Reversible unfolding of single RNA molecules by mechanical force

Here we use mechanical force to induce the unfolding and refolding of single RNA molecules: a simple RNA hairpin, a molecule containing a three-helix junction, and the P5abc domain of the Tetrahymena thermophila ribozyme. All three molecules (P5abc only in the absence of Mg2+) can be mechanically unfolded at equilibrium, and when kept at constant force within a critical force range, are bi-stable and hop between folded and unfolded states. We determine the force-dependent equilibrium constants for folding/unfolding these single RNA molecules and the positions of their transition states along the reaction coordinate.     

智利小植绥螨对土耳其斯坦叶螨、截形叶螨、朱砂叶螨的捕食作用

[目的]研究智利小植绥螨(Phytoseiulus persimilis)雌成螨对新疆三种主要害螨土耳其斯坦叶螨(Tetranychus turkestani Ugarov et Nikolski)、截形叶螨(Tetranychus truncatctus Ehara)、朱砂叶螨(Tetranychus cinnabarinus)的捕食功能反应,以及智利小植绥螨个体之间捕食的干扰反应.[方法]在27℃条件下,采用实验生态学方法,用HoUingⅡ型圆盘方程进行拟合.[结果]智利小植绥螨雌成螨和若螨对的反应均属于HollingⅡ型.智利小植绥螨雌成螨的捕食能力(a/Th值)远大于若螨;智利小植绥螨雌成螨对土耳其斯坦叶螨的攻击率最高为0.665,对朱砂叶螨的攻击率最低为0.214;对土耳其斯坦叶螨的捕食能力最强为10.726,对朱砂叶螨的捕食能力最弱为3.754;智利小植绥螨的寻找效应随着天敌密度的增加而降低.[结论]智利小植绥螨对土耳其斯坦叶螨、截形叶螨、朱砂叶螨捕食作用均属于HollingⅡ型,其捕食作用同时受到猎物密度和自身密度的影响.     

油菜抗(耐)菌核病两种鉴定方法的效果

采用草酸浸根法和菌丝接种法。对15个油菜品种组合进行抗菌核病的鉴定，结果表明：①苗期草酸浸根法与终花期植株菌丝接种法的鉴定结果存在一定的正相关，说明在苗期用草酸浸根法鉴定品种的抗病性有一定可靠性；②参试品种组合间抗性差异达显著水平，其中恢复系5609R、5546R和不育系8078A、5862A的抗性接近对照(中油821)水平，可作育种抗源材料加以利用。     

Stereochemical course of catalysis by the Tetrahymena ribozyme

The group I intron from Tetrahymena catalyzes phosphodiester transfer reactions on various RNA substrates. A modified RNA substrate with a phosphorothioate group in one stereoisomeric form at the site of reaction was synthesized in order to determine the stereochemical course of an RNA-catalyzed reaction. The reaction product was digested with a stereospecific nuclease to determine the configuration of the product phosphorothioate. The reaction occurs with inversion of configuration at phosphorus, implying an in-line pathway for the reaction.     

湖南省蔬菜出口技术壁垒分析

蔬菜在湖南农产品出口中占有重要地位,连续三年出口总额排名第一,但增长幅度有所下降,主要原因是遭遇到了较强的技术壁垒。为此,应该在蔬菜的种植、运输和深加工方面采取相应的对策。     

网络环境下用户获取信息的障碍研究

伴随《第27次中国互联网络发展状况统计报告》的发布,我国网民数量已成为全球之最,互联网普及率已攀升至34.3%。如何使这么多的网民,这么多的用户有效地获取网络信息资源是必须要面对的问题。文章从用户自身因素、网络信息资源障碍、网络环境障碍、网络服务的障碍、网络检索工具的障碍5个方面,对网络环境下影响用户获取信息的障碍进行了研究和分析,并提出了改进建议。     

烟草赤星病抗病性鉴定方法研究进展

现阶段有代表性的烟草赤星病抗病性鉴定方法主要有大田人工病圃鉴定法、离体叶片鉴定法和赤星病菌毒素抗性鉴定法等，本文对这几种鉴定方法的优缺点进行了分析，并在此基础上结合其它作物品种的抗病性鉴定方法提出了一些新的设想。     

