A large population of 'Lyman-break' galaxies in a protocluster at redshift z approximately 4.1

The most massive galaxies and the richest clusters are believed to have emerged from regions with the largest enhancements of mass density relative to the surrounding space. Distant radio galaxies may pinpoint the locations of the ancestors of rich clusters, because they are massive systems associated with 'overdensities' of galaxies that are bright in the Lyman-alpha line of hydrogen. A powerful technique for detecting high-redshift galaxies is to search for the characteristic 'Lyman break' feature in the galaxy colour, at wavelengths just shortwards of Lyalpha, which is due to absorption of radiation from the galaxy by the intervening intergalactic medium. Here we report multicolour imaging of the most distant candidate protocluster, TN J1338-1942 at a redshift z approximately 4.1. We find a large number of objects with the characteristic colours of galaxies at that redshift, and we show that this excess is concentrated around the targeted dominant radio galaxy. Our data therefore indicate that TN J1338-1942 is indeed the most distant cluster progenitor of a rich local cluster, and that galaxy clusters began forming when the Universe was only ten per cent of its present age.     

轿车车门断面设计的原则和方法

介绍了轿车车门断面设计的基本方法,探讨了控制断面的作用和设置原则．结合车门间隙、车门密封、车门运动校核及车门结构件连接等具体设计过程,分析了车门断面设计中需要考虑的控制要素,阐述了轿车车门断面设计的基本思路和程序．     

Cis-trans isomerization at a proline opens the pore of a neurotransmitter-gated ion channel

5-hydroxytryptamine type 3 (5-HT3) receptors are members of the Cys-loop receptor superfamily. Neurotransmitter binding in these proteins triggers the opening (gating) of an ion channel by means of an as-yet-uncharacterized conformational change. Here we show that a specific proline (Pro 8*), located at the apex of the loop between the second and third transmembrane helices (M2-M3), can link binding to gating through a cis-trans isomerization of the protein backbone. Using unnatural amino acid mutagenesis, a series of proline analogues with varying preference for the cis conformer was incorporated at the 8* position. Proline analogues that strongly favour the trans conformer produced non-functional channels. Among the functional mutants there was a strong correlation between the intrinsic cis-trans energy gap of the proline analogue and the activation of the channel, suggesting that cis-trans isomerization of this single proline provides the switch that interconverts the open and closed states of the channel. Consistent with this proposal, nuclear magnetic resonance studies on an M2-M3 loop peptide reveal two distinct, structured forms. Our results thus confirm the structure of the M2-M3 loop and the critical role of Pro 8* in the 5-HT3 receptor. In addition, they suggest that a molecular rearrangement at Pro 8* is the structural mechanism that opens the receptor pore.