Mutational analysis of the Mu transposase. Contributions of two distinct regions of domain II to recombination

Mu transposase is a member of a protein family that includes many transposases and the retroviral integrases. These recombinases catalyze the DNA cleavage and joining reactions essential for transpositional recombination. Here we demonstrate that, consistent with structural predictions, aspartate 336 of Mu transposase is required for catalysis of both DNA cleavage and DNA joining. This residue, although located 55 rather than 35 residues NH2-terminal of the essential glutamate, is undoubtedly the analog of the second aspartate of the Asp-Asp-35-Glu motif found in other family members. The core domain of Mu transposase consists of two subdomains: the NH2-terminal subdomain (IIA) contains the conserved Asp-Asp-Glu motif residues, whereas the smaller COOH-terminal subdomain (IIB) contains a large positively charged region exposed on its surface. To probe the function of domain IIB, we constructed mutant proteins carrying deletion or substitution mutations within this region. The activity of the deletion proteins revealed that domains IIA and IIB can be provided by different subunits in the transposase tetramer. Substitution mutations at two pairs of exposed lysine residues within the positively charged surface of domain IIB render transposase defective in transposition at a reaction step after DNA cleavage but prior to DNA joining. The severity of this defect depends on the structure of the DNA flanking the cleavage site. Thus, these data suggest that domain IIB is involved in manipulating the DNA near the cleavage site and that this function is important during the transition between the DNA cleavage and the DNA joining steps of recombination.     

Correlation and prediction of dense fluid transport coefficients. I. n-alkanes

Recent accurate measurements of the self-diffusion coefficient for n-hexadecane and n-octane and of the viscosity coefficient for n-heptane, n-nonane, and n-undecane over wide pressure ranges have been used to provide a critical test of a previously described method, based on consideration of hard-sphere theory, for the correlation of transport coefficient data. It is found that changes are required to the universal curve for the reduced viscosity coefficient as a function of reduced volume and, also, to the parameters R _D, R , and R which were introduced to account for effects of nonspherical molecular shape. The scheme now accounts most satisfactorily for the self-diffusion, viscosity, and thermal conductivity coefficient data for all n-alkanes from methane to hexadecane at densities greater than the critical density.     

基于信息检索课的大学生信息检索行为调查研究

信息检索课程的系统化讲授,对于大学生优化信息检索行为,提高信息检索使用效率具有不容忽视的教学辅助作用。立足问卷调查形成的一系列相关数据,探讨大学生信息检索行为存在问题,最终在课程教学中提出优化大学生信息检索的对策：明确信息检索目的,提升实施效果;有效提升检索行为实施频率,增强综合质量;使得信息获取多元发展,优化成效凸显。     

Structural evolution in gel-derived mullite precursors

The evolution of mullite from organo-metal gel precursors above 700 °C is found to be strongly influenced in both gel pieces and powdered samples by the thermal pretreatment at lower temperatures. Under the present conditions, the optimum preheating temperature was found to be 350 °C, at which temperature an anomalously high concentration was found of an Al species with a characteristic ²⁷Al magic-angle spinning NMR resonance at about 30 ppm. Such Al sites are often described as pentaco-ordinated, but an alternative assignment is considered. The optimum temperature for the formation of this Al site is also optimal for the catalytic formation of aromatic molecules from the residual organic fragments and/or solvent present. Mass spectrometry shows that under the present reaction conditions, these aromatics are thermally stable up to at least 900 °C in air, and the prolonged presence of their decomposition products (CO and water) could facilitate the transformation of the gel to crystalline mullite. The ²⁹Si NMR spectra indicate at least three different Si environments, including one which may arise from the formation of silicon oxycarbide glasses in these gels.     

The distribution and phylogenetic significance of desmethylsterols inChenopodium andAtriplex: Coexistence of Δ7- and Δ5 -sterols

Twenty-one species in the Chenopodiaceae were analyzed for sterol composition. In ten of eleven species ofChenopodium, the major desmethylsterols were Δ⁷-sterols accompanied by lower proportions of Δ⁵-sterols. InC. fremontii this pattern was reversed. The sterol profiles of five species ofAtriplex were characterized by the coexistence of Δ⁷- and Δ⁵-sterols in ratios of 0.3∶1 to 0.4∶1. MaleAtriplex plants contained higher proportions of Δ⁵-sterols than femaleAtriplex plants. OneCeratoides and twoSalicornia species contained Δ⁵-sterols as their predominant sterols.     

