NBE), self-activated (SA) and donor-acceptor (DA) emission. No new Sm sites have been created by the implantation and the direct 4f-4f excitation channel is still efficient, whereas the energy transfer (ET) via the ZnS lattice is disturbed leading to an intensity decrease. These results are taken as an evidence for the complex nature and the relative stability of RE centers in a II–VI lattice.

Localization of pre-messenger RNA at discrete nuclear sites.

We have studied the nuclear localization of rhodamine-labeled pre-mRNA after microinjection into nuclei of cultured rat kidney epithelial cells. Intranuclear localization of the injected RNA was followed in the living cells by fluorescence microscopy and digital image processing. Injected human beta-globin pre-mRNA became localized in 30-60 discrete nuclear sites that were coincident with loci defined by monoclonal antibodies against small nuclear ribonucleoproteins (Sm) or another spliceosome component (SC-35) in parallel immunocytochemical studies on the same nuclei. Similar patterns of nuclear localization were observed with a rat proenkephalin pre-mRNA. Nuclear microinjection of an intronlacking beta-globin RNA, a splicing-defective beta-globin mutant pre-mRNA, or an antisense beta-globin pre-mRNA did not result in localization at discrete sites. These results indicate that pre-mRNA binds preferentially to nuclear Sm and SC-35 antibody-reactive sites in vivo and that the binding requires intron sequences.

Temperature and pressure dependence of magnetic and electric hyperfine interactions in metallic Samarium

The magnetic and electric hyperfine interaction at111Cd impurities in Samarium has been investigated by TDPAC measurements. The quadrupole frequency is νQ=20.0(2) MHz at 290 K and has a linear temperature dependence with the same slope (dlnνQ/dT)290K=−7.3(2) 10−4 K−1 in the rhombohedral and the hcp phase. The pressure dependence up to 7 kbar is (dlnνQ/dT)=+8.7(1.4) 10−3 kbar−1. The magnetic hyperfine field of111Cd in Sm is Hhf=242(6) kG at 4.2 K. Its temperature dependence confirms the existence of 2 different magnetic phases in Sm. The crystal field parameters B20 and B40 have been estimated from a comparison of Hhf(T) with molecular field models. The TDPAC spectra in the magnetic phases suggest that the impurities preferentially occupy the hexagonal Sm sites.

Coherent potential approximation treatment of intermediate valence in rare-earth compounds induced by alloying

The coherent potential approximation (CPA) is applied to the description of intermediate valence induced by alloying as observed in the solid solutions Sm1−xRxS where R denotes a rare-earth element which is trivalent in the monosulfide. The description is based on the Falicov-Kimball model for metal-insulator transitions generalized to take into account the disorder associated with the R substitution. This disorder is treated within the CPA while the intra-atomic correlation on the Sm sites is also treated by the CPA in analogy with an alloy of divalent and trivalent Sm ions. The transition to an intermediate valence for Sm as a function of alloy composition is discussed in terms of the results of this treatment.

Giant intrinsic magnetic hardness due to randomized crystal field interactions in SmNi5−xCux

Giant intrinsic magnetic hardness is found at low temperatures in structurally homogeneous materials SmNi5−xCux. Magnetic hardness peaks at a composition SmNi2.5Cu2.5 where the crystal field disorder due to compositional randomization is at a maximum. By comparison with SmCo5−xNix, where giant intrinsic magnetic hardness originates, in part, from fluctuations from site to site of the magnitude of moment and strength of exchange of the transition metal, magnetic hardness in the SmNi5−xCux is predominantly due to anisotropy and concomitant exchange fluctuations on Sm sites.