Letters in 2010

Nuclei with equal neutron (N) and proton (Z) numbers show enhanced correlations that have been predicted to favour an unusual type of pairing, distinct from normal nuclear superfluidity. Here, technically challenging observations are reported of excited states in the N = Z = 46 nucleus ⁹²Pd, from which evidence is inferred for a type of spin-aligned structure in the ground and low-lying excited states, not established in nuclei before and differing from previous predictions.

The unusual capability of solid crystalline materials to deform plastically (known as superplasticity) has been found in metals and even ceramics; however, no experimental studies have yet to demonstrate this behaviour in geological materials. It is now demonstrated that some synthetic rocks, which are good analogues for Earth's mantle, undergo homogeneous elongation up to 500% under subsolidus conditions. Calculations show that such superplastic flow in the mantle is inevitably accompanied by significant grain growth that can change fine-grained rocks to coarse-grained aggregates, resulting in increasing mantle viscosity and finally termination of superplastic flow.

The initial crystal structure of LeuT, together with subsequent functional and structural studies, provided direct evidence for a single, high-affinity substrate-binding site. Recent binding, flux and molecular simulation studies, however, have been interpreted in terms of a model where there are two high-affinity binding sites: the second (S2) site is believed to be located within the extracellular vestibule. Here, direct measurement is performed of substrate binding to wild-type LeuT and to S2 site mutants using isothermal titration calorimetry, equilibrium dialysis and scintillation proximity assays. The conclusion is made that LeuT harbours a single, centrally located, high-affinity substrate-binding site.

Motion of electrons can influence their spins through a fundamental effect called the spin–orbit interaction. Here, a spin–orbit quantum bit (qubit) is implemented in an indium arsenide nanowire, which should offer significant advantages for quantum computing. The spin–orbit qubit is electrically controllable, and information can be stored in the spin. Moreover, nanowires can serve as one dimensional templates for scalable qubit registers, and are suited for both electronic and optical devices.

Place cells in the hippocampus track an animal's position as it travels through space. Previous work contends that sequential place cell maps are produced upon the initial navigation of a new area and subsequently consolidated at rest or during sleep. Here, place-cell firing patterns during rest or sleep are observed before a novel spatial experience, a phenomenon termed 'preplay'. These sequences were separate from the replay of pervious experience and suggest that internal dynamics during rest may organize cell assemblies to be ready for any novel encoding that may occur in the immediate future.

The enzyme eNOS is crucial for regulating vascular function as it can produce both the vasodilator nitric oxide and the vasoconstrictor superoxide. Here it is shown that a modification associated with oxidant stress, S-glutathionylation, switches the enzyme from forming nitric oxide to forming superoxide. In hypertensive vessels, S-glutathionylation of eNOS is increased and this is associated with impaired endothelium-dependent vasodilation.

Ubiquitination of histone H2A has been implicated in polycomb-mediated transcriptional silencing, but its precise functions are unclear. Here, ZRF1 is shown to be recruited to ubiquitinated H2A and to function in displacing polycomb-repressive complex 1 (PRC1) from chromatin to facilitate transcriptional activation.

During periods of fasting the liver produces ketone bodies, which the peripheral tissues can use as a source of energy. Here it is shown that fasting inhibits multi-component mTOR complex 1 (mTORC1) in the liver. Inhibition of mTORC1 is required for activation of PPARα, a master regulator that switches on genes involved in ketogenesis. Livers from aged mice have increased mTORC1 signalling, reduced PPARα activity, and reduced ketone production. The observation that mTORC1 promotes an ageing phenotype in the liver fits well with the observation that inhibition of this pathway increases lifespan in several organisms.

The histone variant mH2A is shown to be expressed at reduced levels in many melanomas. Loss of mH2A promotes tumour growth and metastasis via transcriptional upregulation of CDK8, a known oncogene. This study therefore reveals a new tumour suppression mechanism exerted by epigenetic modifications.

This study finds frequent mutations in MYD88 in the activated B-cell-like subtype of diffuse large B-cell lymphoma and, with lower frequency, in mucosa-associated lymphoid tissue lymphomas. MYD88 mediates signalling by Toll-like receptors, and the mutations, most of which affect the same amino acid, are shown to activate the pathway and promote cancer cell survival.