Chemical cartography: finding the keys to the kinetic labyrinth

Very high resolution lasers allow spectroscopic pictures to be taken following a collision between two molecular reactants. The features of these "pictures" are the electronic, vibrational, rotational, and translational motions of the atomic particles, which relate the quantum states of the reactants to the quantum states of the products. Such state-to-state kinetic information can be used to test the shape and nature of the interaction potential that controls the collision process. The potential itself is akin to a map of the terrain through mountains and valleys where elevation is a measure of energy instead of height. Accurate mapping of this potential surface leads to an understanding of the forces which control rates and mechanisms of chemical reactions. The application of four different advanced laser techniques to the study of collisions between "hot" hydrogen(H) atoms and carbon dioxide(CO(2)) molecules has provided a wealth of information about both reactive and nonreactive collisions for this system. The availability of data for rotationally, vibrationally, and translationally inelastic excitation of CO(2) by H atoms, when compared with data for reactive events producing OH + CO, provides insights into the dynamics of collisions between H and CO(2), and illustrates the future promise of these powerful techniques for elucidating features of potential energy surfaces.     

Genome-wide experimental determination of barriers to horizontal gene transfer

Horizontal gene transfer, in which genetic material is transferred from the genome of one organism to that of another, has been investigated in microbial species mainly through computational sequence analyses. To address the lack of experimental data, we studied the attempted movement of 246,045 genes from 79 prokaryotic genomes into Escherichia coli and identified genes that consistently fail to transfer. We studied the mechanisms underlying transfer inhibition by placing coding regions from different species under the control of inducible promoters. Our data suggest that toxicity to the host inhibited transfer regardless of the species of origin and that increased gene dosage and associated increased expression may be a predominant cause for transfer failure. Although these experimental studies examined transfer solely into E. coli, a computational analysis of gene-transfer rates across available bacterial and archaeal genomes supports that the barriers observed in our study are general across the tree of life.     

技术性贸易壁垒对福建食用菌出口的影响

本文阐述了技术性贸易壁垒的概念,分析了技术性贸易壁垒对食用菌出口的经济效应。在理论分析的基础上,探讨了技术性贸易壁垒对福建食用菌出口产生的影响,并分析产生这些影响的原因,提出如何突破技术性贸易壁垒、促进福建食用菌出口的建议。     

Neuroscience: breaking down scientific barriers to the study of brain and mind

In this month's essay, Eric R. Kandel and Larry R. Squire chronicle how brain research has migrated from the peripheries of biology and psychology to assume a central position within those disciplines. The multidiscipline of neuroscience that emerged from this process now ranges from genes to cognition, from molecules to minds.     

金融危机背景下技术贸易壁垒的趋势与对策研究

通过对金融危机下技术贸易壁垒的新趋势、对我国贸易负面影响的分析,提出新形势下我国政府和企业应对策略。     

不同含量烟酸铬日粮对奥尼罗非鱼的影响

在日粮中分别添加0．75，1．50，2．25，3．00mg／kg的烟酸铬来饲养奥尼罗非鱼，试验结果表明各试验组均能提高鱼增重、降低血糖浓度，其中添加1．75mg／kg烟酸铬组的效果较好，铬主要在肾脏积累，肌肉、肝脏积累较少．添加烟酸铬对鱼的胰岛组织细胞没有影响。     

Identifying the driver of pulsating aurora

Pulsating aurora, a spectacular emission that appears as blinking of the upper atmosphere in the polar regions, is known to be excited by modulated, downward-streaming electrons. Despite its distinctive feature, identifying the driver of the electron precipitation has been a long-standing problem. Using coordinated satellite and ground-based all-sky imager observations from the THEMIS mission, we provide direct evidence that a naturally occurring electromagnetic wave, lower-band chorus, can drive pulsating aurora. Because the waves at a given equatorial location in space correlate with a single pulsating auroral patch in the upper atmosphere, our findings can also be used to constrain magnetic field models with much higher accuracy than has previously been possible.     

General acid-base catalysis in the mechanism of a hepatitis delta virus ribozyme

Many protein enzymes use general acid-base catalysis as a way to increase reaction rates. The amino acid histidine is optimized for this function because it has a pK(a) (where K(a) is the acid dissociation constant) near physiological pH. The RNA enzyme (ribozyme) from hepatitis delta virus catalyzes self-cleavage of a phosphodiester bond. Reactivity-pH profiles in monovalent or divalent cations, as well as distance to the leaving-group oxygen, implicate cytosine 75 (C75) of the ribozyme as the general acid and ribozyme-bound hydrated metal hydroxide as the general base in the self-cleavage reaction. Moreover, C75 has a pK(a) perturbed to neutrality, making it "histidine-like." Anticooperative interaction is observed between protonated C75 and a metal ion, which serves to modulate the pK(a) of C75. General acid-base catalysis expands the catalytic repertoire of RNA and may provide improved rate acceleration.