2007年10月25日Nature中文摘要

封面故事：固体中电子运动时间差别的实验研究
Attosecond spectroscopy in condensed matter
电子在固体中能以非常高的速度运动，能在几十至几百阿秒（一阿秒等于一秒的
十亿分之一的十亿分之一，即10-18秒）内穿过原子层及界面。这些非常短的运
动时间最终将限制未来电子装置的运行速度。现在，物理学家对这种电子动态进
行了实时实验探索。本期封面所示为在一种固体中第一次进行的阿秒光谱测量工
作，显示了在一个钨晶体中进行光激发之后两种不同类型的电子在运动时间上有
一个110阿秒的差别。能够对在固体中、在相距仅几个原子的距离内运动的电子
进行时间测定的能力，使得我们有可能对以极限速度发生的固态电子过程进行研
究，从而帮助推进如计算、数据存储和光伏发电等技术，这些技术都依赖于对在
越来越小的固体物质结构中电子传输进行极为精确的控制。（Letter p. 1029;
News & Views）封面图片：Barbara Ferus，马普学会量子光学研究所阿秒及强
场物理实验室。

比《京都议定书》更激进的减排措施（AFTER KYOTO）
Time to ditch Kyoto
Gwyn Prins 和 Steve Rayner富有挑战性地提出，《京都议定书》正在走向失
败。他们认为，现在是在气候政策方面采取更为激进的措施的时候了。他们提出
的替代方案包括对排放交易采取一种从上向下的办法、政府出巨资支持能源技术
研究以及增加应对气候变化的投资等。（Commentary p. 973）

结肠癌干细胞的标记（The mark of a stem cell）
Identification of stem cells in small intestine and colon by marker
gene Lgr5
小肠和结肠壁上的细胞在整个生命中能够迅速再生。虽然它们是所有干细胞中功
能最明确的细胞之一，但要分离出纯粹的或高度富集的小肠上皮干细胞群一直被
证明是一个难以实现的目标。现在，这个目标已经实现。Lgr5基因（Wnt细胞信
号系统）已经被确定为几种成年组织中和癌症中干细胞的一个独特标记。Lgr5最
初是在结肠癌细胞中发现的，现在在恶化前的小鼠腺瘤中也发现了，这说明
Lgr5+也许是结肠癌干细胞的一个标记。（Article p. 1003）

关于生长素与植物发育的一个新模型（A PLETHORA of gradients）
Auxin transport is sufficient to generate a maximum and gradient
guiding root growth / PLETHORA proteins as dose-dependent master
regulators of Arabidopsis root development
“生长素极性输送”是植物所有发育过程中的一个因素，但关于这个因素是怎样
发挥作用的仍有很多东西需要了解。在细胞中，“生长素定向输送”由PIN蛋白
载体复合物控制。现在，研究人员已经开发出一个新的计算模型，来描述根系中
细胞内和细胞间的扩散及由PIN帮助进行的生长素输送。该模型涉及植物发育的
两大问题。第一，它支持关于生长素在引导根生长方面有一个最大值和一个梯度
的概念。第二，它反映了一个分子何以能够产生一系列信息：整个器官生长区的
一个浓度梯度与生长素最大值密切相关。另一篇相关的论文表明，４个PLETHORA
基因是根发育所必不可少的，是生长素梯度的候选输出因子。（Article p.
1008; Letter p. 1053; News & Views）

土星A-环中新发现８个推进器形状的扰动（Propelled to prominence）
A belt of moonlets in Saturn's A ring
在土星的A-环中观测到４个推进器形状的扰动（是在2004年由“卡西尼”拍摄的
图片中发现的），这是在该环中存在小型顽石大小的嵌入式卫星的证据。过去科
学家认为，该环中的这种卫星将不止４个。实际上的确有更多的这种卫星：现在
科学家在后来的一组“卡西尼”图片中发现了８个新的推进器形状的特征，这组
图片覆盖了整个Ａ-环。引起这些扰动的小卫星半径在30米到70米之间，可能是
一颗较大的嵌入式卫星分解后形成的，后者曾经在这个位置绕土星的环运行。
（Letter p. 1019）

