Features of structure, magnetic state and electrodynamic performance of SrFe12-xInxO19.

Indium-substituted strontium hexaferrites were prepared by the conventional solid-phase reaction method. Neutron diffraction patterns were obtained at room temperature and analyzed using the Rietveld methods. A linear dependence of the unit cell parameters is found. In3+ cations are located mainly in octahedral positions of 4fVI and 12 k. The average crystallite size varies within 0.84-0.65 µm. With increasing substitution, the TC Curie temperature decreases monotonically down to ~ 520 K. ZFC and FC measurements showed a frustrated state. Upon substitution, the average and maximum sizes of ferrimagnetic clusters change in the opposite direction. The Mr remanent magnetization decreases down to ~ 20.2 emu/g at room temperature. The Ms spontaneous magnetization and the keff effective magnetocrystalline anisotropy constant are determined. With increasing substitution, the maximum of the ε/ real part of permittivity decreases in magnitude from ~ 3.3 to ~ 1.9 and shifts towards low frequencies from ~ 45.5 GHz to ~ 37.4 GHz. The maximum of the tg(α) dielectric loss tangent decreases from ~ 1.0 to ~ 0.7 and shifts towards low frequencies from ~ 40.6 GHz to ~ 37.3 GHz. The low-frequency maximum of the µ/ real part of permeability decreases from ~ 1.8 to ~ 0.9 and slightly shifts towards high frequencies up to ~ 34.7 GHz. The maximum of the tg(δ) magnetic loss tangent decreases from ~ 0.7 to ~ 0.5 and shifts slightly towards low frequencies from ~ 40.5 GHz to ~ 37.7 GHz. The discussion of microwave properties is based on the saturation magnetization, natural ferromagnetic resonance and dielectric polarization types.

Rise of nations: Why do empires expand and fall?

We consider centralized networks composed of multiple satellites arranged around a few dominating super-egoistic centers. These so-called empires are organized using a divide and rule framework enforcing strong center-satellite interactions while keeping the pairwise interactions between the satellites sufficiently weak. We present a stochastic stability analysis, in which we consider these dynamical systems as stable if the centers have sufficient resources while the satellites have no value. Our model is based on a Hopfield type network that proved its significance in the field of artificial intelligence. Using this model, it is shown that the divide and rule framework provides important advantages: it allows for completely controlling the dynamics in a straight-forward way by adjusting center-satellite interactions. Moreover, it is shown that such empires should only have a single ruling center to provide sufficient stability. To survive, empires should have switching mechanisms implementing adequate behavior models by choosing appropriate local attractors in order to correctly respond to internal and external challenges. By an analogy with Bose-Einstein condensation, we show that if the noise correlations are negative for each pair of nodes, then the most stable structure with respect to noise is a globally connected network. For social systems, we show that controllability by their centers is only possible if the centers evolve slowly. Except for short periods when the state approaches a certain stable state, the development of such structures is very slow and negatively correlated with the size of the system's structure. Hence, increasing size eventually ends up in the "control trap."

Method of surface energy investigation by lateral AFM: application to control growth mechanism of nanostructured NiFe films.

A new method for the specific surface energy investigation based on a combination of the force spectroscopy and the method of nanofriction study using atomic force microscopy was proposed. It was shown that air humidity does not affect the results of investigation by the proposed method as opposed to the previously used methods. Therefore, the method has high accuracy and repeatability in air without use of climate chambers and liquid cells. The proposed method has a high local resolution and is suitable for investigation of the specific surface energy of individual nanograins or fixed nanoparticles. The achievements described in the paper demonstrate one of the method capabilities, which is to control the growth mechanism of thin magnetic films. The conditions for the transition of the growth mechanism of thin Ni80Fe20 films from island to layer-by-layer obtained via electrolyte deposition have been determined using the proposed method and the purpose made probes with Ni coating.

Optimized RNA targets of two closely related triple KH domain proteins, heterogeneous nuclear ribonucleoprotein K and alphaCP-2KL, suggest Distinct modes of RNA recognition.

The KH domain mediates RNA binding in a wide range of proteins. Here we investigate the RNA-binding properties of two abundant RNA-binding proteins, alphaCP-2KL and heterogeneous nuclear ribonucleoprotein (hnRNP) K. These proteins constitute the major poly(C) binding activity in mammalian cells, are closely related on the basis of the structures and positioning of their respective triplicated KH domains, and have been implicated in a variety of post-transcriptional controls. By using SELEX, we have obtained sets of high affinity RNA targets for both proteins. The primary and secondary structures necessary for optimal protein binding were inferred in each case from SELEX RNA sequence comparisons and confirmed by mutagenesis and structural mapping. The target sites for alphaCP-2KL and hnRNP K were both enriched for cytosine bases and were presented in a single-stranded conformation. In contrast to these shared characteristics, the optimal target sequence for hnRNP K is composed of a single short "C-patch" compatible with recognition by a single KH domain whereas that for alphaCP-2KL encompassed three such C-patches suggesting more extensive interactions. The binding specificities of the respective SELEX RNAs were confirmed by testing their interactions with native proteins in cell extracts, and the importance of the secondary structure in establishing an optimized alphaCP-2KL-binding site was supported by comparison of SELEX target structure with that of the native human alpha-globin 3'-untranslated region. These data indicate that modes of macromolecular interactions of arrayed KH domains can differ even among closely related KH proteins and that binding affinities are substantially dependent on the presentation of the target site within the RNA secondary structure.

