469 research outputs found
Bacteriophage lambda cro mutations: effects on activity and intracellular degradation.
Topological effects in ring polymers: A computer simulation study
Unconcatenated, unknotted polymer rings in the melt are subject to strong
interactions with neighboring chains due to the presence of topological
constraints. We study this by computer simulation using the bond-fluctuation
algorithm for chains with up to N=512 statistical segments at a volume fraction
\Phi=0.5 and show that rings in the melt are more compact than gaussian chains.
A careful finite size analysis of the average ring size R \propto N^{\nu}
yields an exponent \nu \approx 0.39 \pm 0.03 in agreement with a Flory-like
argument for the topologica interactions. We show (using the same algorithm)
that the dynamics of molten rings is similar to that of linear chains of the
same mass, confirming recent experimental findings. The diffusion constant
varies effectively as D_{N} \propto N^{-1.22(3) and is slightly higher than
that of corresponding linear chains. For the ring sizes considered (up to 256
statistical segments) we find only one characteristic time scale \tau_{ee}
\propto N^{2.0(2); this is shown by the collapse of several mean-square
displacements and correlation functions onto corresponding master curves.
Because of the shrunken state of the chain, this scaling is not compatible with
simple Rouse motion. It applies for all sizes of ring studied and no sign of a
crossover to any entangled regime is found.Comment: 20 Pages,11 eps figures, Late
On a Generalization of Zaslavsky's Theorem for Hyperplane Arrangements
We define arrangements of codimension-1 submanifolds in a smooth manifold
which generalize arrangements of hyperplanes. When these submanifolds are
removed the manifold breaks up into regions, each of which is homeomorphic to
an open disc. The aim of this paper is to derive formulas that count the number
of regions formed by such an arrangement. We achieve this aim by generalizing
Zaslavsky's theorem to this setting. We show that this number is determined by
the combinatorics of the intersections of these submanifolds.Comment: version 3: The title had a typo in v2 which is now fixed. Will appear
in Annals of Combinatorics. Version. 2: 19 pages, major revision in terms of
style and language, some results improved, contact information updated, final
versio
Volume Effects on the Glass Transition Dynamics
The role of jamming (steric constraints) and its relationship to the
available volume is addressed by examining the effect that certain
modifications of a glass-former have on the ratio of its isochoric and isobaric
activation enthalpies. This ratio reflects the relative contribution of volume
(density) and temperature (thermal energy) to the temperature-dependence of the
relaxation times of liquids and polymers. We find that an increase in the
available volume confers a stronger volume-dependence to the relaxation
dynamics, a result at odds with free volume interpretations of the glass
transition.Comment: 9 pages 5 figure
DNA looping provides stability and robustness to the bacteriophage lambda switch
The bistable gene regulatory switch controlling the transition from lysogeny
to lysis in bacteriophage lambda presents a unique challenge to quantitative
modeling. Despite extensive characterization of this regulatory network, the
origin of the extreme stability of the lysogenic state remains unclear. We have
constructed a stochastic model for this switch. Using Forward Flux Sampling
simulations, we show that this model predicts an extremely low rate of
spontaneous prophage induction in a recA mutant, in agreement with experimental
observations. In our model, the DNA loop formed by octamerization of CI bound
to the O_L and O_R operator regions is crucial for stability, allowing the
lysogenic state to remain stable even when a large fraction of the total CI is
depleted by nonspecific binding to genomic DNA. DNA looping also ensures that
the switch is robust to mutations in the order of the O_R binding sites. Our
results suggest that DNA looping can provide a mechanism to maintain a stable
lysogenic state in the face of a range of challenges including noisy gene
expression, nonspecific DNA binding and operator site mutations.Comment: In press on PNAS. Single file contains supplementary inf
Evolution favors protein mutational robustness in sufficiently large populations
BACKGROUND: An important question is whether evolution favors properties such
as mutational robustness or evolvability that do not directly benefit any
individual, but can influence the course of future evolution. Functionally
similar proteins can differ substantially in their robustness to mutations and
capacity to evolve new functions, but it has remained unclear whether any of
these differences might be due to evolutionary selection for these properties.
RESULTS: Here we use laboratory experiments to demonstrate that evolution
favors protein mutational robustness if the evolving population is sufficiently
large. We neutrally evolve cytochrome P450 proteins under identical selection
pressures and mutation rates in populations of different sizes, and show that
proteins from the larger and thus more polymorphic population tend towards
higher mutational robustness. Proteins from the larger population also evolve
greater stability, a biophysical property that is known to enhance both
mutational robustness and evolvability. The excess mutational robustness and
stability is well described by existing mathematical theories, and can be
quantitatively related to the way that the proteins occupy their neutral
network.
