Structures of SAS-6 suggest its organization in centrioles

Centrioles are cylindrical, ninefold symmetrical structures with peripheral triplet microtubules strictly required to template cilia and flagella. The highly conserved protein SAS-6 constitutes the center of the cartwheel assembly that scaffolds centrioles early in their biogenesis. We determined the x-ray structure of the amino-terminal domain of SAS-6 from zebrafish, and we show that recombinant SAS-6 self-associates in vitro into assemblies that resemble cartwheel centers. Point mutations are consistent with the notion that centriole formation in vivo depends on the interactions that define the self-assemblies observed here. Thus, these interactions are probably essential to the structural organization of cartwheel centers.     

Single-crystal-like diffraction data from polycrystalline materials

A method for solving structures from powder diffraction data was developed, and its validity was demonstrated on three complex structures. The method uses a textured sample and exploits the high intensity and parallel nature of synchrotron radiation. In principle, crystal structures as complex as those routinely solved by single-crystal methods can be determined with this approach. For example, the as-synthesized form of the zeolite UTD-1, with 69 nonhydrogen atoms in the asymmetric unit, could be solved directly. With this method, a larger range of structural complexity becomes accessible to scientists interested in the structures of polycrystalline materials that cannot be grown as single crystals.     

云计算数据中心节能调度算法改进研究

针对传统方法对云计算数据中心调度时能耗过高,虚拟机迁移次数过多等问题,本文提出一种新的云计算数据中心节能调度算法.采用面向实时任务的云计算数据中心节能调度算法,保证云数据中心任务完成率;为了进一步降低云数据中心的能耗,构建云计算数据中心功耗模型,分析云数据中心SLA协议违约率和虚拟机迁移功耗对数据中心的能耗影响,采用基于能耗感知的虚拟机节能调度算法合理安置和迁移虚拟机,使虚拟机安装在物理机后无任何负载较低节点,实现云计算数据中心的节能调度.模拟实验结果表明,本文算法与对比算法相比,大幅度降低了云计算数据中心的能耗,且虚拟机迁移次数少,可有效降低虚拟机多次迁移带来的额外支出,是一种有效的云数据中心节能调度算法.     

网络中心农业机理分析

研究基于互联网的农业系统运行机理是农业生产科学管理中必须解决的重要问题之一,可以借助结构化模块分析方法,运用资源共享原理,通过构造一种网络中心农业体系结构分析省级农业生产管理系统来研究这种机理,初步的成果表明:以互联网为核心的农业是农业机械化和信息化共同发展、相互作用的产物,它使传统农业生产管理层次减少,从多层次结构向偏平化结构方向发展,使农业生产呈现出明显的集中管理和分散执行的新格局,研究其运行机理对于制订适应这种新型农业发展模式的农业资源的科学配置与管理的对策具有重要的意义。     

计算非饱和土壤水流问题的一种新差分格式

将求解对流占优Burgers方程的随流格式推广应用于计算非饱和土壤水流入渗问题。在Euler坐标系中,扩散项取中心差分格式,对流项中的空间偏导数取迎风格式,而对流速度取为随流迎风格式。算例表明,这样构造的差分格式精度高于传统格式。     

An inward-facing conformation of a putative metal-chelate-type ABC transporter

The crystal structure of a putative metal-chelate-type adenosine triphosphate (ATP)-binding cassette (ABC) transporter encoded by genes HI1470 and HI1471 of Haemophilus influenzae has been solved at 2.4 angstrom resolution. The permeation pathway exhibits an inward-facing conformation, in contrast to the outward-facing state previously observed for the homologous vitamin B12 importer BtuCD. Although the structures of both HI1470/1 and BtuCD have been solved in nucleotide-free states, the pairs of ABC subunits in these two structures differ by a translational shift in the plane of the membrane that coincides with a repositioning of the membrane-spanning subunits. The differences observed between these ABC transporters involve relatively modest rearrangements and may serve as structural models for inward- and outward-facing conformations relevant to the alternating access mechanism of substrate translocation.     

