Photoionization of tungsten ions: Experiment and theory for W5+

Experimental and theoretical cross sections are reported for single-photon single ionization of W5+ ions. Absolute measurements were conducted employing the photon-ion merged-beams technique. Detailed photon-energy scans were performed at (67 ± 10) meV resolution in the 20-160 eV range. In contrast to photoionization of tungsten ions in lower charge states, the cross section is dominated by narrow, densely-spaced resonances. Theoretical results were obtained from a Dirac-Coulomb R-matrix approach employing a basis set of 457 levels providing cross sections for photoionization of W5+ ions in the ground level as well as the and metastable excited levels. Considering the complexity of the electronic structure of tungsten ions in low charge states, the agreement between theory and experiment is satisfactory

The Escherichia coli transcriptome mostly consists of independently regulated modules

Underlying cellular responses is a transcriptional regulatory network (TRN) that modulates gene expression. A useful description of the TRN would decompose the transcriptome into targeted effects of individual transcriptional regulators. Here, we apply unsupervised machine learning to a diverse compendium of over 250 high-quality Escherichia coli RNA-seq datasets to identify 92 statistically independent signals that modulate the expression of specific gene sets. We show that 61 of these transcriptomic signals represent the effects of currently characterized transcriptional regulators. Condition-specific activation of signals is validated by exposure of E. coli to new environmental conditions. The resulting decomposition of the transcriptome provides: a mechanistic, systems-level, network-based explanation of responses to environmental and genetic perturbations; a guide to gene and regulator function discovery; and a basis for characterizing transcriptomic differences in multiple strains. Taken together, our results show that signal summation describes the composition of a model prokaryotic transcriptome

Mice Doubly-Deficient in Lysosomal Hexosaminidase A and Neuraminidase 4 Show Epileptic Crises and Rapid Neuronal Loss

Tay-Sachs disease is a severe lysosomal disorder caused by mutations in the HexA gene coding for the α-subunit of lysosomal β-hexosaminidase A, which converts GM2 to GM3 ganglioside. Hexa−/− mice, depleted of β-hexosaminidase A, remain asymptomatic to 1 year of age, because they catabolise GM2 ganglioside via a lysosomal sialidase into glycolipid GA2, which is further processed by β-hexosaminidase B to lactosyl-ceramide, thereby bypassing the β-hexosaminidase A defect. Since this bypass is not effective in humans, infantile Tay-Sachs disease is fatal in the first years of life. Previously, we identified a novel ganglioside metabolizing sialidase, Neu4, abundantly expressed in mouse brain neurons. Now we demonstrate that mice with targeted disruption of both Neu4 and Hexa genes (Neu4−/−;Hexa−/−) show epileptic seizures with 40% penetrance correlating with polyspike discharges on the cortical electrodes of the electroencephalogram. Single knockout Hexa−/− or Neu4−/− siblings do not show such symptoms. Further, double-knockout but not single-knockout mice have multiple degenerating neurons in the cortex and hippocampus and multiple layers of cortical neurons accumulating GM2 ganglioside. Together, our data suggest that the Neu4 block exacerbates the disease in Hexa−/− mice, indicating that Neu4 is a modifier gene in the mouse model of Tay-Sachs disease, reducing the disease severity through the metabolic bypass. However, while disease severity in the double mutant is increased, it is not profound suggesting that Neu4 is not the only sialidase contributing to the metabolic bypass in Hexa−/− mice

Thymic Alterations in GM2 Gangliosidoses Model Mice

BACKGROUND: Sandhoff disease is a lysosomal storage disorder characterized by the absence of β-hexosaminidase and storage of GM2 ganglioside and related glycolipids. We have previously found that the progressive neurologic disease induced in Hexb(-/-) mice, an animal model for Sandhoff disease, is associated with the production of pathogenic anti-glycolipid autoantibodies. METHODOLOGY/PRINCIPAL FINDINGS: In our current study, we report on the alterations in the thymus during the development of mild to severe progressive neurologic disease. The thymus from Hexb(-/-) mice of greater than 15 weeks of age showed a marked decrease in the percentage of immature CD4(+)/CD8(+) T cells and a significantly increased number of CD4(+)/CD8(-) T cells. During involution, the levels of both apoptotic thymic cells and IgG deposits to T cells were found to have increased, whilst swollen macrophages were prominently observed, particularly in the cortex. We employed cDNA microarray analysis to monitor gene expression during the involution process and found that genes associated with the immune responses were upregulated, particularly those expressed in macrophages. CXCL13 was one of these upregulated genes and is expressed specifically in the thymus. B1 cells were also found to have increased in the thy mus. It is significant that these alterations in the thymus were reduced in FcRγ additionally disrupted Hexb(-/-) mice. CONCLUSIONS/SIGNIFICANCE: These results suggest that the FcRγ chain may render the usually poorly immunogenic thymus into an organ prone to autoimmune responses, including the chemotaxis of B1 cells toward CXCL13

