1,861 research outputs found
Stochastic dynamics of macromolecular-assembly networks
The formation and regulation of macromolecular complexes provides the
backbone of most cellular processes, including gene regulation and signal
transduction. The inherent complexity of assembling macromolecular structures
makes current computational methods strongly limited for understanding how the
physical interactions between cellular components give rise to systemic
properties of cells. Here we present a stochastic approach to study the
dynamics of networks formed by macromolecular complexes in terms of the
molecular interactions of their components. Exploiting key thermodynamic
concepts, this approach makes it possible to both estimate reaction rates and
incorporate the resulting assembly dynamics into the stochastic kinetics of
cellular networks. As prototype systems, we consider the lac operon and phage
lambda induction switches, which rely on the formation of DNA loops by proteins
and on the integration of these protein-DNA complexes into intracellular
networks. This cross-scale approach offers an effective starting point to move
forward from network diagrams, such as those of protein-protein and DNA-protein
interaction networks, to the actual dynamics of cellular processes.Comment: Open Access article available at
http://www.nature.com/msb/journal/v2/n1/full/msb4100061.htm
The Lazy Bureaucrat Scheduling Problem
We introduce a new class of scheduling problems in which the optimization is
performed by the worker (single ``machine'') who performs the tasks. A typical
worker's objective is to minimize the amount of work he does (he is ``lazy''),
or more generally, to schedule as inefficiently (in some sense) as possible.
The worker is subject to the constraint that he must be busy when there is work
that he can do; we make this notion precise both in the preemptive and
nonpreemptive settings. The resulting class of ``perverse'' scheduling
problems, which we denote ``Lazy Bureaucrat Problems,'' gives rise to a rich
set of new questions that explore the distinction between maximization and
minimization in computing optimal schedules.Comment: 19 pages, 2 figures, Latex. To appear, Information and Computatio
Probabilistic Bounds on the Length of a Longest Edge in Delaunay Graphs of Random Points in d-Dimensions
Motivated by low energy consumption in geographic routing in wireless
networks, there has been recent interest in determining bounds on the length of
edges in the Delaunay graph of randomly distributed points. Asymptotic results
are known for random networks in planar domains. In this paper, we obtain upper
and lower bounds that hold with parametric probability in any dimension, for
points distributed uniformly at random in domains with and without boundary.
The results obtained are asymptotically tight for all relevant values of such
probability and constant number of dimensions, and show that the overhead
produced by boundary nodes in the plane holds also for higher dimensions. To
our knowledge, this is the first comprehensive study on the lengths of long
edges in Delaunay graphsComment: 10 pages. 2 figures. In Proceedings of the 23rd Canadian Conference
on Computational Geometry (CCCG 2011). Replacement of version 1106.4927,
reference [5] adde
Lambda-prophage induction modeled as a cooperative failure mode of lytic repression
We analyze a system-level model for lytic repression of lambda-phage in E.
coli using reliability theory, showing that the repressor circuit comprises 4
redundant components whose failure mode is prophage induction. Our model
reflects the specific biochemical mechanisms involved in regulation, including
long-range cooperative binding, and its detailed predictions for prophage
induction in E. coli under ultra-violet radiation are in good agreement with
experimental data.Comment: added referenc
Analytical study of an exclusive genetic switch
The nonequilibrium stationary state of an exclusive genetic switch is
considered. The model comprises two competing species and a single binding site
which, when bound to by a protein of one species, causes the other species to
be repressed. The model may be thought of as a minimal model of the power
struggle between two competing parties. Exact solutions are given for the
limits of vanishing binding/unbinding rates and infinite binding/unbinding
rates. A mean field theory is introduced which is exact in the limit of
vanishing binding/unbinding rates. The mean field theory and numerical
simulations reveal that generically bistability occurs and the system is in a
symmetry broken state. An exact perturbative solution which in principle allows
the nonequilibrium stationary state to be computed is also developed and
computed to first and second order.Comment: 28 pages, 6 figure
Enhancement of the stability of genetic switches by overlapping upstream regulatory domains
We study genetic switches formed from pairs of mutually repressing operons.
The switch stability is characterised by a well defined lifetime which grows
sub-exponentially with the number of copies of the most-expressed transcription
factor, in the regime accessible by our numerical simulations. The stability
can be markedly enhanced by a suitable choice of overlap between the upstream
regulatory domains. Our results suggest that robustness against biochemical
noise can provide a selection pressure that drives operons, that regulate each
other, together in the course of evolution.Comment: 4 pages, 5 figures, RevTeX
Stochastic Simulations of the Repressilator Circuit
The genetic repressilator circuit consists of three transcription factors, or
repressors, which negatively regulate each other in a cyclic manner. This
circuit was synthetically constructed on plasmids in {\it Escherichia coli} and
was found to exhibit oscillations in the concentrations of the three
repressors. Since the repressors and their binding sites often appear in low
copy numbers, the oscillations are noisy and irregular. Therefore, the
repressilator circuit cannot be fully analyzed using deterministic methods such
as rate-equations. Here we perform stochastic analysis of the repressilator
circuit using the master equation and Monte Carlo simulations. It is found that
fluctuations modify the range of conditions in which oscillations appear as
well as their amplitude and period, compared to the deterministic equations.
The deterministic and stochastic approaches coincide only in the limit in which
all the relevant components, including free proteins, plasmids and bound
proteins, appear in high copy numbers. We also find that subtle features such
as cooperative binding and bound-repressor degradation strongly affect the
existence and properties of the oscillations.Comment: Accepted to PR
Draft Genome Sequence for Desulfovibrio africanus Strain PCS.
Desulfovibrio africanus strain PCS is an anaerobic sulfate-reducing bacterium (SRB) isolated from sediment from Paleta Creek, San Diego, CA. Strain PCS is capable of reducing metals such as Fe(III) and Cr(VI), has a cell cycle, and is predicted to produce methylmercury. We present the D. africanus PCS genome sequence
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