“中子滴线”及两种多中子镁和铝同位素的检测（The edge of stability）
Discovery of 40Mg and 42Al suggests neutron drip-line slant towards
heavier isotopes
一定数量的质子可以结合多少个中子？这个极限值被称为“中子滴线”
（neutron drip line），它在核物理中具有根本性的重要性。物理学家只知道
最轻元素的这个参数，这是由于单个粒子之间存在复杂的相互作用以及它们所具
有的集体性量子效应，但两种富含中子的镁和铝同位素的检测，是科学家向确定
这一极限所迈出的一大步。这两种同位素分别是40Mg 和 42Al，是预测存在于稳
定性边缘的“滴线”核，是在美国国家超导回旋加速器实验室的耦合回旋加速器
装置上产生的。42Al的出现是第一次通过实验表明，中子非常多的铝同位素的稳
定性相对于镁被增强，它也表明，“中子滴线”与稳定同位素的距离可能要比人
们所预料的更远。这些发现说明，对于原子序数在12以上的原子核来说，“滴
线”可能是当前最新的和预测中的核科学装置所不能探测到的。（Letter p.
1022; News & Views）

关于超固体氦热力学特征的新证据（Supersolid signature）
Probable heat capacity signature of the supersolid transition
2004年，科学家在实验中明显观测到超固体氦的存在，这在物理学界引起人们很
大兴趣，也在物理学界之外引起人们的好奇。但预计将伴随这种转变出现的热力
学特征却没有被发现。现在，科学家提出了关于这一问题的新证据。这种现象
（它与当液体氦进入超流体状态时所表现出的现象有相似之处）是在当固体的一
部分要与周围晶格的运动解除耦合时出现的。新的实验显示了一个热容峰值，它
的出现时间与物质解除耦合的开始时间巧合，这表明在氦-4的正常固体相与超固
体相之间存在一个真正的相变。（Letter p. 1025）

一种用于分析生物分子的全新质谱方法（Explosive spectrometry）
Clathrate nanostructures for mass spectrometry
质谱在检测生物分子方面有很大潜力，但现有方法存在几个缺陷，其中包括缺少
所需的灵敏度和需要基质分子促使分析对象发生离子化等。一种利用激光或离子
束来从纳米尺度的小囊中气化材料的全新方法有可能克服这些局限性。该方法涉
及一种特制的表面，它含有“启动分子”，这些“启动分子”在受到激光或离子
束照射时会猛烈爆发。这种爆发释放出离子化的分析物分子，它们被吸收到表面
上，使其能够被检测到。这种被称为NIMS（即“纳米结构启动质谱”），能以极
高的灵敏度分析非常小的区域，从而允许对肽阵列、血液、尿和单个细胞进行分
析，而且甚至还能用于组织成像。（Letter p. 1033）

关于异常3He/4He比的“矛盾”得到解释（Paradox? What paradox?）
Non-equilibrium degassing and a primordial source for helium in
ocean-island volcanism
地球含有两种氦同位素：一种是原始的3He，它是在地球形成过程中进入地球
的；另一种是4He，主要是在地球漫长的历史中由铀和钍衰变产生的。从夏威夷
等热点地区喷发出的岩浆具有相对较高的3He/4He比，因此有人提出，它们来自
一个没有发生过气体溢出的、原始的地幔库。然而矛盾的是，在热点岩浆中所观
测到的总氦浓度要比所预料的低很多。这使得关于地球地幔结构和动态的传统观
点受到了质疑。现在，Helge Gonnermann 和 Sujoy Mukhopadhyay发现，这一明
显的“矛盾”可以用稀有气体扩散进来自喷发的熔岩的气泡中的不同速度来解
释。结果证实，认为存在一个原始地幔库的观点的确有可能解释在这样的热点岩
浆中所观测到的异常3He/4He比。（Letter p. 1037）

