Research articles

Following the formation of a DNA double-strand break (DSB), cells activate the DNA-damage response and recruit a number of proteins to the lesion. Some of these proteins are modified by the attachment of small ubiquitin-related modifier (SUMO). Here, SUMO1, SUMO2 and SUMO3 are shown to accumulate at DSB sites in mammalian cells. SUMO1 and SUMO2/3 accrual requires the E3 ligase enzymes PIAS4 and PIAS1, which promote DSB repair.

The in vivo function of E2f transcription factors has been a matter of debate. The effects of E2f1, E2f2 and E2f3 triple deficiency are now examined in murine embryonic stem cells, embryos and small intestines. E2f1–3 are shown to function as transcriptional activators in normal dividing progenitor cells; however, contrary to the current view, they are dispensable for cell division but are necessary for cell survival.

Most schemes for three-dimensional (3D) structure determination of an object require multiple measurements over various orientations, or a means of scanning it section by section. A 3D imaging modality, termed ankylography, is now presented that under certain circumstances enables complete 3D structure determination from a single diffraction measurement. This approach could find broad applications in the physical and life sciences.

Why hosts of brood parasites generally fail to recognize parasitic offspring after they have hatched from the egg, even when the host and parasitic chicks are very different, remains a puzzle. American coots are now shown to use first-hatched chicks in a brood as referents to learn to recognize their own chicks and discriminate against later-hatched parasitic chicks. A lack of reliable information may explain why the evolution of such chick recognition is not more common.

Sister chromatids are generally assumed to be genetically and functionally identical, with segregation to daughter cells being a random process; however, some evidence contradicts both of these assumptions. A technique is now developed to observe chromatid segregation in mitotic cells in vivo. Whereas many sister chromatids appeared to be randomly distributed between daughter cells, non-random sister chromatid segregation is observed in a subset of cells.

Over the past decade, several techniques have been developed to improve the detection of small amounts of DNA and RNA molecules, but detection of DNA molecules at concentrations below the femtomolar level requires amplified detection schemes. A unique nanomechanical response of hybridized DNA and RNA molecules that serves as an intrinsic molecular label is now reported; nanomechanical measurements allow direct detection and counting of hybridized molecules.

Expression of the embryonic M2 isoform of pyruvate kinase (PKM2) by tumour cells promotes aerobic glycolysis, whereas the normal adult isoform, PKM1, promotes oxidative phosphorylation. Expression of these isoforms is regulated by alternative splicing; here, aberrant expression of three heterogeneous nuclear ribonucleoprotein splicing factors, which are themselves regulated by the c-Myc oncogene, is shown to be responsible for the M1 to M2 switch in cancer.

Dendritic spines carry the majority of excitatory synapses, and spine morphology and distribution are critical for synaptic transmission, synaptic integration and plasticity. Here, signalling by the secreted semaphorin Sema3F is shown to control spine distribution along select dendritic processes, and distinct secreted semaphorin signalling events orchestrate CNS connectivity through the differential control of a variety of processes.

Lymphomas often contain translocations that link c-myc to the immunoglobulin heavy chain locus (encoded by Igh), but the nature of the elements that activate oncogenes within such translocations is unknown. Transgenic mice with Igh 3' regulatory region (Igh3' RR) sequences fused to c-myc are predisposed to B lymphomas. Here, the oncogenic role of the Igh3' RR is investigated by inactivating it in two distinct mouse models for B-cell lymphoma with Igh–c-myc translocations.

All hard, convex shapes pack more densely than spheres, although for tetrahedra this was demonstrated only very recently. Here, tetrahedra are shown to pack even more densely than previously thought. Thermodynamic computer simulations allow the system to evolve naturally towards high-density states, showing that a fluid of hard tetrahedra undergoes a first-order phase transition to a dodecagonal quasicrystal, and yielding the highest packing fractions yet observed for tetrahedra.