Structures from anomalous diffraction of native biological macromolecules

Crystal structure analyses for biological macromolecules without known structural relatives entail solving the crystallographic phase problem. Typical de novo phase evaluations depend on incorporating heavier atoms than those found natively; most commonly, multi- or single-wavelength anomalous diffraction (MAD or SAD) experiments exploit selenomethionyl proteins. Here, we realize routine structure determination using intrinsic anomalous scattering from native macromolecules. We devised robust procedures for enhancing the signal-to-noise ratio in the slight anomalous scattering from generic native structures by combining data measured from multiple crystals at lower-than-usual x-ray energy. Using this multicrystal SAD method (5 to 13 equivalent crystals), we determined structures at modest resolution (2.8 to 2.3 angstroms) for native proteins varying in size (127 to 1148 unique residues) and number of sulfur sites (3 to 28). With no requirement for heavy-atom incorporation, such experiments provide an attractive alternative to selenomethionyl SAD experiments.     

Automatic determination of crystal structure

Several crystallographic computer programs have been organized into one large automatic program for solving crystal structures. The emphasis of this organization has been to produce a noninteractive system, that is, to have all decisions made by the computer. Input data are the raw intensity data, cell constants, space group, chemical formula, and other miscellaneous items. The output is a stereo picture of the contents in a unit cell. The program, operating in a noninteractive mode, has successfully solved compounds of unknown structure; in addition, for a test compound of completely unknown composition, this program deduced the correct structure with an average error in bond distance of 0.05 angstrom and an average error in bond angle of 7 degrees .     

A photoinduced persistent structural transformation of the special pair of a bacterial reaction center

Structural modification of photosynthetic reaction centers is an important approach for understanding their charge-separation processes. An unprecedented persistent structural transformation of the special pair (dimer) of bacteriochlorophyll molecules can be produced by light absorption alone. The nonphotochemical hole-burned spectra for the reaction center of Rhodopseudomonas viridis show that the phototransformation leads to a red shift of 150 wave numbers for the special pair's lowest energy absorption band, P960, and a comparable blue shift for a state at 850 nanometers, which can now be definitively assigned as being most closely associated with the upper dimer component. Additional insights on excited-state electronic structure include the identification of a new state.     

基于模糊层次分析法的成型钢筋配送中心选址

针对成型钢筋配送中心服务对象的不确定性提出采用模糊层次分析法进行选址评价.从经济、社会、生态三方面分析选址的影响因素,并建立起成型钢筋配送中心选址评价指标体系层次结构;着重阐述了利用模糊层次分析法进行成型钢筋配送中心选址决策时模糊权值及模糊指标值的计算方法.通过实例应用证实了模糊层次分析法在成型钢筋配送中心选址中具有较高的可行性与准确性.     

Territorial organization of the lowland classic maya

Thus far I have discussed ancient Maya sociopolitical structure from the upper levels of the hierarchy downward. Let me now summarize their territorial organization from the bottom upward, starting at the hamlet level (Fig. 8). The smallest unit of settlement-one usually overlooked by archeological surveys in the lowland rain forest-was probably a cluster of thatched huts occupied by a group of related families; larger clusters may have been divided into four quadrants along the lines suggested by Coe (26). Because of the long fallow period (6 to 8 years) characteristic of slash-and-burn agriculture in the Petén, these small hamlets are presumed to have changed location over the years, although they probably shifted in a somewhat circular fashion around a tertiary ceremonial-civic center for whose maintenance they were partly responsible. These tertiary centers were spaced at fairly regular intervals around secondary ceremonial-civic centers with pyramids, carved monuments, and palace-like residences. In turn, the secondary centers occurred at such regular intervals as to form hexagonal patterns around primary centers, which were still larger, with acropolises, multiple ceremonial plazas, and greater numbers of monuments. In some cases, the distance between secondary centers was roughly twice the distance between secondary and tertiary centers, creating a lattice of nested hexagonal cells. This pattern, which conforms to a Western theoretical construct, was presumably caused by factors of service function, travel, and transport. The pattern was not recognized by the Maya at all. They simply recognized that a whole series of smaller centers were dependent on a primary center and therefore mentioned its emblem glyph. Linking the centers of the various hexagons were marriage alliances between members of royal dynasties, who had no kinship ties with the farmers in the hamlets. Out of the large number of primary centers available to them, the Maya selected four as regional capitals. True to their cosmology, the Maya regarded these capitals as associated with the four quadrants of their realm, regardless of their actual location. Each was the home city for a very important dynasty whose junior members probably ruled secondary centers. Since the hexagonal lattices were probably adjusted to variations in population density, each of the four quadrants of the Maya realm probably controlled a comparable number of persons. So strong was the cognized model that, despite the rise and fall of individual centers, there seem always to have been four capitals, each associated with a direction and, presumably, with a color. There is still a great deal to learn about the social, political, and territorial organization of the lowland Maya, and parts of the picture presented here need far more data for their confirmation. What seems likely is that the Maya had an overall quadripartite organization (rather than a core and buffer zone) and that within each quadrant there was at least a five-tiered administrative hierarchy of capital, secondary center, tertiary center, village, and hamlet. Perhaps most significant, there was no real conflict between the lattice-like network predicted by locational analysis and the cosmological four-part structure predicted by epigraphy and ethnology.     

