Letters in 2007

Understanding morphogen activity is one of the longest standing problems in developmental biology. Both the duration of exposure to the secreted morphogen Sonic hedgehog (Shh) and the concentration of Shh are important in determining the fate of cells in the neural tube and in the limb bud. This paper clarifies the relationship between time and concentration in Shh signalling during patterning of the neural tube.

Radio-sounding results from the first Venus Express Radio Science (VeRa) occultation season are reported, which determine the fine structure in temperatures at upper cloud-deck altitudes, detect a distinct day–night temperature difference in the southern middle atmosphere, and track day-to-day changes in Venus' ionosphere.

Venus is completely covered by a thick cloud layer, with the cloud tops in fast retrograde rotation. Global and small scale properties of these clouds and their temporal and latitudinal variations are investigated, and the wind velocities are derived. The southern polar region is highly variable and can change dramatically on time scales as short as one day, perhaps arising from the injection of SO₂ into the mesosphere.

The nutrient sensor mTOR controls mitochondrial gene expression and oxidative function via an interaction between the transcriptional regulators PGC-1α and YY1.

On Venus, the bulk of O and O₂ are gravitationally bound, but heavy ions have been observed to escape, though their composition could not be determined. Venus Express measurements report that the dominant escaping ions are O⁺, He⁺ and H⁺. The escape of H⁺ and O⁺, together with the estimated neutral hydrogen and oxygen escape, currently takes place near the stoichometric ratio corresponding to water.

Electron microscopy is combined with optical single-molecule tracking to visualize molecular diffusion in a nanoporous material. For the first time, guest molecules can be 'seen' changing speed or direction in response to structural features of the host.

This work describes the identification and characterization of novel small molecule activators of SIRT1, an NAD⁺-dependent deacetylase that mediates the beneficial effects of caloric restriction. These small molecules are structurally unrelated to, and much more potent than, resveratrol, and improve metabolic function in animal models of diabetes and obesity.

Crystal structures of complexes between the epidermal growth factor receptor (EGFR) kinase domain and a fragment of its feedback inhibitor MIG6 reveal an allosteric inhibition mechanism. MIG6 binds the cyclin/CDK-like asymmetric dimer interface of EGFR and blocks formation of the activating dimer.

Immature thymic T cells receive signals through the T cell receptor which determine whether they undergo positive or negative selection. Calcineurin/NFAT signalling in preselection thymocytes produces a temporal window with enhanced signalling sensitivity paving the way for positive selection.

This paper addresses the question of how centromere architecture affects spindle formation, and presents evidence supporting the claim that centromeres are malleable with respect to tangential forces. Once deformed, they remain in this position until they are straightened by external forces applied along microtubules.

Observations of infrared emission from CO₂, O₂ and NO established that photochemical and dynamic activity controls the structure of the upper atmosphere of Venus, but were unable to identify the altitude of the emission. But it is reported here that day-side CO₂ emission extends from 90–120 km altitude, with a peak at ∼115 km. Night-side O₂ emission peaks at 96 km and is visible over the range 95–100 km.

Venus has a bright 'dipole' double-eye feature at the centre of a vast vortex that rotates around the north pole, and is surrounded by a cold 'collar'. Observations of Venus' south polar region are reported, where clouds with morphology much like those around the north pole are seen, but rotating somewhat faster.

The p53 gene has been extensively studied for its role in tumour prevention but little is known about its normal physiological function. A crucial role of this factor in fecundity and reproduction is now reported.

Conventional antibiotics, which target the infectious agent, are subject to the development of antibiotic resistance. An alternative approach is to focus on host factors that support pathogen growth. A kinase inhibitor screen identified AKT1 as a central molecule essential for intracellular survival of Salmonella typhimurium and Mycobacterium tuberculosis and thus presenting a putative drug target.

It is shown for the first time that a vertebrate-specific member of the TATA-box-binding protein (TBP) family, called TRF3, controls haematopoiesis in zebrafish embryogenesis.

This study presents the first large scale transcriptional analysis of malaria parasites isolated from human patients, and defines three distinct transcriptional patterns that can be described as active growth, response to starvation and environmental stress response.

A relatively new imaging technique is used to reveal that some Cretaceous gymnosperm seeds have evolutionary links with Gnetales (an evolutionarily hard-to-place gymnosperm group with three living genera) and the Bennetitales (an extinct group of cycad-like plants). The link between Gnetales and Bennetitales may have important consequences for our understanding of the evolution of seed plants, including flowering plants.

The use of time series data from 522 remote lakes and streams in North America and northern Europe and a simple model shows that dissolved organic carbon concentrations between 1990– 2004 have increased in proportion to the rates at which atmospherically deposited anthropogenic sulphur and sea salt have declined. It is suggested that acid deposition to these ecosystems has been partially buffered by changes in organic acidity and that the rise in dissolved organic carbon is integral to recovery from acidification.

The discovery of several white dwarfs with atmospheres primarily composed of carbon, with little or no trace of hydrogen or helium is reported. These stars do not fit satisfactorily in any of the currently known theories of post-asymptotic giant branch evolution.