捕食者和被捕食者种群稳定性之谜被揭开（Super groups）
Group formation stabilizes predator–prey dynamics
很多、甚至大多数捕食者和被捕食者种群之稳定让人不解，尽管事实上这种稳定
性预计是基于一般会出现的生态互动的种类。Fryxell等人提出一个假说：有助
于这种稳定性的一个因素是捕食者搜索效率的降低，而这种降低是由捕食者和被
捕食者的群体形成引起的。关于来自Serengeti国家公园中的狮子和它们的大型
食草动物猎物的野外观测数据表明，捕食者和被捕食者的群体形成都对种群动态
有巨大影响，从而使本来可能会非常不稳定的一种状况产生稳定性。（Letter
p. 1041; News & Views）

原肠胚形成过程的实验观测研究（Gastrulation revealed）
The amniote primitive streak is defined by epithelial cell
intercalation before gastrulation
原肠胚形成是胚胎形成中的一个关键事件。控制羊膜动物中原肠胚形成运动的机
制不是很清楚，并且经常是从关于其他物种的研究来推断的。现在，Voiculescu
等人报告了对在原肠胚形成过程中活鸡胚胎中各种细胞运动的首次时差分析
（time-lapse analysis）。多光子时差显微镜分析显示了与在鱼类和两栖类中
所描述的运动不同的运动，这对脊椎动物胚胎学研究及原肠胚形成的演化研究有
重要意义。尤其是，这项工作显示了一个原肠胚形成之前的早期插入事件，它可
能是回答关于羊膜动物原始条纹是怎样从远古胚孔演化而来的这样一个经典问题
的关键。（Letter p. 1049）

----------------------
NEWS AND VIEWS
----------------------
Plant biology: Plumbing the pattern of roots pp991 - 992
The results of a powerful combination of computer modelling and
experimental tests can account for the establishment of gradients of
the plant molecule auxin and for major patterning elements in the
plant root.
Bruce Veit
doi:10.1038/449991a
http://www.nature.com/nature/journal/v449/n7165/full/449991a.html

Nuclear physics: Neutrons cross the line pp992 - 993
For most atomic nuclei, the maximum number of neutrons that can be
bound is unknown. The discovery of two neutron-rich nuclei a?” and
the confirmed absence of others ― might help solve this conundrum.
Paul-Henri Heenen
doi:10.1038/449992a
http://www.nature.com/nature/journal/v449/n7165/full/449992a.html

Cancer: Mixing cocktails pp993 - 996
A major hurdle in treating cancer is that tumour cells acquire drug
resistance. To overcome this problem, one strategy might be to
fine-tune the right mixture of drugs that target specific molecules.
Charles L. Sawyers
doi:10.1038/449993a
http://www.nature.com/nature/journal/v449/n7165/full/449993a.html

50 & 100 Years Ago p995
doi:10.1038/449995a
http://www.nature.com/nature/journal/v449/n7165/full/449995a.html

Population ecology: Group living and hungry lions pp996 - 997
Ecologists have necessarily had to simplify matters in looking at
predator-prey dynamics. Study of a situation in which predator and
prey live in groups reveals that a key process was previously
overlooked.
Tim Coulson
doi:10.1038/449996a
http://www.nature.com/nature/journal/v449/n7165/full/449996a.html

Attophysics: At a glance pp997 - 999
Measurements on the attosecond timescale had been limited to the
dynamics of electrons in an atomic gas. But a record has now been set
in a quite different context ― the photoemission of electrons from a
surface.
David M. Villeneuve
doi:10.1038/449997a
http://www.nature.com/nature/journal/v449/n7165/full/449997a.html

Biochemistry: Indifferent chaperones pp999 - 1000
How do nonspecific enzymes that help to correct RNA folding identify
misfolded structures among similar, properly folded RNAs? It seems
that careful discrimination has little to do with it.
Eckhard Jankowsky
doi:10.1038/449999a
http://www.nature.com/nature/journal/v449/n7165/full/449999a.html

