Translational control of intron splicing in eukaryotes

Most eukaryotic genes are interrupted by non-coding introns that must be accurately removed from pre-messenger RNAs to produce translatable mRNAs. Splicing is guided locally by short conserved sequences, but genes typically contain many potential splice sites, and the mechanisms specifying the correct sites remain poorly understood. In most organisms, short introns recognized by the intron definition mechanism cannot be efficiently predicted solely on the basis of sequence motifs. In multicellular eukaryotes, long introns are recognized through exon definition and most genes produce multiple mRNA variants through alternative splicing. The nonsense-mediated mRNA decay (NMD) pathway may further shape the observed sets of variants by selectively degrading those containing premature termination codons, which are frequently produced in mammals. Here we show that the tiny introns of the ciliate Paramecium tetraurelia are under strong selective pressure to cause premature termination of mRNA translation in the event of intron retention, and that the same bias is observed among the short introns of plants, fungi and animals. By knocking down the two P. tetraurelia genes encoding UPF1, a protein that is crucial in NMD, we show that the intrinsic efficiency of splicing varies widely among introns and that NMD activity can significantly reduce the fraction of unspliced mRNAs. The results suggest that, independently of alternative splicing, species with large intron numbers universally rely on NMD to compensate for suboptimal splicing efficiency and accuracy.     

定量和定性风险评估

风险评估工作在海事工作运用广泛,其中包括定量和定性两种方式.对两种方法的多种具体分析工具加以研究,在此基础上对两者的重要性以及具体分析过程进行分析,最后总结出定量和定性风险评估的关系、各自优点以及局限性.     

定性与定量风险评估方法之比较

分别以危害及可操作性研究和事故树为例,比较定性与定量风险评估方法在应用领域、应用特点等方面的优缺点,得出这两种方法应在事故分析的不同阶段应用于不同领域内的结论.     

The topological configuration and conformational analysis of mRNA in translation

The theoretical model construction of mRNA hairpin structure and single-stranded structure as well as the simulation studies on RNA structure determined by the X-ray crystal diffraction and nuclear magnetic resonance revealed that in translation, after mRNA being unfolded into single-stranded structure, its topological configuration was closely correlative with the original hairpin structure. The conformational features of single-stranded mRNA appeared as helical regions alternating with curly regions to different extents, which might exert the influence on the folding of nascent polypeptide by various regulating effects including different translational rates.     

CPEB and two poly(A) polymerases control miR-122 stability and p53 mRNA translation

Cytoplasmic polyadenylation-induced translation controls germ cell development, neuronal synaptic plasticity and cellular senescence, a tumour-suppressor mechanism that limits the replicative lifespan of cells. The cytoplasmic polyadenylation element binding protein (CPEB) promotes polyadenylation by nucleating a group of factors including defective in germline development 2 (Gld2), a non-canonical poly(A) polymerase, on specific messenger RNA (mRNA) 3' untranslated regions (UTRs). Because CPEB regulation of p53 mRNA polyadenylation/translation is necessary for cellular senescence in primary human diploid fibroblasts, we surmised that Gld2 would be the enzyme responsible for poly(A) addition. Here we show that depletion of Gld2 surprisingly promotes rather than inhibits p53 mRNA polyadenylation/translation, induces premature senescence and enhances the stability of CPEB mRNA. The CPEB 3' UTR contains two miR-122 binding sites, which when deleted, elevate mRNA translation, as does an antagomir of miR-122. Although miR-122 is thought to be liver specific, it is present in primary fibroblasts and destabilized by Gld2 depletion. Gld4, a second non-canonical poly(A) polymerase, was found to regulate p53 mRNA polyadenylation/translation in a CPEB-dependent manner. Thus, translational regulation of p53 mRNA and cellular senescence is coordinated by Gld2/miR-122/CPEB/Gld4.     

5'-Terminal 7-methylguanosine in eukaryotic mRNA is required for translation.

Unmethylated reovirus and VSV mRNAs are specifically methylated to form 5'-terminal structures of the type, m-7-G(5')ppp(5')N by protein synthesising extracts prepared from wheat germ and mouse L cells. Reticulocyte mRNA also contains 5'-terminal m-7-G. MRNAs having 5'-terminal m-7-G stimulate protein synthesis in vitro. Removal of m-7-G by beta-elimination abolishes translation of the mRNAs.     

定性分析和定量分析在海事风险评估中的应用

本文讨论定性分析和定量分析的定义、特点,并结合实例分析他们的优点、缺点及在海事风险评估中的实际应用.     

定性和定量评估在航行安全风险分析中的应用

介绍了定性和定量风险评估方法,利用隐患可行性研究法(HAZOPs)对珠江口水域锚地设置的风险进行定性分析,采用事故树法对一起船舶碰撞事故进行风险定量分析,并对定性和定量风险评估方法的优缺点进行比较.     

对2-λ的定性解释和定量计算

首先对输运过程中穿过参考截面△S的气体分子自由程的平均值-λ穿为什么不等于-λ而是大于-λ作出了定性解释,然后通过定量计算求出了-λ穿=2-λ.     

对2-λ的定性解释和定量计算

首先对输运过程中穿过参考截面△S的气体分子自由程的平均值-λ穿为什么不等于-λ而是大于-λ作出了定性解释,然后通过定量计算求出了-λ穿=2-λ.     

截叶铁扫帚提取物的GC-MS分析及羽扇烯酮的定性定量分析

采用乙醇超声提取三个不同月份的截叶铁扫帚,并运用GC MS法分析截叶铁扫帚乙醚供试液的化学成分和羽扇烯酮的相对含量.结果显示从截叶铁扫帚中共检测出61种成分,5月份截叶铁扫帚的成分有42种,8月份有39种,11月份有42种,均主要为烷烃、萜类、芳香类物质,并首次发现截叶铁扫帚中含有羽扇烯酮,测得5月、8月、11月截叶铁扫帚中羽扇烯酮的含量分别为1.734mg/g、2.235mg/g、1.111mg/g.通过比较三个不同月份截叶铁扫帚中的化学成分及其含量,8月份截叶铁扫帚中化学成分种类最少,但羽扇烯酮含量最高.