Photoionisation of ions with synchrotron radiation: from ions in space to atoms in cages

The photon-ion merged-beams technique for the photoionisation of mass/charge selected ionised atoms, molecules and clusters by x-rays from synchrotron radiation sources is introduced. Examples for photoionisation of atomic ions are discussed by going from outer shell ionisation of simple few electron systems to inner shell ionisation of complex many electron ions. Fundamental ionisation mechanisms are elucidated and the importance of the results for applications in astrophysics and plasma physics is pointed out. Finally, the unique capabilities of the photon-ion merged-beams technique for the study of photoabsorption by nanoparticles are demonstrated by the example of endohedral fullerene ions

Experimental studies on photoabsorption by endohedral fullerene ions with a focus on Xe@C60+confinement resonances

A brief overview on the status of experimental investigations into photoionization and fragmentation of fullerenes and endohedral fullerenes by absorption of a single short-wavelength photon is presented. The focus of this paper is on the endohedral Xe@C60+ molecular ion in which a xenon atom is centrally encapsulated inside a C+60 fullerene cage. Confinement resonances that result from the interference of Xe photoelectron matter waves emerging from the C+60 cavity were studied by exposing Xe@C60+ ions to synchrotron radiation of 60 to 150 eV energy which is the region of the well-known giant atomic Xe 4d excitation resonance. Photoions Xe@C n q+ (with final charge states q = 2,3,4 of the product ions and numbers of carbon atoms left in the cage) were recorded as a function of photon energy and cross sections for the individual reaction channels were determined. In addition to previous work, new final channels (q = 2; n = 60, 58 and q = 4; n = 58, 56) were observed, and thus, about 70% of the oscillator strength expected for the encapsulated Xe atom in the investigated energy range could be recovered. The present results establish the first and, so far, only conclusive experimental observation of confinement resonances in an endohedral fullerene. The data are in remarkable agreement with relativistic R-matrix calculations that accompanied the previous experimental work

Direct double ionization of the Ar+ M shell by a single photon

Direct double ionization of the Ar+(3p-1) ion by a single photon is investigated both experimentally and theoretically. The photon-ion merged-beams technique was employed at the Advanced Light Source in Berkeley, USA, to measure absolute cross sections in the energy range from 60 to 150 eV. In this range, three contributions to the double ionization of Ar+ are to be expected: the removal of two 3p electrons, of a 3s and a 3p electron, and of two 3s electrons. Among the possible mechanisms leading to double ionization, the TS1 (two-step one) process dominates in the near-threshold region. In TS1, a photoelectron is ejected and, on its way out, knocks out a secondary electron. This two-step mechanism is treated theoretically by multiplying the calculated cross section for direct single photoionization of a given subshell with the calculated (e,2e) ionization probability for the ejected photoelectron to knock off a secondary electron. The calculated cross section is in very good agreement with the experiment

Direct double ionization of the Ar+ M shell by a single photon

Direct double ionization of the Ar+(3p-1) ion by a single photon is investigated both experimentally and theoretically. The photon-ion merged-beams technique was employed at the Advanced Light Source in Berkeley, USA, to measure absolute cross sections in the energy range from 60 to 150 eV. In this range, three contributions to the double ionization of Ar+ are to be expected: the removal of two 3p electrons, of a 3s and a 3p electron, and of two 3s electrons. Among the possible mechanisms leading to double ionization, the TS1 (two-step one) process dominates in the near-threshold region. In TS1, a photoelectron is ejected and, on its way out, knocks out a secondary electron. This two-step mechanism is treated theoretically by multiplying the calculated cross section for direct single photoionization of a given subshell with the calculated (e,2e) ionization probability for the ejected photoelectron to knock off a secondary electron. The calculated cross section is in very good agreement with the experiment

Photoionization of tungsten ions: Experiment and theory for W4+

Experimental and theoretical results are reported for single-photon single ionization of the tungsten ion W4+. Absolute cross sections have been measured employing the photon-ion merged-beams setup at the Advanced Light Source in Berkeley. Detailed photon-energy scans were performed at 200 meV bandwidth in the 40-105 eV range. Theoretical results have been obtained from a Dirac-Coulomb R-matrix approach employing basis sets of 730 levels for the photoionization of W4+. Calculations were carried out for the 4f14 5s2 5p6 5d23FJ, J = 2, ground level and the associated fine-structure levels with J = 3 and 4 for the W4+ ions. In addition, cross sections have been calculated for the metastable levels 4f14 5s2 5p6 5d23P0,1,2, 1D2, 1G4, 1S0. A very satisfying agreement of theory and experiment is found for the photoionization cross section of W4+ which is remarkable given the complexity of the electronic structure of tungsten ions in low charge states