Environmental science: Nutrients in synergy pp1000 - 1001
A literature meta-analysis of the effects of nitrogen and phosphorus
on plant growth prompts a thought-provoking inference ― that the
supply of, and demand for, these nutrients are usually in close
balance.
Eric A. Davidson and Robert W. Howarth
doi:10.1038/4491000a
http://www.nature.com/nature/journal/v449/n7165/full/4491000a.html

----------------------
ARTICLES
----------------------
Identification of stem cells in small intestine and colon by marker
gene Lgr5 pp1003 - 1007
The Lgr5 (also known as Gpr49) gene is identified as a unique, single
marker gene for adult stem cells and provides the first lineage
tracing data for long-lived pluripotent stem cells in the small
intestine and colon. These findings will have great impact on stem
cell and cancer research.
Nick Barker et al.
doi:10.1038/nature06196
Abstract:
http://www.nature.com/nature/journal/v449/n7165/abs/nature06196.html
Article:
http://www.nature.com/nature/journal/v449/n7165/full/nature06196.html

Auxin transport is sufficient to generate a maximum and gradient
guiding root growth pp1008 - 1013
A computational model that spans molecular and cellular levels
describing diffusion and PIN-facilitated auxin transport in and across
cells within the root system is experimentally tested. The model
strongly supports that plant roots are able to generate an auxin
maximum and a highly robust auxin gradient with morphogenic
properties.
Veronica A. Grieneisen et al.
doi:10.1038/nature06215
Abstract:
http://www.nature.com/nature/journal/v449/n7165/abs/nature06215.html
Article:
http://www.nature.com/nature/journal/v449/n7165/full/nature06215.html

Kinetic redistribution of native and misfolded RNAs by a DEAD-box
chaperone pp1014 - 1018
Hari Bhaskaran and Rick Russell
doi:10.1038/nature06235
Abstract:
http://www.nature.com/nature/journal/v449/n7165/abs/nature06235.html
Article:
http://www.nature.com/nature/journal/v449/n7165/full/nature06235.html

----------------------
LETTERS
----------------------
A belt of moonlets in Saturn's A ring pp1019 - 1021
The detection of eight new propellers in Saturn's A ring is reported,
indicating embedded moonlets with radii between 30-70m. The moonlets
found are concentrated in a narrow 3,000 km wide annulus at 130,000km
distance from Saturn.
Miodrag Sremcevic et al.
doi:10.1038/nature06224
Abstract:
http://www.nature.com/nature/journal/v449/n7165/abs/nature06224.html
Article:
http://www.nature.com/nature/journal/v449/n7165/full/nature06224.html

Discovery of 40Mg and 42Al suggests neutron drip-line slant towards
heavier isotopes pp1022 - 1024
The paper reports a significant advance in the determination of the
limit of how many neutrons a given number of protons can bind to
(known as the neutron drip line), with the discovery of two new
neutron-rich isotopes predicted to be drip line nuclei. Nuclei at the
drip line gain stability from an unpaired proton, which narrows the
shell gaps and provides the opportunity to bind many more neutrons.
T. Baumann et al.
doi:10.1038/nature06213
Abstract:
http://www.nature.com/nature/journal/v449/n7165/abs/nature06213.html
Article:
http://www.nature.com/nature/journal/v449/n7165/full/nature06213.html

Probable heat capacity signature of the supersolid transition pp1025
- 1028
In solid 4He, a fraction of the solid seems to decouple from the
motion of the surrounding lattice when subjected to oscillatory
motion. However, no thermodynamic signature of the possible supersolid
transition has been seen. This paper reports the finding of a heat
capacity peak that coincides with the onset of mass decoupling. This
complementary experimental evidence supports the existence of a
genuine transition between the normal solid and supersolid phases of
4He.
X. Lin, A. C. Clark and M. H. W. Chan
doi:10.1038/nature06228
Abstract:
http://www.nature.com/nature/journal/v449/n7165/abs/nature06228.html
Article:
http://www.nature.com/nature/journal/v449/n7165/full/nature06228.html