To shed light on the natural history of Precambrian life, the evolutionary history of almost 4,000 gene families across the three domains of life are mapped onto a geological timeline. Over one-quarter of modern gene families arose during a period of rapid diversification of bacterial lineages. Functionally, these genes are likely to be involved in electron transport and respiratory pathways, whereas those that arose later are implicated in functions consistent with an increasingly oxygenating biosphere.

Pathogenic Escherichia coli translocate many proteins into the host cell to promote virulence. It is now shown that one of these proteins, EspG, which is present in enterohaemorrhagic E. coli, interferes with the host signalling network.

In the retina, highly selective wiring from inhibitory cells contributes to determine the direction-selection characteristics of an individual ganglion cell, yet how the asymmetric wiring inherent to these connections is established was unknown. Here, two independent studies using complementary techniques, including pharmacology, electrophysiology and optogenetics, find that although inhibitory inputs to both sides of the direction-selective cell are uniform early in development, by the second postnatal week, inhibitory synapses on the null side strengthen whereas those on the preferred side remain constant. These plasticity changes occur independent of neural activity, indicating that a specific developmental program is executed to produce the direction-selective circuitry in the retina.

Electron microscopy has advanced to the stage where individual elements can be identified with atomic resolution. Here it is shown to be possible to get fine-structure spectroscopic information of individual light atoms such as those of carbon, and so also probe their chemical state. This capability is illustrated by investigating the edges of a graphene sample, where it is possible to discriminate between single-, double- and triple-coordinated carbon atoms.

Although loss of XLF, a classical non-homologous DNA end-joining (NHEJ) repair factor, shows strong effects in non-lymphoid cells, in lymphoid cells its absence has only modest effects on V(D)J recombination. This study now shows that in lymphoid cells, two other repair factors — ATM kinase and histone protein H2AX — have functional redundancy with XLF. Thus, mice deficient in both ATM and XLF have compromised conventional NHEJ, although alternative end-joining is retained. The results hint that the redundant function in end-joining that XLF has with both ATM and H2AX may have to do with an ATM role in chromatin accessibility.

Here, an indirect estimate for the magnetic field strength within the Earth's core from measurements of tidal dissipation is presented. Previously reported evidence of anomalous dissipation in the Earth's nutations can be explained with a core-averaged field of 2.5 mT, eliminating the need for high fluid viscosity or a stronger magnetic field at the inner-core boundary.

Hexagons can easily tile a flat surface, but not a curved one. Defects with topological charge (such as heptagons and pentagons) make it easier to tile curved surfaces, such as soccer balls. Here, a new type of defect is reported that accommodates curvature in the same way as fabric pleats. The appearance of such defects on the negatively curved surfaces of stretched colloidal crystals are observed. The results will facilitate the exploration of general theories of defects in curved spaces, the engineering of curved structures and novel methods for soft lithography and directed self-assembly.

Antigen receptor loci contain numerous gene segments that are recombined in response to antigen stimulation. The RAG endonuclease makes the double-strand breaks that initiate recombination. The ends of these breaks are hairpins that can only be cleaved by the Artemis nuclease. Here, it is shown that the specificity for Artemis is dictated by the histone protein H2AX, in cooperation with the repair protein MDC-1. In the absence of H2AX, another nuclease, CtIP, can open the ends but they are not joined efficiently; this leads to genomic instability.

How new phenotypes can be introduced during evolution without losses of fitness remains largely unexplained at the molecular level. By comparing the molecular details of a well known process — mating type determination — across a large diversity of yeast species, the network rewiring event of the intercalation of a new level of gene transcription control into an ancient regulatory circuit is shown, which allowed for the creation of a new phenotype — taking food availability into account when deciding to mate.

The dramatic loss of Arctic sea ice with climate change has led to the prediction of a tipping point beyond which ice loss is irreversible and polar bear habitat will be catastrophically lost. By contrast, this study shows a linear relationship between temperature and sea-ice coverage that overcomes the albedo effect that would cause a tipping point. As a result, reducing greenhouse gas emissions can have a positive effect on polar bear populations.