Male animals are typically more elaborately ornamented than females, probably because females make more of an energetic investment in raising young. However, this generality may not apply in cooperatively breeding vertebrates, where the energetic load is similar in males and females. The socially diverse African starlings are now used to study this issue, revealing that where intrasexual competition among females may be intense, female trait elaboration is selected for.

Here, the link between non-coding RNA and chromatin regulation is investigated through analysis of FLC — a regulator of flowering time in Arabidopsis and a target of several chromatin pathways. FLC is silenced by prolonged cold in a Polycomb-mediated process called vernalization. Upregulation of long non-coding antisense transcripts covering the entire FLC locus are now suggested to have an early role in the cold-induced silencing mechanism.

The nature of the jets and the role of magnetic fields in gamma-ray bursts (GRBs) remain unclear. There are two possible models, one of which suggests that magnetic fields are critical; to distinguish between these models the degree of polarization in early-time emission must be measured. The early optical emission from GRB 090102 is now reported to be polarized at 10 ± 1 per cent, indicating the presence of large-scale fields originating in the expanding fireball.

5.33 million years ago, in an event known as the Zanclean flood, Atlantic waters refilled a mostly desiccated Mediterranean Sea which had become disconnected from the world's oceans; however, the nature, abruptness and evolution of this flood remain poorly constrained. Using borehole and seismic data and a model study, it is now suggested that 90 per cent of the water was transferred in a short period of a few months to two years, with peak rates of sea level rise in the Mediterranean possibly reaching more than 10 metres per day.

The atomic ratio of carbon:nitrogen:phosphorus can be used to describe biota in terms of elemental composition, and this stoichiometry is fundamental for understanding the production dynamics and biogeochemical cycles of ecosystems. Heterotrophic microbial communities from terrestrial soils and fresh-water sediments are now shown to share a common functional stoichiometry in relation to organic nutrient acquisition, reflecting the similar scaling relationships of four key ecoenzymes.

Seismic detectability of the boundaries and convection in the mantle is strongly influenced by the thicknesses and Clapeyron slopes of mantle phase boundaries. The unusually large positive Clapeyron slope found for the boundary between perovskite and post-perovskite (the 'pPv boundary') would destabilize high-temperature anomalies in the lowermost mantle, in disagreement with the seismic observations. Here, new studies of the thickness and Clapeyron slope of the pPv boundary shed light on this matter.

Alanyl-tRNA synthetases (AlaRSs) may confuse glycine or serine with alanine, potentially causing mistranslation and thus profound functional consequences, with serine posing a bigger challenge than glycine. AlaXps — free-standing, genome-encoded editing proteins — represent one editing checkpoint to prevent this from occurring. Nine crystal structures, together with kinetic and mutational analysis, now show how AlaXps solve the serine misactivation problem.

Chiral spin liquids are a hypothetical class of spin liquids in which time-reversal symmetry is macroscopically broken even in the absence of an applied magnetic field or any magnetic dipole long-range order. Although such spin-liquid states were proposed more than two decades ago, they remain elusive. Here, evidence is presented that the time-reversal symmetry can be broken spontaneously on a macroscopic scale in the absence of magnetic dipole long-range order, suggesting the emergence of a chiral spin liquid.

The Red Queen metaphor has species accumulating small changes to keep up with a continually changing environment, with speciation occurring at a constant rate. This constant-rate claim is now tested against four competing models, using 101 phylogenies of animal, plant and fungal taxa. The results provide a new interpretation of the Red Queen; a view linking speciation to rare stochastic events that cause reproductive isolation.

Placing a macroscopic object in its quantum-mechanical ground state of motion is an exciting experimental target that should reveal counterintuitive physical behaviour — such as the existence of states in which the mechanical system is located in two places simultaneously. A nanomechanical resonator is now cooled to a point where the probability of its residing in the quantum ground state of motion is 0.21; this level of cooling should allow a series of fundamental quantum mechanical observations.