秸秆过剩原因解析及对秸秆利用途径的思考

秸秆焚烧或随意遗弃是困扰我国的一大生态环境问题。对秸秆出现区域性、季节性和结构性过剩的客观原因进行了分析,并从技术和研究的角度,提出了对秸秆利用途径与方法的思考。产生秸秆区域性、季节性和结构性过剩的客观原因主要是秸秆产生量增加、传统利用方式消耗的秸秆数量减少、秸秆利用机会成本不断增加与种植制度改变加剧了收种季节矛盾等。采用改良品种、优化作物布局与调整种植方式等措施减少秸秆产生量,大力发展秸秆生物质能产业增加秸秆就近消纳,加快配套机械设备研发以提高秸秆收集效率,优先发展统筹兼顾经济、社会及生态效益的可持续农业技术等途径有利于解决秸秆过剩问题,促进秸秆全量利用。     

通过信息技术提升西部森林旅游的途径探讨

随着西部大开发战略和可持续发展战略逐步推进,森林旅游业成为西部地区在森林资源保护和生态环境建设前提下发展经济的重要产业.建设网络电视传播中心、网络景区预览中心、网络真人导游中心和定位向导服务中心,是利用现代信息技术提升和改造传统森林旅游产业的重要途径.该文从功能描述和服务方式两个方面详细介绍了这些中心的构成要素.      

Multis: a new class of multiprocessor computers

Multis are a new class of computers based on multiple microprocessors. The small size, low cost, and high performance of microprocessors allow the design and construction of computer structures that offer significant advantages in manufacture, price-performance ratio, and reliability over traditional computer families. Currently, commercial multis consist of 4 to 28 modules, which include microprocessors, common memories, and input-output devices, all of which communicate through a single set of wires called a bus. Adding microprocessors together increases the performance of multis in direct proportion to their price and allows multis to offer a performance range that spans that of small minicomputers to mainframe computers. Multis are commercially available for applications ranging from real-time industrial control to transaction processing. Traditional batch, time-sharing, and transaction systems process a number of independent jobs that can be distributed among the microprocessors of a multi with a resulting increased throughput (number of jobs completed per unit of time). Many scientific applications (such as the solving of partial differential equations) and engineering applications (such as the checking of integrated circuit designs) are speeded up by this parallel computation; thus, multis produce results at supercomputer speed but at a fraction of the cost. Multis are likely to be the basis for the next, the fifth, generation of computers-a generation based on parallel processing.     