Glutarimide acrylic copolymer: A new route to high heat PVC

PVC is the second largest polymer produced and used in the U.S. It is a self-extinguishing material with excellent chemical resistance and good dimensional stability. It's greatest asset, however, is it's versatility. By specifically formulating PVC with the numerous types of additives available, a balance of properties can be tailored for a wide range of applications. The most serious disadvantage of PVC is it's low heat resistance which limits the service temperature of finished articles to about 70°C. Now an additive is available that can be formulated with PVC to form miscible, transparent blends with increased service temperature. With proper formulating, the higher HDT can be achieved without sacrificing other properties. It is especially important in clear PVC applications where no increase in haze was noted with the addition of the acrylic imide copolymer. Indications are that substantial market opportunities exist for a high heat PVC. Major market segments include clear packaging with it's need for hot-fill containers, building products where the desire to use darker colors provides the driving force, and in custom injection molding applications where improved heat resistance would allow vinyl to compete more effectively with FR-styrenics.     

Quantitative Characterization of Indentation Crack Path in a Cubic Zirconia-10 vol% Alumina Composite

Stereological measurements were performed to characterize the indentation crack path in a cubic zirconia-10 vol% alumina (c-ZrO₂-10 vol% Al₂O₃) composite. Cracks were generated using Vickers indentation, and the crack propagation behavior was characterized as a function of the indentation loading/unloading rates. Cracks that were produced by Vickers indentation formed at higher crack velocities as the loading/unloading rates increased. The amount of contact between the crack and the Al₂O₃ particles increased as the indentation rate decreased. The total number of crack-particle interactions per unit crack length also increased as the indentation rate decreased, because of an increase in the number of particles that were fractured per unit crack length, whereas the number of particles that were debonded remained relatively constant as the indentation rate changed. These results suggest that residual thermal mismatch stresses have predominant control of the crack path at lower crack velocities (low indentation loading/unloading rate), whereas elastic mismatch stresses predominate at higher crack velocities (high indentation loading/unloading rate).     

Sterols of the oyster,Crassostrea virginica

A commericial sample of the oyster,Crassostrea virginica, obtained from Maryland waters of the Chesapeake Bay, contained 31 desmethylesterols and at least eight 4-monomethylsterols. The combined gas liquid chromatography-mass spectra of the minor components showed the presence of 6 unusual sterols, 24-ethylcholest-22-en-3β ol, 4α-methyl-24-ethylcholestan-3β-ol, occelasterol, (24E)-24-propylidene-cholest-5-en-3β-ol, (24ZO-24-propylidene-cholest-5-en-3β-ol, and 24-methylene-cholestanol. The C-24 configuration of 24-ethylcholest-5-enol, 24-methyl-cholesta-5,22-dienol, and 24-ethylcholesta-5,22 dienol were elucidated by 220 MHz nuclear magnetic resonance spectrometry.     

Analysis of polymer blend morphology by transmission and reflection light scattering techniques

Optical inspection is a potentially powerful and practical tool to collect information on polymer blend morphology for on-line quality and process control. This paper describes two approaches for the characterization of polymer blend materials by multiple light scattering. A quantitative evaluation of spectral turbidimetry (scattering of transmitted light) is presented and an adaptation of this technique for monitoring particle size, independent of the volume fraction is demonstrated. Such a procedure is applicable to materials in film form. A different approach is presented for the analysis of nearly spherical particles in optically thick samples: the diffuse reflection pattern surrounding the position of incidence of a focused light beam on the surface of the turbid medium is scanned by a single side imaging system. A spectral pseudo-transmittance signal processing technique provides particle size information again independent of the volume fraction: Both techniques have been tested using well characterized polystyrene microspheres suspended in water as well as poly(methyl) (methacrylate)/polystyrene polymer blends.     

混凝土坝裂缝变形的不确定性分析模型

混凝土坝裂缝的发生和发展受到不确定因素影响。将大坝系统及其安全监控看作一个受诸多因素影响的不确定性系统,结合裂缝实测资料,应用近似推理模型和信息分配法,建立了裂缝变形的不确定性分析模型。