Attosecond spectroscopy in condensed matter pp1029 - 1032
When exposing a tungsten crystal to intense light, the travel times of
emitted electrons differ by 110 attoseconds, depending on whether they
were originally tightly bound to one atom in the crystal or
delocalized over many atoms. This ability to directly probe
fundamental aspects of solid-state electron dynamics could aid the
further development of modern technologies such as electronics,
information processing and photovoltaics.
A. L. Cavalieri et al.
doi:10.1038/nature06229
Abstract:
http://www.nature.com/nature/journal/v449/n7165/abs/nature06229.html
Article:
http://www.nature.com/nature/journal/v449/n7165/full/nature06229.html

Clathrate nanostructures for mass spectrometry pp1033 - 1036
Trent R. Northen et al.
doi:10.1038/nature06195
Abstract:
http://www.nature.com/nature/journal/v449/n7165/abs/nature06195.html
Article:
http://www.nature.com/nature/journal/v449/n7165/full/nature06195.html

Non-equilibrium degassing and a primordial source for helium in
ocean-island volcanism pp1037 - 1040
Helium concentrations and elemental ratios of He/Ne and He/Ar in
ocean-island basalts (such as Hawaii and Iceland) are an order of
magnitude lower than in mid-ocean ridge basalts. These observations
can be self-consistently explained by a higher CO2 content of
ocean-island basalts and disequilibrium, open-system degassing of the
erupting magma.
Helge M. Gonnermann and Sujoy Mukhopadhyay
doi:10.1038/nature06240
Abstract:
http://www.nature.com/nature/journal/v449/n7165/abs/nature06240.html
Article:
http://www.nature.com/nature/journal/v449/n7165/full/nature06240.html

Group formation stabilizes predator-prey dynamics pp1041 - 1043
Group formation by both predators and prey dramatically reduces
predator search efficiency. Field data on lions and their large
herbivore prey from Serengeti National Park suggest that group
formation by both predators and prey has an enormous impact on
Serengeti population dynamics, lending a stabilizing influence to what
would be otherwise be a highly unstable situation.
John M. Fryxell, Anna Mosser, Anthony R. E. Sinclair and Craig Packer
doi:10.1038/nature06177
Abstract:
http://www.nature.com/nature/journal/v449/n7165/abs/nature06177.html
Article:
http://www.nature.com/nature/journal/v449/n7165/full/nature06177.html

Revisiting Levy flight search patterns of wandering albatrosses,
bumblebees and deer pp1044 - 1048
Levy flights are random walks characterised by many short steps and
rare long steps. They display fractal properties, have no typical
scale, and occur in physical and chemical systems. On the basis of
high-resolution data and new analyses, previous claims that wandering
albatrosses perform Levy flights when searching for prey on the ocean
surface are shown to be unfounded. A new method is proposed for use
when testing for power-law distributions.
Andrew M. Edwards et al.
doi:10.1038/nature06199
Abstract:
http://www.nature.com/nature/journal/v449/n7165/abs/nature06199.html
Article:
http://www.nature.com/nature/journal/v449/n7165/full/nature06199.html

The amniote primitive streak is defined by epithelial cell
intercalation before gastrulation pp1049 - 1052
The first time-lapse analysis of individual cell movements in living
chick embryos during gastrulation are reported, and details that these
movements differ from those described in fish and amphibians, with
important implications for embryology and evolution of gastrulation in
vertebrates.
Octavian Voiculescu et al.
doi:10.1038/nature06211
Abstract:
http://www.nature.com/nature/journal/v449/n7165/abs/nature06211.html
Article:
http://www.nature.com/nature/journal/v449/n7165/full/nature06211.html

PLETHORA proteins as dose-dependent master regulators of Arabidopsis
root development pp1053 - 1057
The PLETHORA (PLT) family of transcription factors are essential for
defining the root stem cell niche. The PLT proteins are expressed in
gradients with a high point in the stem cell area and act in a
dose-dependent manner to dictate the size of the stem cell area and
the size of the transit amplifying cells that make up the meristem.
High levels of PLT activity promote stem cell identity and
maintenance; lower levels promote division of stem cell daughters; and
further reduced levels allow cell differentiation.
Carla Galinha et al.
doi:10.1038/nature06206
Abstract:
http://www.nature.com/nature/journal/v449/n7165/abs/nature06206.html
Article:
http://www.nature.com/nature/journal/v449/n7165/full/nature06206.html

