64,119 research outputs found
Protein Mobility in the Cytoplasm of Escherichia coli
The rate of protein diffusion in bacterial cytoplasm may constrain a variety of cellular functions and limit the rates of many biochemical reactions in vivo. In this paper, we report noninvasive measurements of the apparent diffusion coefficient of green fluorescent protein (GFP) in the cytoplasm of Escherichia coli. These measurements were made in two ways: by photobleaching of GFP fluorescence and by photoactivation of a red-emitting fluorescent state of GFP (M. B. Elowitz, M. G. Surette, P. E. Wolf, J. Stock, and S. Leibler, Curr. Biol. 7:809-812, 1997). The apparent diffusion coefficient, Da, of GFP in E. coli DH5alpha was found to be 7.7 ± 2.5 µm^2/s. A 72-kDa fusion protein composed of GFP and a cytoplasmically localized maltose binding protein domain moves more slowly, with Da of 2.5 ± 0.6 µm^2/s. In addition, GFP mobility can depend strongly on at least two factors: first, Da is reduced to 3.6 ± 0.7 µm^2/s at high levels of GFP expression; second, the addition to GFP of a small tag consisting of six histidine residues reduces Da to 4.0 ± 2.0 µm^2/s. Thus, a single effective cytoplasmic viscosity cannot explain all values of Da reported here. These measurements have implications for the understanding of intracellular biochemical networks
Non-stationary patterns of isolation-by-distance: inferring measures of local genetic differentiation with Bayesian kriging
Patterns of isolation-by-distance arise when population differentiation
increases with increasing geographic distances. Patterns of
isolation-by-distance are usually caused by local spatial dispersal, which
explains why differences of allele frequencies between populations accumulate
with distance. However, spatial variations of demographic parameters such as
migration rate or population density can generate non-stationary patterns of
isolation-by-distance where the rate at which genetic differentiation
accumulates varies across space. To characterize non-stationary patterns of
isolation-by-distance, we infer local genetic differentiation based on Bayesian
kriging. Local genetic differentiation for a sampled population is defined as
the average genetic differentiation between the sampled population and fictive
neighboring populations. To avoid defining populations in advance, the method
can also be applied at the scale of individuals making it relevant for
landscape genetics. Inference of local genetic differentiation relies on a
matrix of pairwise similarity or dissimilarity between populations or
individuals such as matrices of FST between pairs of populations. Simulation
studies show that maps of local genetic differentiation can reveal barriers to
gene flow but also other patterns such as continuous variations of gene flow
across habitat. The potential of the method is illustrated with 2 data sets:
genome-wide SNP data for human Swedish populations and AFLP markers for alpine
plant species. The software LocalDiff implementing the method is available at
http://membres-timc.imag.fr/Michael.Blum/LocalDiff.htmlComment: In press, Evolution 201
The mean, variance and limiting distribution of two statistics sensitive to phylogenetic tree balance
For two decades, the Colless index has been the most frequently used
statistic for assessing the balance of phylogenetic trees. In this article,
this statistic is studied under the Yule and uniform model of phylogenetic
trees. The main tool of analysis is a coupling argument with another well-known
index called the Sackin statistic. Asymptotics for the mean, variance and
covariance of these two statistics are obtained, as well as their limiting
joint distribution for large phylogenies. Under the Yule model, the limiting
distribution arises as a solution of a functional fixed point equation. Under
the uniform model, the limiting distribution is the Airy distribution. The
cornerstone of this study is the fact that the probabilistic models for
phylogenetic trees are strongly related to the random permutation and the
Catalan models for binary search trees.Comment: Published at http://dx.doi.org/10.1214/105051606000000547 in the
Annals of Applied Probability (http://www.imstat.org/aap/) by the Institute
of Mathematical Statistics (http://www.imstat.org
Modular properties of two-loop maximal supergravity and connections with string theory
The low-momentum expansion of the two-loop four-graviton scattering amplitude
in eleven-dimensional supergravity compactified on a circle and a two-torus is
considered up to terms of order S^6R^4 (where S is a Mandelstam invariant and R
is the linearized Weyl curvature). In the case of the toroidal compactification
the coefficient of each term in the low energy expansion is generically a sum
of a number of SL(2,Z)-invariant functions of the complex structure of the
torus. Each such function satisfies a separate Poisson equation on moduli space
with particular source terms that are bilinear in coefficients of lower order
terms, consistent with qualitative arguments based on supersymmetry. Comparison
is made with the low-energy expansion of type II string theories in ten and
nine dimensions. Although the detailed behaviour of the string amplitude is not
generally expected to be reproduced by supergravity perturbation theory to all
orders, for the terms considered here we find agreement with direct results
from string perturbation theory. These results point to a fascinating pattern
of interrelated Poisson equations for the IIB coefficients at higher orders in
the momentum expansion which may have a significance beyond the particular
methods by which they were motivated.Comment: 79 pages, 4 figures. Latex format. v2: Small corrections made,
version to appear in JHE
Government Solvency: Revisiting some EMU Countries
Corsetti and Roubini (1991) reported that the government finances of Greece, Ireland, Italy and the Netherlands (now all EMU countries) did not satisfy the intertemporal budget constraint (IBC). We re-examine this issue by utilizing a new empirical approach and extended data set. Structural shifts, an issue which Corsetti and Roubini were unable to address due to the lack of suitable econometric methods, are tackled. We find that: (i) multiple structural shifts, most of which correspond to important policy changes, did occur in the fiscal path of these countries; (ii) the effect of the majority of structural shifts has been to strengthen the evidence supporting IBC; and (iii) government finances of all four countries satisfy the IBC and this finding is robust to different time horizons. We also find a clear positive Maastricht effect on IBC for all countries.intertemporal budget constraints, strong and weak form sustainability, structural breaks
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