Assembly of the alpha-globin mRNA stability complex reflects binary interaction between the pyrimidine-rich 3' untranslated region determinant and poly(C) binding protein alphaCP.

Globin mRNAs accumulate to 95% of total cellular mRNA during terminal erythroid differentiation, reflecting their extraordinary stability. The stability of human alpha-globin mRNA is paralleled by formation of a sequence-specific RNA-protein (RNP) complex at a pyrimidine-rich site within its 3' untranslated region (3'UTR), the alpha-complex. The proteins of the alpha-complex are widely expressed. The alpha-complex or a closely related complex also assembles at pyrimidine-rich 3'UTR segments of other stable mRNAs. These data suggest that the alpha-complex may constitute a general determinant of mRNA stability. One or more alphaCPs, members of a family of hnRNP K-homology domain poly(C) binding proteins, are essential constituents of the alpha-complex. The ability of alphaCPs to homodimerize and their reported association with additional RNA binding proteins such as AU-rich binding factor 1 (AUF1) and hnRNP K have suggested that the alpha-complex is a multisubunit structure. In the present study, we have addressed the composition of the alpha-complex. An RNA titration recruitment assay revealed that alphaCPs were quantitatively incorporated into the alpha-complex in the absence of associated AUF1 and hnRNP K. A high-affinity direct interaction between each of the three major alphaCP isoforms and the alpha-globin 3'UTR was detected, suggesting that each of these proteins might be sufficient for alpha-complex assembly. This sufficiency was further supported by the sequence-specific binding of recombinant alphaCPs to a spectrum of RNA targets. Finally, density sedimentation analysis demonstrated that the alpha-complex could accommodate only a single alphaCP. These data established that a single alphaCP molecule binds directly to the alpha-globin 3'UTR, resulting in a simple binary structure for the alpha-complex.

Characterization of an unusual, sequence-specific termination signal for T7 RNA polymerase.

We have characterized an unusual type of termination signal for T7 RNA polymerase that requires a conserved 7-base pair sequence in the DNA (ATCTGTT in the non-template strand). Each of the nucleotides within this sequence is critical for function, as any substitutions abolish termination. The primary site of termination occurs 7 nucleotides downstream from this sequence but is context-independent (that is, the sequence around the site of termination, and in particular the nucleotide at the site of termination, need not be conserved). Termination requires the presence of the conserved sequence and its complement in duplex DNA and is abolished or diminished if the signal is placed downstream of regions in which the non-template strand is missing or mismatched. Under the latter conditions, much of the RNA product remains associated with the template. The latter results suggest that proper resolution of the transcription bubble at its trailing edge and/or displacement of the RNA product are required for termination at this class of signal.

Pausing and termination by bacteriophage T7 RNA polymerase.

Two types of sites are known to cause pausing and/or termination by bacteriophage T7 RNA polymerase (RNAP). Termination at class I sites (typified by the signal found in the late region of T7 DNA, TPhi) involves the formation of a stable stem-loop structure in the nascent RNA ahead of the point of termination, and results in termination near runs of U. Class II sites, typified by a signal first identified in the cloned human preproparathyroid hormone (PTH) gene, generate no evident structure in the RNA but contain a conserved sequence ahead of the point of termination, and also contain runs of U. Termination at class I and class II sites may involve non-equivalent mechanisms, as mutants of T7 RNA polymerase have been identified that fail to recognize class II sites yet continue to recognize class I sites. In this work, we have analyzed pausing and termination at several class II sites, and variants of them. We conclude that the 7 bp sequence ATCTGTT (5' to 3' in the non-template strand) causes transcribing T7 or T3 RNA polymerase to pause. Termination 6 to 8 bp past this sequence is favored by the presence of runs of U, perhaps because they destabilize an RNA:DNA hybrid. The effects of T7 lysozyme on pausing and termination are consistent with the idea that termination involves a reversion of the polymerase from the elongation to the initiation conformation, and that lysozyme inhibits the return to the elongation conformation. A kinetic model of pausing and termination is presented that provides a consistent interpretation of our results.

Mutant bacteriophage T7 RNA polymerases with altered termination properties.

