Distinctive termini characterize two families of human endogenous retroviral sequences

Human DNA contains many copies of endogenous retroviral sequences. Characterization of molecular clones of these structures reveals the existence of two related families. One family consists of full-length (8.8 kilobases) proviral structures, with typical long terminal repeates (LTR's). The other family consists of structures, which contain only 4.1 kilobases of gag-pol sequences, bounded by a tandem array of imperfect repeats 72 to 76 base pairs in length. Typical LTR sequences that exist as solitary elements in the genome were cloned and characterized.     

X chromosome-linked transmission and expression of retroviral genomes microinjected into mouse zygotes

A genomic clone consisting of the Moloney leukemia proviral genome with moderately repetitive mouse sequences was microinjected into the pronucleus of a mouse zygote. An animal was derived that carried multiple copies of proviral DNA in a tandem array. No evidence for homologous recombination was obtained. The viral genome was expressed in this animal and was transmitted as a single unit to its offspring. Subsequent breeding studies revealed that the proviral DNA had integrated on an X chromosome.     

Construction and recovery of viable retroviral genomes carrying a bacterial suppressor transfer RNA gene

The integration of retroviral genomes into cellular DNA can induce mutations by altering the expression of nearby cellular genes and can serve to identify the gene affected. The construction of a retrovirus that stably carries a suppressor transfer RNA gene from Escherichia coli has allowed facile recovery of the viral genome in vectors marked with amber mutations. This virus can be used for rapid isolation of cellular sequences at the site of proviral insertion.     

英语教学中的几点新做法

英语作为一种交流工具 ,是当今社会对高素质人才要求的必备条件之一。本文从 5个方面阐述了如何把好课堂这一关键环节 ,为学生学习英语找到一条正确途径。     

Untangling genomes from metagenomes: revealing an uncultured class of marine Euryarchaeota

Ecosystems are shaped by complex communities of mostly unculturable microbes. Metagenomes provide a fragmented view of such communities, but the ecosystem functions of major groups of organisms remain mysterious. To better characterize members of these communities, we developed methods to reconstruct genomes directly from mate-paired short-read metagenomes. We closed a genome representing the as-yet uncultured marine group II Euryarchaeota, assembled de novo from 1.7% of a metagenome sequenced from surface seawater. The genome describes a motile, photo-heterotrophic cell focused on degradation of protein and lipids and clarifies the origin of proteorhodopsin. It also demonstrates that high-coverage mate-paired sequence can overcome assembly difficulties caused by interstrain variation in complex microbial communities, enabling inference of ecosystem functions for uncultured members.     

携带人血清白蛋白cDNA的逆转录病毒质粒载体的构建

采用粘平端定向基因克隆技术,成功地构建了携带ＨＳＡｃＤＮＡ基因片段的重组逆转录病毒质粒载体ＧＩＮＨＳＡａ。经限制性酶切鉴定和菌落原位杂交证明ＨＳＡｃＤＮＡ已克隆到Ｇ１Ｎａ当中并得到９个重组病毒质粒载体阳性克隆。     

A role for DNA-PK in retroviral DNA integration

Retroviral DNA integration is catalyzed by the viral protein integrase. Here, it is shown that DNA-dependent protein kinase (DNA-PK), a host cell protein, also participates in the reaction. DNA-PK-deficient murine scid cells infected with three different retroviruses showed a substantial reduction in retroviral DNA integration and died by apoptosis. Scid cell killing was not observed after infection with an integrase-defective virus, suggesting that abortive integration is the trigger for death in these DNA repair-deficient cells. These results suggest that the initial events in retroviral integration are detected as DNA damage by the host cell and that completion of the integration process requires the DNA-PK-mediated repair pathway.     

Defective presentation of endogenous antigen by a cell line expressing class I molecules

Cytotoxic T lymphocytes (CTLs) recognize class I major histocompatibility complex (MHC) molecules associated with antigenic peptides derived from endogenously synthesized proteins. Binding to such peptides is a requirement for class I assembly in the endoplasmic reticulum (ER). A mutant human cell line, T2, assembles and transports to its surface some, but not all, class I MHC molecules. The class I molecules expressed on the surface of T2 do not present peptides derived from cytosolic antigens, although they can present exogenously added peptides to CTL. The transported class I molecules may interact weakly with an unknown retaining factor in the ER such that they can assemble despite the relative shortage of peptides.     

河北农业大学2008级本科新生A班听说课现状调查分析和教学方法探索

2008年,河北农业大学对本科非英语专业新生大学英语教学进行了新的探索和实践,实行了A、B班分级教学体制,以最大程度提高学生的英语水平和应用能力。通过问卷调查及其描述性统计和定量分析,对非英语专业A班新生的英语听力学习现状做了探讨,结果发现：（1）学生听力基础较差,且心理焦躁,负面情绪较重;（2）口语被学生给予了极大的关注,学生希望学会用英语与人交流;（3）听说课堂中听力材料很受关注;（4）学生的课后听力练习时间不够,需要监控。在此基础上,对A班听说教学有了新的认识,形成了新的思路。     

Ctenocystoidea: new class of primitive echinoderms

The oldest known "carpoids" of the echinoderm subphylum Homalozoa have been discovered in lower Middle Cambrian rocks of northern Utah. They were free-living benthonic animals characterized by a unique ctenoid feeding apparatus and flattened flexible theca with near bilateral symmetry. A new class, Ctenocystoidea, is proposed to include the new genus and new species, Ctenocystis utahensis.     

当代中国新阶层阶层意识的朦胧与觉醒

改革开放以来,我国的社会结构发生了深刻变化,出现了许多新的社会阶层,新阶层又逐渐产生了阶层意识。这些新阶层的阶层意识目前尚处于自发和朦胧状态,远未形成整体意义上的共同阶层意识和整体的社会责任,共同的阶层意识对社会主义现代化建设意义重大,我们应着力培育和健全他们共同的阶层意识。     

A class I antigen, HLA-G, expressed in human trophoblasts

The alpha chain of the human histocompatibility antigen HLA-G was identified as an array of five 37- to 39-kilodalton isoforms by the use of two-dimensional gel electrophoresis. Both cell-associated and secreted HLA-G antigens are prominent in first trimester villous cytotrophoblasts and are greatly reduced in third trimester cytotrophoblasts. Allelic variation was not detected, an indication that HLA-G is not obviously polymorphic in cytotrophoblasts. Among the following choriocarcinoma cell lines studied, HLA-G is expressed in JEG but not in Jar or BeWo. Expression of endogenous HLA-G genes has not been found in normal lymphoid cells. Thus, HLA-G is subject to both cell type-specific and developmental regulation and is expressed in early gestation human cytotrophoblasts.