Purine-mediated signalling triggers eye development pp1058 - 1062
Understanding the signalling pathways that initiate organ development
is of fundamental importance to human medicine. A mechanism that turns
on the expression of the genes necessary to make the eye via the
purinergic signalling has now been identified.
Karine Masse et al.
doi:10.1038/nature06189
Abstract:
http://www.nature.com/nature/journal/v449/n7165/abs/nature06189.html
Article:
http://www.nature.com/nature/journal/v449/n7165/full/nature06189.html

Functional architecture of the retromer cargo-recognition
complex pp1063 - 1067
The retromer is a complex of Vacuolar protein sorting proteins that
mediate retrieval of lysosomal hydrolases from endosomes to the trans
golgi network. The human retromer complex can be separated into two
smaller complexes, a membrane targeting sorting nexin dimer and a
heterotrimeric cargo recognition complex. New and previously solved
structures, bioinformatics, interaction studies, molecular modelling
and electron microscopy of the entire complex are combined to develop
a structural model of the complex.
Aitor Hierro et al.
doi:10.1038/nature06216
Abstract:
http://www.nature.com/nature/journal/v449/n7165/abs/nature06216.html
Article:
http://www.nature.com/nature/journal/v449/n7165/full/nature06216.html

Regulation of cell cycle progression and gene expression by H2A
deubiquitination pp1068 - 1072
An enzyme, Ubp-M, is identified that removes the ubiquitin
modification from histone H2A. Depletion of Ubp-M leads to slower cell
growth and abnormal chromosome segregation in mitosis. In addition,
depletion of Ubp-M causes defects in development and dysregulation of
Hox gene expression.
Heui-Yun Joo et al.
doi:10.1038/nature06256
Abstract:
http://www.nature.com/nature/journal/v449/n7165/abs/nature06256.html
Article:
http://www.nature.com/nature/journal/v449/n7165/full/nature06256.html

An elaborate pathway required for Ras-mediated epigenetic
silencing pp1073 - 1077
A genome-wide RNAi screen identifies several genes required for
Ras-mediated epigenetic silencing that encode cell signalling
molecules, chromatin modifiers, transcriptional activators and
repressors, and the DNA methyltransferase DNMT1.
Claude Gazin et al.
doi:10.1038/nature06251
Abstract:
http://www.nature.com/nature/journal/v449/n7165/abs/nature06251.html
Article:
http://www.nature.com/nature/journal/v449/n7165/full/nature06251.html

Open-to-closed transition in apo maltose-binding protein observed by
paramagnetic NMR pp1078 - 1082
It is well known that large-scale rearrangements of domains of a
protein can play an important role in ligand binding and recognition,
catalysis, and regulation. Though X-ray crystal structures can provide
a static picture of the apo (usually open) and holo (usually closed)
states of a protein, it is not usually clear whether the apo state
exists as a single species where the closed state is energetically
inaccessible and interdomain rearrangement is induced by ligand or
substrate binding or whether the predominantly open form already
co-exists in rapid equilibrium with a minor closed species. In this
paper, the authors obtained paramagnetic relaxation enhancement data
that indicate that there is a rapidly exchanging mixture of a
predominantly open form of the maltose-binding protein and a minor
(partially-closed) form of the protein. Ensemble simulated annealing
refinement was used to determine an ensemble average structure of the
minor apo species and demonstrate that it is distinct from the
sugar-bound state.
Chun Tang, Charles D. Schwieters and G. Marius Clore
doi:10.1038/nature06232
Abstract:
http://www.nature.com/nature/journal/v449/n7165/abs/nature06232.html
Article:
http://www.nature.com/nature/journal/v449/n7165/full/nature06232.html
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2007年10月25日Nature中文摘要