We have identified mutants of bacteriophage T7 RNA polymerase (RNAP) that are altered in their ability to pause or terminate at a variety of signals. These signals include a terminator found fortuitously in the human preproparathyroid hormone (PTH) gene, a pause site found in the concatamer junction (CJ) of replicating T7 DNA, and termination signals that are also utilized by Escherichia coli RNAP (e.g. rrnB T1 and T2). Whereas the mutant enzymes terminate normally at the late terminator in T7 DNA (T(phi)) and rrnB T2, they fail to terminate at one of the termination sites of rrnB T1, and also fail to recognize the PTH and CJ signals. The mutant enzymes exhibit normal processivity on linear templates, but show a slightly reduced processivity on supercoiled templates and terminate more efficiently when synthesizing poly(U) tracts. The mutant enzymes also show a decreased tendency to produce aberrant transcription products from DNA templates having protruding 3' ends. T7 lysozyme (an inhibitor of T7 RNAP) has been shown to exert its action by preventing the transition of the RNAP from an unstable initiation complex (IC) to a stable elongation complex (EC). We have found that T7 lysozyme enhances recognition of CJ by wild-type T7 RNAP, and that mutant T7 RNAPs that show increased sensitivity to lysozyme show enhanced recognition of this signal, even in the absence of lysozyme. These results, together with the observation that the mutations that result in the termination-deficient phenotype affect a region of the RNAP that has been implicated in RNA binding and upstream promoter contacts, support the hypothesis that, in some cases, termination represents a reversal of the events that occur during initiation.

Rapid mutagenesis and purification of phage RNA polymerases.

We have developed plasmid-based expression systems that encode modified forms of T7 RNA polymerase (RNAP) having 6-12 histidine residues fused to the amino terminus. The histidine-tagged RNAPs (His-T7 RNAPS) are indistinguishable from the wild-type (WT) enzyme in nearly all biochemical assays. Similar plasmids that encode His-tagged T3 and SP6 RNAPs have also been constructed. To facilitate site-directed mutagenesis of the RNAP gene, the size of the target plasmid was minimized by using T7 RNAP itself as a selectable marker. BL21 (DCAT4) cells (which carry a chromosomal copy of the chloramphenicol acetyltransferase cat gene under control of a T7 promoter) are resistant to chloramphenicol when functional T7 RNAP is expressed, thus allowing the selection and maintenance of the target plasmid in these cells. Mutagenesis is accomplished by denaturing the plasmid, annealing mutagenic DNA primers, and repairing the plasmid with T4 DNA polymerase. Two DNA primers are used: one corrects a defect in the bla gene, the other introduces the desired mutation into the RNAP gene; 30-85% of the ampicillin-resistant transformants carry the desired mutation in the RNAP gene. By using BL21 (DCAT4) cells as a recipient for transformation the functional integrity of the RNAP gene may conveniently be monitored by assessing the level of chloramphenicol resistance in vivo. Methods for rapid, simultaneous purification of multiple samples of modified (His-tagged) and conventional RNAPs are described. Together, these developments greatly enhance our ability to characterize this important class of enzymes.

Mutants of T7 RNA polymerase that are able to synthesize both RNA and DNA.

A mutant T7 RNA polymerase (T7 RNAP) having two amino-acid substitutions (Y639F and S641A) is altered in its specificity towards nucleotide substrates, but is not affected in the specificity of its interaction with promoter and terminator sequences. The mutant enzyme gains the ability to utilize dNTPs and catalyze RNA and DNA synthesis from circular supercoiled plasmid DNA. DNA synthesis can also be initiated from a single stranded template using a DNA primer. Another T7 RNAP mutant having only the single substitution S641A loses RNA polymerase activity but is able to synthesize DNA.

Random mutagenesis of the gene for bacteriophage T7 RNA polymerase.

Random mutagenesis of the gene for bacteriophage T7 RNA polymerase was used to identify functionally essential amino acid residues of the enzyme. A two-plasmid system was developed that permits the straightforward isolation of T7 RNA polymerase mutants that had lost almost all catalytic activity. It was shown that substitutions of Thr and Ala for Pro at the position 563, Ser for Tyr571, Pro for Thr636, Asp for Tyr639 and of Cys for Phe646 resulted in inactivation of the enzyme. It is noteworthy that all these mutations are limited to two short regions that are highly conservative in sequences of monomeric RNA polymerases.

Lys631 residue in the active site of the bacteriophage T7 RNA polymerase. Affinity labeling and site-directed mutagenesis.

A highly selective affinity labeling of T7 RNA polymerase with the o-formylphenyl ester of GMP and [alpha-32P]UTP was carried out. The site of the labeling was located using limited cleavages with hydroxylamine, bromine, N-chlorosuccinimide and cyanogene bromide and was identified as the Lys631 residue. Site-directed mutagenesis using synthetic oligonucleotides was used to substitute Lys631 by a Gly, Leu or Arg residue. Kinetic studies of the purified mutant enzymes showed alterations of their polymerizing activity. For the Lys----Gly mutant enzyme, anomalous template binding was observed.