Crystallography of the hexagonal ferrites

The hexagonal ferrites form an unusual group of complex, ferrimagnetic oxides embodying some 60 known crystal structures. These include phases for which the structural unit cell is larger than that in any known inorganic materials. The various hexagonal ferrite modifications fall into two distinct structural series, each formed by the ordered interlayering (stacking) of two discrete building blocks; these blocks stack along the c crystallographic axis in varying ratios and varying permutations to form strictly coherent, reproducible crystal structures. This mixed-layering aspect of the hexagonal ferrites permits direct, visual observation of the sequence of their subunit-cell stacking elements, after etching, by means of electron microscopy. The sequence of stacked blocks in such structures constitutes the only information lacking for a complete, three-dimensional structure determination. Direct access to this information provides an immediate, unique solution of the crystal structure problem in each case and thereby avoids the dilemmas of a classical diffraction approach to such large unit cells. Ferrite structures with hexagonal c dimensions of 1455 and 1577 angstroms have been uniquely solved by direct electron microscopic readout of surface etch features. One must exercise caution, however, in generalizing these findings to other materials. The method is successful in the case of the hexagonal ferrites because these are mixed-layer structures, wherein the building blocks react at different rates to a specific etchant. Mixed-layer systems are not uncommon in crystallography, and it is likely that similar techniques can be developed for other such materials. Regardless of the validity of this prognosis, however, it is quite evident that high-resolution replica electron microscopy is a most promising tool for the direct observation of surface structure on an ultramicro scale. During the studies reported here replica resolution capability was improved to about 10 angstroms; final resolution is limited by the particle size of the platinum shadowing material. Careful control of experimental conditions during replica preparation or an alternate choice of shadowing material, or both, might reasonably improve the resolution by a factor of 2. This resolution is within the range of most unit cell dimensions and approaches interatomic distances in solid-state materials. The potential of such an experimental capability needs no elaboration.     

Solution of a protein crystal structure with a model obtained from NMR interproton distance restraints

Model calculations were performed to test the possibility of solving crystal structures of proteins by Patterson search techniques with three-dimensional structures obtained from nuclear magnetic resonance (NMR) interproton distance restraints. Structures for crambin obtained from simulated NMR data were used as the test system; the root-mean-square deviations of the NMR structures from the x-ray structure were 1.5 to 2.2 A for backbone atoms and 2.0 to 2.8 A for side-chain atoms. Patterson searches were made to determine the orientation and position of the NMR structures in the unit cell. The correct solution was obtained by comparing the rotation function results of several of the NMR structures and the average structure derived from them. Conventional refinement techniques reduced the R factor from 0.43 at 4 A resolution to 0.27 at 2 A resolution without inclusion of water molecules. The partially refined structure has root-mean-square backbone and side-chain atom deviations from the x-ray structure of 0.5 and 1.3 A, respectively.     

Crystal structure of bee-venom phospholipase A2 in a complex with a transition-state analogue

The 2.0 angstroms crystal structure of a complex containing bee-venom phospholipase A2 (PLA2) and a phosphonate transition-state analogue was solved by multiple isomorphous replacement. The electron-density map is sufficiently detailed to visualize the proximal sugars of the enzyme's N-linked carbohydrate and a single molecule of the transition-state analogue bound ot its active center. Although bee-venom PLA2 does not belong to the large homologous Class I/II family that encompasses most other well-studied PLA2s, there is segmental sequence similarity and conservation of many functional substructures. Comparison of the bee-venom enzyme with other phospholipase structures provides compelling evidence for a common catalytic mechanism.     

淮南市耕地数量变化与经济增长的结构性分析

经济发展过程中建设用地与耕地保护之间的矛盾普遍存在,如何解决这一矛盾,为经济的快速发展提供更多的土地空间是当前研究的重点.通过分析淮南市经济增长的结构性变化,发现非农化产值率是影响经济增长的重要因子,并进一步阐述耕地数量变化与经济增长结构性变化之间的关系,指出经济发展与耕地保护具有一定的协同性,还针对经济增长过程中耕地资源面临的问题提出一些对策和建议.     

试验统计方法教学方法改革

试验统计方法是生物学专业的专业基础课,具有较强的理论性和实践性。传统的教学只重视理论知识的传授,忽视了学生分析和解决问题能力的培养。结合多年的教学实践和试验统计方法教学的特点,提出在教学中应选择合适的教材和教学内容,采用多元化的教学方法以激发学生的学习兴趣,加深对课程内容的理解和掌握,同时应强化计算机辅助教学,以培养学生应用统计分析软件的能力。     

阿魏的研究进展

本研究从分类学、解剖学、生殖生物学、药用价值及人工繁殖等方面对国内外有关阿魏属植物的研究进行了总结，对其研究中存在的问题进行了分析，并从生物学特性、民族植物学及资源的开发与保护等角度对今后的研究方向进行了探讨，以期为该类资源的可持续利用提供参考。