CONCLUSIONS: Our work is the first experimental demonstration of the general
tendency of evolution to favor mutational robustness and protein stability in
highly polymorphic populations. We suggest that this phenomenon may contribute
to the mutational robustness and evolvability of viruses and bacteria that
exist in large populations
Measurement of XUV-absorption spectra of ZnS radiatively heated foils
Time-resolved absorption of zinc sulfide (ZnS) and aluminum in the XUV-range
has been measured. Thin foils in conditions close to local thermodynamic
equilibrium were heated by radiation from laser-irradiated gold spherical
cavities. Analysis of the aluminum foil radiative hydrodynamic expansion, based
on the detailed atomic calculations of its absorption spectra, showed that the
cavity emitted flux that heated the absorption foils corresponds to a radiation
temperature in the range 55 60 eV. Comparison of the ZnS absorption spectra
with calculations based on a superconfiguration approach identified the
presence of species Zn6+ - Zn8+ and S5+ - S6+. Based on the validation of the
radiative source simulations, experimental spectra were then compared to
calculations performed by post-processing the radiative hydrodynamic
simulations of ZnS. Satisfying agreement is found when temperature gradients
are accounted for
Inferring stabilizing mutations from protein phylogenies : application to influenza hemagglutinin
One selection pressure shaping sequence evolution is the requirement that a protein fold with sufficient stability to perform its biological functions. We present a conceptual framework that explains how this requirement causes the probability that a particular amino acid mutation is fixed during evolution to depend on its effect on protein stability. We mathematically formalize this framework to develop a Bayesian approach for inferring the stability effects of individual mutations from homologous protein sequences of known phylogeny. This approach is able to predict published experimentally measured mutational stability effects (ÎÎG values) with an accuracy that exceeds both a state-of-the-art physicochemical modeling program and the sequence-based consensus approach. As a further test, we use our phylogenetic inference approach to predict stabilizing mutations to influenza hemagglutinin. We introduce these mutations into a temperature-sensitive influenza virus with a defect in its hemagglutinin gene and experimentally demonstrate that some of the mutations allow the virus to grow at higher temperatures. Our work therefore describes a powerful new approach for predicting stabilizing mutations that can be successfully applied even to large, complex proteins such as hemagglutinin. This approach also makes a mathematical link between phylogenetics and experimentally measurable protein properties, potentially paving the way for more accurate analyses of molecular evolution
Extreme genetic fragility of the HIV-1 capsid
Genetic robustness, or fragility, is defined as the ability, or lack thereof, of a biological entity to maintain function in the face of mutations. Viruses that replicate via RNA intermediates exhibit high mutation rates, and robustness should be particularly advantageous to them. The capsid (CA) domain of the HIV-1 Gag protein is under strong pressure to conserve functional roles in viral assembly, maturation, uncoating, and nuclear import. However, CA is also under strong immunological pressure to diversify. Therefore, it would be particularly advantageous for CA to evolve genetic robustness. To measure the genetic robustness of HIV-1 CA, we generated a library of single amino acid substitution mutants, encompassing almost half the residues in CA. Strikingly, we found HIV-1 CA to be the most genetically fragile protein that has been analyzed using such an approach, with 70% of mutations yielding replication-defective viruses. Although CA participates in several steps in HIV-1 replication, analysis of conditionally (temperature sensitive) and constitutively non-viable mutants revealed that the biological basis for its genetic fragility was primarily the need to coordinate the accurate and efficient assembly of mature virions. All mutations that exist in naturally occurring HIV-1 subtype B populations at a frequency >3%, and were also present in the mutant library, had fitness levels that were >40% of WT. However, a substantial fraction of mutations with high fitness did not occur in natural populations, suggesting another form of selection pressure limiting variation in vivo. Additionally, known protective CTL epitopes occurred preferentially in domains of the HIV-1 CA that were even more genetically fragile than HIV-1 CA as a whole. The extreme genetic fragility of HIV-1 CA may be one reason why cell-mediated immune responses to Gag correlate with better prognosis in HIV-1 infection, and suggests that CA is a good target for therapy and vaccination strategies
Correlation of gene expression and protein production rate - a system wide study
<p>Abstract</p> <p>Background</p> <p>Growth rate is a major determinant of intracellular function. However its effects can only be properly dissected with technically demanding chemostat cultivations in which it can be controlled. Recent work on <it>Saccharomyces cerevisiae </it>chemostat cultivations provided the first analysis on genome wide effects of growth rate. In this work we study the filamentous fungus <it>Trichoderma reesei </it>(<it>Hypocrea jecorina</it>) that is an industrial protein production host known for its exceptional protein secretion capability. Interestingly, it exhibits a low growth rate protein production phenotype.</p> <p>Results</p> <p>We have used transcriptomics and proteomics to study the effect of growth rate and cell density on protein production in chemostat cultivations of <it>T. reesei</it>. Use of chemostat allowed control of growth rate and exact estimation of the extracellular specific protein production rate (SPPR). We find that major biosynthetic activities are all negatively correlated with SPPR. We also find that expression of many genes of secreted proteins and secondary metabolism, as well as various lineage specific, mostly unknown genes are positively correlated with SPPR. Finally, we enumerate possible regulators and regulatory mechanisms, arising from the data, for this response.</p> <p>Conclusions</p> <p>Based on these results it appears that in low growth rate protein production energy is very efficiently used primarly for protein production. Also, we propose that flux through early glycolysis or the TCA cycle is a more fundamental determining factor than growth rate for low growth rate protein production and we propose a novel eukaryotic response to this i.e. the lineage specific response (LSR).</p
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