C-terminal processing of yeast Spt7 occurs in the absence of functional SAGA complex

Background  

Spt7 is an integral component of the multi-subunit SAGA complex that is required for the expression of ~10% of yeast genes. Two forms of Spt7 have been identified, the second of which is truncated at its C-terminus and found in the SAGA-like (SLIK) complex.     

Solving modulated structures by X-ray and electron crystallography

X-ray diffraction can be used for accurately determining not only classical, ordinary structures, but also modulated ones. For structures with weak modulations, the modulation induced satellite reflections are often hard to be observed by X-ray diffraction, but they appear clearly in electron diffraction. In these cases, X-ray diffraction will give only average structures whereas electron diffraction will yield information about the modulations. Sr(1.4)Ta(0.6)O(2.9) is a complex modulated compound with weak modulation and small modulated domains. Here we demonstrate the power of combining X-ray and electron crystallography for studying modulated structures on powders. The modulations of Sr(1.4)Ta(0.6)O(2.9) were determined from electron diffraction (SAED) and high resolution electron microscopy (HREM) images. With specially developed image processing techniques, the weak modulations were enhanced, facilitating the interpretation of HREM images in terms of atomic structure.     

A Topochemical Approach to Synthesize Layered Materials Based on the Redox Reactivity of Anionic Chalcogen Dimers

Layered transition metal compounds represent a major playground to explore unconventional electric or magnetic properties. In that framework, topochemical approaches that mostly preserve the topology of layered reactants have been intensively investigated to tune properties and/or design new materials. Topochemical reactions often involve the insertion or deinsertion of a chemical element accompanied by a change of oxidation state of the cations only. Conversely, cases where anions play the role of redox centers are very scarce. Here we show that the insertion of copper into two dimensional precursors containing chalcogen dimers (Q₂)^2? (Q=S, Se) can produce layered materials with extended (CuQ) sheets. The reality of this topochemical reaction is demonstrated here for different pristine materials, namely La₂O₂S₂, Ba₂F₂S₂, and LaSe₂. Therefore, this work opens up a new synthetic strategy to design layered transition metal compounds from precursors containing polyanionic redox centers.     

Using CBED and crystallographic image processing to evidence a structural distortion in a new family of ionic conductor Sr1−xLa1+xAl1−xMgxO4 (0?x?0.7)

A previous study by Raman scattering of the Sr_1−xLa_1+xAl_1−xMg_xO₄ solid solution evidenced a distortion from ideal K₂NiF₄ structure. X-ray powder diffraction and selected area electron diffraction studies were carried out and no lowering of symmetry was observed. All the reflections could be indexed in the space group I4/mmm with a=b=0.38 nm and c=1.27 nm. A coupled study by convergent beam electron diffraction and crystallographic image processing was performed. These techniques have been used to determine a crystal distortion due to small atom displacements from mirrors or axes, which lower the structure symmetry. The mm2 point group symmetry was determined. A microdiffraction study leads to the Imm2 space group. This orthorhombic distortion allows a better understanding of the ionic conductivity behavior of these compounds.     

Effects of water uptake on the inherently oxygen-deficient compounds Ln26O27 square(BO3)8 (Ln=La, Nd)

The inherently oxygen-deficient compounds Ln26O27 square(BO3)8 (Ln=La, Nd) react with water vapor leading to Ln26O26(OH)2(BO3)8 phases, and this reaction is reversible. The crystal structure of Nd26O27 square(BO3)8 has been determined from single-crystal data (space group P with a=6.7643(10) A, b=12.663(2) A, c=14.271(2) A, alpha=90.553(8) degrees, beta=99.778(10) degrees, and gamma=90.511(9) degrees). It is a triclinic distorted version of the monoclinic structure of La26O27 square(BO3)8. The Ln26O26(OH)2(BO3)8 phases both crystallize in the monoclinic system (space group P21/c with a=6.7445(4) A, b=12.6177(9) A, c=14.4947(10) A, and beta=100.168(7) degrees for Nd26O26(OH)2(BO3)8 and a=6.9130(15) A, b=12.896(3) A, c=14.792(4) A, beta=99.698(16) degrees for La26O26(OH)2(BO3)8), and their crystal structure has been determined from single-crystal data, showing that the hydroxyl groups are localized mainly on one of the oxygen sites at room temperature (RT). For the Nd phases, the change in crystal system can result from two different phenomena depending on the atmosphere, either a phase transformation corresponding to a water uptake under wet conditions (triclinic Nd26O27 square(BO3)8 at RT-->monoclinic Nd26O26(OH)2(BO3)8) or a phase transition at approximately 300 degrees C for the anhydrous phase under dry conditions (triclinic Nd26O27 square(BO3)8 at RT-->monoclinic Nd26O27 square(BO3)8 at T>300 degrees C). For Nd26O26(OH)2(BO3)8, the conductivity measured under wet conditions at 300 degrees C is sigma300 degrees C approximately 0.5x10(-5) S cm(-1). Due to the dehydration process, the proton contribution to the total conductivity of the Nd phase is no longer observed above 500 degrees C whereas it was still clearly visible at 600 degrees C for the La phase.