硅谷：美国高科技发展的标志

美国硅谷的发展在整个世界引起了广泛的兴趣和关注 ,硅谷已成为美国高科技发展的标志 ,任何其他国家和地区的高科技的发展都不能和硅谷相比。经济论坛硅谷位于海湾和山谷之间 ,东西宽１６公里 ,南北跨度５０公里 ,并且正在不断延长。硅谷有４０００家公司 ,他们都很小。８０年代后期 ,ＩＢＭ公司总裁访问了我大学 ,他告诉我硅谷３０００多家公司中只有少于１００家的年收入超过１亿美元。硅谷是从事信息技术的、小的、成长型公司的产地。直到硅半导体芯片运用于微电子工业 ,硅谷才成为美国乃至世界的高科技工业的中心。现在 ,许多大公司将生…     

Structure of the S-adenosylmethionine riboswitch regulatory mRNA element

Riboswitches are cis-acting genetic regulatory elements found in the 5'-untranslated regions of messenger RNAs that control gene expression through their ability to bind small molecule metabolites directly. Regulation occurs through the interplay of two domains of the RNA: an aptamer domain that responds to intracellular metabolite concentrations and an expression platform that uses two mutually exclusive secondary structures to direct a decision-making process. In Gram-positive bacteria such as Bacillus species, riboswitches control the expression of more than 2% of all genes through their ability to respond to a diverse set of metabolites including amino acids, nucleobases and protein cofactors. Here we report the 2.9-angstroms resolution crystal structure of an S-adenosylmethionine (SAM)-responsive riboswitch from Thermoanaerobacter tengcongensis complexed with S-adenosylmethionine, an RNA element that controls the expression of several genes involved in sulphur and methionine metabolism. This RNA folds into a complex three-dimensional architecture that recognizes almost every functional group of the ligand through a combination of direct and indirect readout mechanisms. Ligand binding induces the formation of a series of tertiary interactions with one of the helices, serving as a communication link between the aptamer and expression platform domains.