Independent transfer of mitochondrial plasmids in Neurospora crassa

In the ascomycete fungus Neurospora, the distribution of homologous mitochondrial plasmid DNAs in different species and among mitochondrial types of N. crassa suggests that these molecules have moved between lineages of clonally propagated mtDNA. Here we report direct evidence for independent inheritance of mitochondrial plasmids by sexual reproduction which may help explain the distribution of these molecules among mitochondrial lineages.     

Independent evolution of structural and coding regions in a Neurospora mitochondrial intron

The discovery of intervening sequences (introns) in eukaryotic genes has raised questions about the origin and evolution of these sequences. Hypotheses concerning these topics usually consider the intron as a unit that could be lost or gained over time, or as a region within which recombination can occur to facilitate the production of new proteins by exon shuffling. Additional complexities are observed in introns of mitochondrial and chloroplast genes which contain secondary structures required for messenger RNA splicing and open-reading frames encoding proteins. Here we describe differences in the organization of protein-coding sequences in the intron of the mitochondrial ND1 gene in two closely related species of Neurospora. These differences show that intron sequences involved in secondary structure formation and in protein coding can evolve as physically distinct elements. Indeed, the secondary structure elements of the ND1 intron can contain two different coding sequences located at two different positions within the intron.     

Bacterial conjugative plasmids mobilize DNA transfer between bacteria and yeast

Conjugative plasmids of Escherichia coli can mobilize DNA transmission from this bacterium to the yeast Saccharomyces cerevisiae. The process shares some of the features of conjugation between bacteria and could be evolutionarily significant in promoting trans-kingdom genetic exchange.     

Widespread horizontal transfer of mitochondrial genes in flowering plants

Horizontal gene transfer--the exchange of genes across mating barriers--is recognized as a major force in bacterial evolution. However, in eukaryotes it is prevalent only in certain phagotrophic protists and limited largely to the ancient acquisition of bacterial genes. Although the human genome was initially reported to contain over 100 genes acquired during vertebrate evolution from bacteria, this claim was immediately and repeatedly rebutted. Moreover, horizontal transfer is unknown within the evolution of animals, plants and fungi except in the special context of mobile genetic elements. Here we show, however, that standard mitochondrial genes, encoding ribosomal and respiratory proteins, are subject to evolutionarily frequent horizontal transfer between distantly related flowering plants. These transfers have created a variety of genomic outcomes, including gene duplication, recapture of genes lost through transfer to the nucleus, and chimaeric, half-monocot, half-dicot genes. These results imply the existence of mechanisms for the delivery of DNA between unrelated plants, indicate that horizontal transfer is also a force in plant nuclear genomes, and are discussed in the contexts of plant molecular phylogeny and genetically modified plants.     

Nanog promotes transfer of pluripotency after cell fusion

Through cell fusion, embryonic stem (ES) cells can erase the developmental programming of differentiated cell nuclei and impose pluripotency. Molecules that mediate this conversion should be identifiable in ES cells. One candidate is the variant homeodomain protein Nanog, which has the capacity to entrain undifferentiated ES cell propagation. Here we report that in fusions between ES cells and neural stem (NS) cells, increased levels of Nanog stimulate pluripotent gene activation from the somatic cell genome and enable an up to 200-fold increase in the recovery of hybrid colonies, all of which show ES cell characteristics. Nanog also improves hybrid yield when thymocytes or fibroblasts are fused to ES cells; however, fewer colonies are obtained than from ES x NS cell fusions, consistent with a hierarchical susceptibility to reprogramming among somatic cell types. Notably, for NS x ES cell fusions elevated Nanog enables primary hybrids to develop into ES cell colonies with identical frequency to homotypic ES x ES fusion products. This means that in hybrids, increased Nanog is sufficient for the NS cell epigenome to be reset completely to a state of pluripotency. We conclude that Nanog can orchestrate ES cell machinery to instate pluripotency with an efficiency of up to 100% depending on the differentiation status of the somatic cell.     

聚变堆包层第一壁流道换热性能研究

以聚变堆包层第一壁内流道作为研究对象,设计了以空气为介质的包层第一壁U型流道换热性能实验台架.通过测量第一壁流道沿流动方向的温度和压力分布,研究了在不同管径和雷诺数下,温度、流速和弯头形状等因素对第一壁流道换热性能的影响,并与数值模拟结果进行了对比分析.实验结果表明:30mm×30mm最大的U型方管可以在不增加流动阻力的情况下,提高流体与管壁之间换热强度23%,并且通过弯头处渐缩的优化改进可进一步提高换热强度15%,数值分析结果与之也较符合.本研究表明通过改变包层第一壁流道的形状和尺寸可以有效提高第一壁流道的换热性能.     

