Prevalent positive epistasis in Escherichia coli and Saccharomyces cerevisiae metabolic networks

Epistasis refers to the interaction between genes. Although high-throughput epistasis data from model organisms are being generated and used to construct genetic networks(1-3), the extent to which genetic epistasis reflects biologically meaningful interactions remains unclear(4-6). We have addressed this question through in silico mapping of positive and negative epistatic interactions amongst biochemical reactions within the metabolic networks of Escherichia coli and Saccharomyces cerevisiae using flux balance analysis. We found that negative epistasis occurs mainly between nonessential reactions with overlapping functions, whereas positive epistasis usually involves essential reactions, is highly abundant and, unexpectedly, often occurs between reactions without overlapping functions. We offer mechanistic explanations of these findings and experimentally validate them for 61 S. cerevisiae gene pairs.

Fabrication of a Low-Loss SSC Using High-Dose Electron Beam Lithography Exposure With Negative PMMA Resist

A silicon-on-insulator optical fiber-to-waveguide spot-size converter (SSC) using Poly-MethylMethAcrylate (PMMA) is presented for integrated optical circuits. Unlike the conventional use of PMMA as a positive resist, it has been successfully used as a negative resist with high-dose electron exposure for the fabrication of ultrafine silicon wire waveguides. Additionally, this process is able to reduce the side-wall roughness, and substantially depresses the unwanted propagation loss. Exploiting this technology, the authors demonstrated that the SSC can improve coupling efficiency by as much as over 2.5 dB per coupling facet, compared with that of SSC fabricated with PMMA as a positive resist with the same dimension.

Experimental observation of negative lateral displacements of microwave beams transmitting through dielectric slabs

It was theoretically predicted that when a beam of light travels through a thin slab of optically denser medium in the air, the emerging beam from the slab will suffer a lateral displacement that is different from the prediction of geometrical optics, that is, the Snell's law of refraction and can be zero and negative as well as positive. These phenomena have been directly observed in microwave experiments in which large angles of incidence are chosen for the purpose of obtaining negative lateral displacements. (C) 2005 Elsevier B.V. All rights reserved.

High negative ion production yield in 30 keV F2+ + adenine (C5H5N5) collisions

In collisions between slow F2+ ions (30 keV) and molecular targets, adenine, scattered particle production yields have been measured directly by simultaneous detection of neutrals, positive and negative ions. The relative cross-section for a negative ion formation channel was measured to be 1%. Despite a slight decrease compared to a larger target, the fullerene C-60, the measured negative ion formation cross section is still at least one order of magnitude larger than the yield in ion-atom interactions.

Can the nuclear symmetry potential at supra-saturation densities be negative?

In the framework of an isospin-dependent Boltzmann-Uehling-Uhlenbeck (IBUU) transport model, for the central Au-197 + Au-197 reaction at an incident beam energy of 400 MeV/nucleon, the effect of nuclear symmetry potential at supra-saturation densities on the preequilibrium clusters emission is studied. It is found that for the positive symmetry potential at supra-saturation densities the neutron-to-proton ratio of lighter clusters with mass number A less than or similar to 3 [(n/p)(A less than or similar to 3)] is larger than that of the heavier clusters with mass number A > 3 [(n/p)(A>3)], whereas for the negative symmetry potential at supra-saturation densities the (n/p)(A less than or similar to 3) is smaller than the (n/p)(A>3). This may be considered as a probe of the negative symmetry potential at supra-saturation densities.

Origin of negative differential resistance and memory characteristics in organic devices based on tris(8-hydroxyquinoline) aluminum

Negative differential resistance (NDR) and memory phenomenon have been realized in current-voltage (I-V) characteristics of indium tin oxide/tris(8-hydroxyquinoline) aluminum/aluminum devices. The I-V curves have been divided into three operational regions that are associated with different working regimes of the devices: (i) bistable region, (ii) NDR region, and (iii) monotonic region. The bistable region disappeared after a couple of voltage sweeps from zero to a positive voltage. The bistable nature can be reinstated by applying a suitable negative voltage.

Direct patterning of negative nanostructures on self-assembled monolayers of 16-mercaptohexadecanoic acid on Au(111) substrate via dip-pen nanolithography

Both bare and self-assembled monolayer (SAM) protected gold substrate could be etched by allyl bromide according to atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS) and inductively coupled plasma mass spectrometric (ICPMS) analysis results. With this allyl bromide ink material, negative nanopatterns could be fabricated directly by dip-pen nanolithography (DPN) on SAMs of 16-mercaptohexadecanoic acid (MHA) on Au(111) substrate. A tip-promoted etching mechanism was proposed where the gold-reactive ink could penetrate the MHA resist film through tip-induced defects resulting in local corrosive removal of the gold substrate. The fabrication mechanism was also confirmed by electrochemical characterization, energy dispersive spectroscopy (EDS) analysis and fabrication of positive nanopatterns via a used DPN tip.

Global negative vegetation feedback to climate warming responses of leaf litter decomposition rates in cold biomes

Whether climate change will turn cold biomes from large long-term carbon sinks into sources is hotly debated because of the great potential for ecosystem-mediated feedbacks to global climate. Critical are the direction, magnitude and generality of climate responses of plant litter decomposition. Here, we present the first quantitative analysis of the major climate-change-related drivers of litter decomposition rates in cold northern biomes worldwide. Leaf litters collected from the predominant species in 33 global change manipulation experiments in circum-arctic-alpine ecosystems were incubated simultaneously in two contrasting arctic life zones. We demonstrate that longer-term, large-scale changes to leaf litter decomposition will be driven primarily by both direct warming effects and concomitant shifts in plant growth form composition, with a much smaller role for changes in litter quality within species. Specifically, the ongoing warming-induced expansion of shrubs with recalcitrant leaf litter across cold biomes would constitute a negative feedback to global warming. Depending on the strength of other (previously reported) positive feedbacks of shrub expansion on soil carbon turnover, this may partly counteract direct warming enhancement of litter decomposition.