Structure of a tyrosyl-tRNA synthetase splicing factor bound to a group I intron RNA

The 'RNA world' hypothesis holds that during evolution the structural and enzymatic functions initially served by RNA were assumed by proteins, leading to the latter's domination of biological catalysis. This progression can still be seen in modern biology, where ribozymes, such as the ribosome and RNase P, have evolved into protein-dependent RNA catalysts ('RNPzymes'). Similarly, group I introns use RNA-catalysed splicing reactions, but many function as RNPzymes bound to proteins that stabilize their catalytically active RNA structure. One such protein, the Neurospora crassa mitochondrial tyrosyl-tRNA synthetase (TyrRS; CYT-18), is bifunctional and both aminoacylates mitochondrial tRNA(Tyr) and promotes the splicing of mitochondrial group I introns. Here we determine a 4.5-A co-crystal structure of the Twort orf142-I2 group I intron ribozyme bound to splicing-active, carboxy-terminally truncated CYT-18. The structure shows that the group I intron binds across the two subunits of the homodimeric protein with a newly evolved RNA-binding surface distinct from that which binds tRNA(Tyr). This RNA binding surface provides an extended scaffold for the phosphodiester backbone of the conserved catalytic core of the intron RNA, allowing the protein to promote the splicing of a wide variety of group I introns. The group I intron-binding surface includes three small insertions and additional structural adaptations relative to non-splicing bacterial TyrRSs, indicating a multistep adaptation for splicing function. The co-crystal structure provides insight into how CYT-18 promotes group I intron splicing, how it evolved to have this function, and how proteins could have incrementally replaced RNA structures during the transition from an RNA world to an RNP world.     

香港对新移民的管理和安置

与复杂的跨国网络与跨界互动相比,公民身份却划分了清晰的边界。香港回归中国主权后,对其公民身份的类别划分带来了极大的挑战。依照香港基本法中对居留权的定义,很大一部分——近一百六十七万人——可以享有这个权利,反映出香港与内地的跨界社会关系有着复杂历史。尽管香港与内地的社会和经济高度统一,但其边界仍然严格限制了人口从内地向香港流动。这种既统一又分离的状况导致了一种复杂的对立,部分地反映在关于居留权的争论。即使那些已经成功移民香港的内地居民也面临着各种挑战,其中最严峻的是怎样在香港这样一个住房价格最高的城市中获得可以负担的住所。因为新移民无法申请公营房屋,但这些房屋却占香港房屋总数的一半。而其私人住房的价格又是世界上最贵的。     

Rapid discharge connects Antarctic subglacial lakes

The existence of many subglacial lakes provides clear evidence for the widespread presence of water beneath the East Antarctic ice sheet, but the hydrology beneath this ice mass is poorly understood. Such knowledge is critical to understanding ice flow, basal water transfer to the ice margin, glacial landform development and subglacial lake habitats. Here we present ice-sheet surface elevation changes in central East Antarctica that we interpret to represent rapid discharge from a subglacial lake. Our observations indicate that during a period of 16 months, 1.8 km3 of water was transferred over 290 km to at least two other subglacial lakes. While viscous deformation of the ice roof above may moderate discharge, the intrinsic instability of such a system suggests that discharge events are a common mode of basal drainage. If large lakes, such as Lake Vostok or Lake Concordia, are pressurizing, it is possible that substantial discharges could reach the coast. Our observations conflict with expectations that subglacial lakes have long residence times and slow circulations, and we suggest that entire subglacial drainage basins may be flushed periodically. The rapid transfer of water between lakes would result in large-scale solute and microbe relocation, and drainage system contamination from in situ exploration is, therefore, a distinct risk.     

Thermodynamic control of asymmetric amplification in amino acid catalysis

Ever since Pasteur noticed that tartrate crystals exist in two non-superimposable forms that are mirror images of one another--as are left and right hands--the phenomenon of chirality has intrigued scientists. On the molecular level, chirality often has a profound impact on recognition and interaction events and is thus important to biochemistry and pharmacology. In chemical synthesis, much effort has been directed towards developing asymmetric synthesis strategies that yield product molecules with a significant excess of either the left-handed or right-handed enantiomer. This is usually achieved by making use of chiral auxiliaries or catalysts that influence the course of a reaction, with the enantiomeric excess (ee) of the product linearly related to the ee of the auxiliary or catalyst used. In recent years, however, an increasing number of asymmetric reactions have been documented where this relationship is nonlinear, an effect that can lead to asymmetric amplification. Theoretical models have long suggested that autocatalytic processes can result in kinetically controlled asymmetric amplification, a prediction that has now been verified experimentally and rationalized mechanistically for an autocatalytic alkylation reaction. Here we show an alternative mechanism that gives rise to asymmetric amplification based on the equilibrium solid-liquid phase behaviour of amino acids in solution. This amplification mechanism is robust and can operate in aqueous systems, making it an appealing proposition for explaining one of the most tantalizing examples of asymmetric amplification-the development of high enantiomeric excess in biomolecules from a presumably racemic prebiotic world.     

Observation of the radiative decay mode of the free neutron

The theory of quantum electrodynamics (QED) predicts that beta decay of the neutron into a proton, electron and antineutrino should be accompanied by a continuous spectrum of soft photons. While this inner bremsstrahlung branch has been previously measured in nuclear beta and electron capture decay, it has never been observed in free neutron decay. Recently, the photon energy spectrum and branching ratio for neutron radiative decay have been calculated using two approaches: a standard QED framework and heavy baryon chiral perturbation theory (an effective theory of hadrons based on the symmetries of quantum chromodynamics). The QED calculation treats the nucleons as point-like, whereas the latter approach includes the effect of nucleon structure in a systematic way. Here we observe the radiative decay mode of free neutrons, measuring photons in coincidence with both the emitted electron and proton. We determined a branching ratio of (3.13 +/- 0.34) x 10(-3) (68 per cent level of confidence) in the energy region between 15 and 340 keV, where the uncertainty is dominated by systematic effects. The value is consistent with the predictions of both theoretical approaches; the characteristic energy spectrum of the radiated photons, which differs from the uncorrelated background spectrum, is also consistent with the calculated spectrum. This result may provide opportunities for more detailed investigations of the weak interaction processes involved in neutron beta decay.     

The finished DNA sequence of human chromosome 12

Human chromosome 12 contains more than 1,400 coding genes and 487 loci that have been directly implicated in human disease. The q arm of chromosome 12 contains one of the largest blocks of linkage disequilibrium found in the human genome. Here we present the finished sequence of human chromosome 12, which has been finished to high quality and spans approximately 132 megabases, representing approximately 4.5% of the human genome. Alignment of the human chromosome 12 sequence across vertebrates reveals the origin of individual segments in chicken, and a unique history of rearrangement through rodent and primate lineages. The rate of base substitutions in recent evolutionary history shows an overall slowing in hominids compared with primates and rodents.