论哈萨克斯坦独立初期的政治转型

哈萨克斯坦宪法规定:国家政体采取立法、司法、行政三权分立的原则,但在独立国家创建初期,总统权力通过各种方式得到扩大,哈萨克斯坦政体逐渐从三权分立的议会民主向总统集权制发展,形成了大总统、弱议会、小政府的威权政体。     

Structural changes of envelope proteins during alphavirus fusion

Alphaviruses are enveloped RNA viruses that have a diameter of about 700?? and can be lethal human pathogens. Entry of virus into host cells by endocytosis is controlled by two envelope glycoproteins, E1 and E2. The E2-E1 heterodimers form 80 trimeric spikes on the icosahedral virus surface, 60 with quasi-three-fold symmetry and 20 coincident with the icosahedral three-fold axes arranged with T = 4 quasi-symmetry. The E1 glycoprotein has a hydrophobic fusion loop at one end and is responsible for membrane fusion. The E2 protein is responsible for receptor binding and protects the fusion loop at neutral pH. The lower pH in the endosome induces the virions to undergo an irreversible conformational change in which E2 and E1 dissociate and E1 forms homotrimers, triggering fusion of the viral membrane with the endosomal membrane and then releasing the viral genome into the cytoplasm. Here we report the structure of an alphavirus spike, crystallized at low pH, representing an intermediate in the fusion process and clarifying the maturation process. The trimer of E2-E1 in the crystal structure is similar to the spikes in the neutral pH virus except that the E2 middle region is disordered, exposing the fusion loop. The amino- and carboxy-terminal domains of E2 each form immunoglobulin-like folds, consistent with the receptor attachment properties of E2.     

Independent recruitment of a conserved developmental mechanism during leaf evolution

Vascular plants evolved in the Middle to Late Silurian period, about 420 million years ago. The fossil record indicates that these primitive plants had branched stems with sporangia but no leaves. Leaf-like lateral outgrowths subsequently evolved on at least two independent occasions. In extant plants, these events are represented by microphyllous leaves in lycophytes (clubmosses, spikemosses and quillworts) and megaphyllous leaves in euphyllophytes (ferns, gymnosperms and angiosperms). Our current understanding of how leaves develop is restricted to processes that operate during megaphyll formation. Because microphylls and megaphylls evolved independently, different mechanisms might be required for leaf formation. Here we show that this is not so. Gene expression data from a microphyllous lycophyte, phylogenetic analyses, and a cross-species complementation experiment all show that a common developmental mechanism can underpin both microphyll and megaphyll formation. We propose that this mechanism might have operated originally in the context of primitive plant apices to facilitate bifurcation. Recruitment of this pathway to form leaves occurred independently and in parallel in different plant lineages.     

急倾斜重复采动软岩巷道失稳破坏分析

基于赵家坝煤矿3964巷道围岩稳定性控制,采用地质雷达现场探测和理论分析结合的手段确定了巷道围岩的松动圈范围,并对多次重复采动条件下急倾斜煤层软岩巷道围岩失稳破坏机理进行分析。结果表明:复杂的地质力学环境、重复采动影响和支护方式及参数不合理是造成巷道呈"底板隆起,顶板下挫"相互错动的非对称性变形破坏的主要原因。在此基础上,提出了以高强高预应力扭矩应力锚杆、高强度让压均压锚索为基础的非对称性多介质结构耦合支护对策,并在生产实践中取得了良好的技术经济效益。     

脉胞菌节律调控网络的鲁棒性

生物钟是生命活动以24小时左右为周期的波动,但是其一旦遭到扰乱,会导致严重后果.因而,保持其振荡的鲁棒性与周期长度(接近24小时)的鲁棒性是生物钟的核心问题.有研究发现,生物钟网络中除了一个主要的负反馈调节之外,通常都有其他的反馈调控机制,如脉胞菌的主要反馈是转录调控,但有额外的转录后调控.我们猜想:这些额外反馈调控主要用来维持生物钟的鲁棒性.我们在原有三变量脉胞菌模型基础上,增加了额外调控项,运用Matlab数值模拟,分别对比是否有额外调控的两种情况对环境的容忍程度,得以验证我们的猜想.进一步我们求出在保持振荡时,脉胞菌所能容忍的环境变化范围的理论值,对比理论值和模拟值,引入Hopf 分叉定理进行理论分析,为理解生物钟鲁棒性机理和相关疾病原因提供理论基础.     

Simulation of heat transfer in steel billets during continuous casting

This work is focused on the development of computational algorithms to create a simulator for solving the heat transfer during the continuous casting process of steel. The temperatures and the solid shell thickness profiles were calculated and displayed on the screen for a billet through a defined continuous casting plant (CCP). The algorithms developed to calculate billet temperatures, involve the solutions of the corresponding equations for the heat removal conditions such as radiation, forced convection, and conduction according to the billet position through the CCP. This is done by a simultaneous comparison with the kinematics model previously developed. A finite difference method known as Crank-Nicholson is applied to solve the two-dimensional computational array (2D model). Enthalpy (H_I,J) and temperature (T_I,J) in every node are updated at each step time. The routines to display the results have been developed using a graphical user interface (GUI) in the programming language C++. Finally, the results obtained are compared with those of industrial trials for the surface temperature of three steel casters with different plant configurations in different casting conditions.