56 research outputs found
Loop models, Marginally Rough Interfaces, and the Coulomb Gas
We develop a coarse-graining procedure for two-dimensional models of
fluctuating loops by mapping them to interface models. The result is an
effective field theory for the scaling limit of loop models, which is found to
be a Liouville theory with imaginary couplings. This field theory is {\it
completely specified} by geometry and conformal invariance {\it alone}, and it
leads to exact results for the critical exponents and the conformal charge of
loop models. A physical interpretation of the Dotsenko-Fateev screening charge
is found.Comment: LaTex, 7 pages, 1 PostScript figure; uses epsf.sty. To appear in the
proceedings of the symposium''Exactly soluable models in Statistical
Mechanics'', March 1996, Northeastern University, Bosto
Critical Dynamics of Dimers: Implications for the Glass Transition
The Adam-Gibbs view of the glass transition relates the relaxation time to
the configurational entropy, which goes continuously to zero at the so-called
Kauzmann temperature. We examine this scenario in the context of a dimer model
with an entropy vanishing phase transition, and stochastic loop dynamics. We
propose a coarse-grained master equation for the order parameter dynamics which
is used to compute the time-dependent autocorrelation function and the
associated relaxation time. Using a combination of exact results, scaling
arguments and numerical diagonalizations of the master equation, we find
non-exponential relaxation and a Vogel-Fulcher divergence of the relaxation
time in the vicinity of the phase transition. Since in the dimer model the
entropy stays finite all the way to the phase transition point, and then jumps
discontinuously to zero, we demonstrate a clear departure from the Adam-Gibbs
scenario. Dimer coverings are the "inherent structures" of the canonical
frustrated system, the triangular Ising antiferromagnet. Therefore, our results
provide a new scenario for the glass transition in supercooled liquids in terms
of inherent structure dynamics
Membrane mechanics as a probe of ion-channel gating mechanisms
The details of conformational changes undergone by transmembrane ion channels in response to stimuli, such as electric fields and membrane tension, remain controversial. We approach this problem by considering how the conformational changes impose deformations in the lipid bilayer. We focus on the role of bilayer deformations in the context of voltage-gated channels because we hypothesize that such deformations are relevant in this case as well as for channels that are explicitly mechanosensitive. As a result of protein conformational changes, we predict that the lipid bilayer suffers deformations with a characteristic free-energy scale of 10kBT. This free energy is comparable to the voltage-dependent part of the total gating energy, and we argue that these deformations could play an important role in the overall free-energy budget of gating. As a result, channel activity will depend upon mechanical membrane parameters such as tension and leaflet thickness. We further argue that the membrane deformation around any channel can be divided into three generic classes of deformation that exhibit different mechanosensitive properties. Finally, we provide the theoretical framework that relates conformational changes during gating to tension and leaflet thickness dependence in the critical gating voltage. This line of investigation suggests experiments that could discern the dominant deformation imposed upon the membrane as a result of channel gating, thus providing clues as to the channel deformation induced by the stimulus
Deciphering transcriptional dynamics in vivo by counting nascent RNA molecules
Transcription of genes is the focus of most forms of regulation of gene
expression. Even though careful biochemical experimentation has revealed the
molecular mechanisms of transcription initiation for a number of different
promoters in vitro, the dynamics of this process in cells is still poorly
understood. One approach has been to measure the transcriptional output
(fluorescently labeled messenger RNAs or proteins) from single cells in a
genetically identical population, which could then be compared to predictions
from models that incorporate different molecular mechanisms of transcription
initiation. However, this approach suffers from the problem, that processes
downstream from transcription can affect the measured output and therefore mask
the signature of stochastic transcription initiation on the cell-to-cell
variability of the transcriptional outputs. Here we show theoretically that
measurements of the cell-to-cell variability in the number of nascent RNAs
provide a more direct test of the mechanism of transcription initiation. We
derive exact expressions for the first two moments of the distribution of
nascent RNA molecules and apply our theory to published data for a collection
of constitutively expressed yeast genes. We find that the measured nascent RNA
distributions are inconsistent with transcription initiation proceeding via one
rate-limiting step, which has been generally inferred from measurements of
cytoplasmic messenger RNA. Instead, we propose a two-step mechanism of
initiation, which is consistent with the available data. These findings for the
yeast promoters highlight the utility of our theory for deciphering
transcriptional dynamics in vivo from experiments that count nascent RNA
molecules in single cells.Comment: 28 pages including S
Antenna mechanism of length control of actin cables
Actin cables are linear cytoskeletal structures that serve as tracks for
myosin-based intracellular transport of vesicles and organelles in both yeast
and mammalian cells. In a yeast cell undergoing budding, cables are in constant
dynamic turnover yet some cables grow from the bud neck toward the back of the
mother cell until their length roughly equals the diameter of the mother cell.
This raises the question: how is the length of these cables controlled? Here we
describe a novel molecular mechanism for cable length control inspired by
recent experimental observations in cells. This antenna mechanism involves
three key proteins: formins, which polymerize actin, Smy1 proteins, which bind
formins and inhibit actin polymerization, and myosin motors, which deliver Smy1
to formins, leading to a length-dependent actin polymerization rate. We compute
the probability distribution of cable lengths as a function of several
experimentally tuneable parameters such as the formin-binding affinity of Smy1
and the concentration of myosin motors delivering Smy1. These results provide
testable predictions of the antenna mechanism of actin-cable length control
Operator Sequence Alters Gene Expression Independently of Transcription Factor Occupancy in Bacteria
A canonical quantitative view of transcriptional regulation holds that the only role of operator sequence is to set the probability of transcription factor binding, with operator occupancy determining the level of gene expression. In this work, we test this idea by characterizing repression in vivo and the binding of RNA polymerase in vitro in experiments where operators of various sequences were placed either upstream or downstream from the promoter in Escherichia coli. Surprisingly, we find that operators with a weaker binding affinity can yield higher repression levels than stronger operators. Repressor bound to upstream operators modulates promoter escape, and the magnitude of this modulation is not correlated with the repressor-operator binding affinity. This suggests that operator sequences may modulate transcription by altering the nature of the interaction of the bound transcription factor with the transcriptional machinery, implying a new layer of sequence dependence that must be confronted in the quantitative understanding of gene expression
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