Multiple ionization of atom clusters by intense soft X-rays from a free-electron laser

Intense radiation from lasers has opened up many new areas of research in physics and chemistry, and has revolutionized optical technology. So far, most work in the field of nonlinear processes has been restricted to infrared, visible and ultraviolet light, although progress in the development of X-ray lasers has been made recently. With the advent of a free-electron laser in the soft-X-ray regime below 100 nm wavelength, a new light source is now available for experiments with intense, short-wavelength radiation that could be used to obtain deeper insights into the structure of matter. Other free-electron sources with even shorter wavelengths are planned for the future. Here we present initial results from a study of the interaction of soft X-ray radiation, generated by a free-electron laser, with Xe atoms and clusters. We find that, whereas Xe atoms become only singly ionized by the absorption of single photons, absorption in clusters is strongly enhanced. On average, each atom in large clusters absorbs up to 400 eV, corresponding to 30 photons. We suggest that the clusters are heated up and electrons are emitted after acquiring sufficient energy. The clusters finally disintegrate completely by Coulomb explosion.     

Respiration in the open ocean

A key question when trying to understand the global carbon cycle is whether the oceans are net sources or sinks of carbon. This will depend on the production of organic matter relative to the decomposition due to biological respiration. Estimates of respiration are available for the top layers, the mesopelagic layer, and the abyssal waters and sediments of various ocean regions. Although the total open ocean respiration is uncertain, it is probably substantially greater than most current estimates of particulate organic matter production. Nevertheless, whether the biota act as a net source or sink of carbon remains an open question.     

Active transport of Ca2+ by an artificial photosynthetic membrane

Transport of calcium ions across membranes and against a thermodynamic gradient is essential to many biological processes, including muscle contraction, the citric acid cycle, glycogen metabolism, release of neurotransmitters, vision, biological signal transduction and immune response. Synthetic systems that transport metal ions across lipid or liquid membranes are well known, and in some cases light has been used to facilitate transport. Typically, a carrier molecule located in a symmetric membrane binds the ion from aqueous solution on one side and releases it on the other. The thermodynamic driving force is provided by an ion concentration difference between the two aqueous solutions, coupling to such a gradient in an auxiliary species, or photomodulation of the carrier by an asymmetric photon flux. Here we report a different approach, in which active transport is driven not by concentration gradients, but by light-induced electron transfer in a photoactive molecule that is asymmetrically disposed across a lipid bilayer. The system comprises a synthetic, light-driven transmembrane Ca2+ pump based on a redox-sensitive, lipophilic Ca2+-binding shuttle molecule whose function is powered by an intramembrane artificial photosynthetic reaction centre. The resulting structure transports calcium ions across the bilayer of a liposome to develop both a calcium ion concentration gradient and a membrane potential, expanding Mitchell's concept of a redox loop mechanism for protons to include divalent cations. Although the quantum yield is relatively low (approximately 1 per cent), the Ca2+ electrochemical potential developed is significant.     

Asymmetric inheritance of centrosomally localized mRNAs during embryonic cleavages

During development, different cell fates are generated by cell-cell interactions or by the asymmetric distribution of patterning molecules. Asymmetric inheritance is known to occur either through directed transport along actin microfilaments into one daughter cell or through capture of determinants by a region of the cortex inherited by one daughter. Here we report a third mechanism of asymmetric inheritance in a mollusc embryo. Different messenger RNAs associate with centrosomes in different cells and are subsequently distributed asymmetrically during division. The segregated mRNAs are diffusely distributed in the cytoplasm and then localize, in a microtubule-dependent manner, to the pericentriolar matrix. During division, they dissociate from the core mitotic centrosome and move by means of actin filaments to the presumptive animal daughter cell cortex. In experimental cells with two interphase centrosomes, mRNAs accumulate on the correct centrosome, indicating that differences between centrosomes control mRNA targeting. Blocking the accumulation of mRNAs on the centrosome shows that this event is required for subsequent cortical localization. These events produce a complex pattern of mRNA localization, in which different messages distinguish groups of cells with the same birth order rank and similar developmental potentials.     

Endocytosis-mediated downregulation of LIN-12/Notch upon Ras activation in Caenorhabditis elegans

The coordination of signals from different pathways is important for cell fate specification during animal development. Here, we define a novel mode of crosstalk between the epidermal growth factor receptor/Ras/mitogen-activated protein kinase cascade and the LIN-12/Notch pathway during Caenorhabditis elegans vulval development. Six vulval precursor cells (VPCs) are initially equivalent but adopt different fates as a result of an inductive signal mediated by the Ras pathway and a lateral signal mediated by the LIN-12/Notch pathway. One consequence of activating Ras is a reduction of LIN-12 protein in P6.p (ref. 2), the VPC believed to be the source of the lateral signal. Here we identify a 'downregulation targeting signal' (DTS) in the LIN-12 intracellular domain, which encompasses a di-leucine-containing endocytic sorting motif. The DTS seems to be required for internalization of LIN-12, and on Ras activation it might mediate altered endocytic routing of LIN-12, leading to downregulation. We also show that if LIN-12 is stabilized in P6.p, lateral signalling is compromised, indicating that LIN-12 downregulation is important in the appropriate specification of cell fates in vivo.