56 research outputs found
How the DNA sequence affects the Hill curve of transcriptional response
The Hill coefficient is often used as a direct measure of the cooperativity
of binding processes. It is an essential tool for probing properties of
reactions in many biochemical systems. Here we analyze existing experimental
data and demonstrate that the Hill coefficient characterizing the binding of
transcription factors to their cognate sites can in fact be larger than one --
the standard indication of cooperativity -- even in the absence of any standard
cooperative binding mechanism. By studying the problem analytically, we
demonstrate that this effect occurs due to the disordered binding energy of the
transcription factor to the DNA molecule and the steric interactions between
the different copies of the transcription factor. We show that the enhanced
Hill coefficient implies a significant reduction in the number of copies of the
transcription factors which is needed to occupy a cognate site and, in many
cases, can explain existing estimates for numbers of the transcription factors
in cells. The mechanism is general and should be applicable to other biological
recognition processes.Comment: 9 pages, 7 figure
Classes of fast and specific search mechanisms for proteins on DNA
Problems of search and recognition appear over different scales in biological
systems. In this review we focus on the challenges posed by interactions
between proteins, in particular transcription factors, and DNA and possible
mechanisms which allow for a fast and selective target location. Initially we
argue that DNA-binding proteins can be classified, broadly, into three distinct
classes which we illustrate using experimental data. Each class calls for a
different search process and we discuss the possible application of different
search mechanisms proposed over the years to each class. The main thrust of
this review is a new mechanism which is based on barrier discrimination. We
introduce the model and analyze in detail its consequences. It is shown that
this mechanism applies to all classes of transcription factors and can lead to
a fast and specific search. Moreover, it is shown that the mechanism has
interesting transient features which allow for stability at the target despite
rapid binding and unbinding of the transcription factor from the target.Comment: 65 pages, 23 figure
Classical R-Matrices and the Feigin-Odesskii Algebra via Hamiltonian and Poisson Reductions
We present a formula for a classical -matrix of an integrable system
obtained by Hamiltonian reduction of some free field theories using pure gauge
symmetries. The framework of the reduction is restricted only by the assumption
that the respective gauge transformations are Lie group ones. Our formula is in
terms of Dirac brackets, and some new observations on these brackets are made.
We apply our method to derive a classical -matrix for the elliptic
Calogero-Moser system with spin starting from the Higgs bundle over an elliptic
curve with marked points. In the paper we also derive a classical
Feigin-Odesskii algebra by a Poisson reduction of some modification of the
Higgs bundle over an elliptic curve. This allows us to include integrable
lattice models in a Hitchin type construction.Comment: 27 pages LaTe
Effects of intersegmental transfers on target location by proteins
We study a model for a protein searching for a target, using facilitated
diffusion, on a DNA molecule confined in a finite volume. The model includes
three distinct pathways for facilitated diffusion: (a) sliding - in which the
protein diffuses along the contour of the DNA (b) jumping - where the protein
travels between two sites along the DNA by three-dimensional diffusion, and
finally (c) intersegmental transfer - which allows the protein to move from one
site to another by transiently binding both at the same time. The typical
search time is calculated using scaling arguments which are verified
numerically. Our results suggest that the inclusion of intersegmental transfer
(i) decreases the search time considerably (ii) makes the search time much more
robust to variations in the parameters of the model and (iii) that the optimal
search time occurs in a regime very different than that found for models which
ignore intersegmental transfers. The behavior we find is rich and shows
surprising dependencies, for example, on the DNA length.Comment: 40 pages, 14 figure
A simple model for DNA bridging proteins and bacterial or human genomes:bridging-induced attraction and genome compaction
Search reliability and search efficiency of combined Lévy–Brownian motion: long relocations mingled with thorough local exploration
A combined dynamics consisting of Brownian motion and Levy flights is exhibited by a variety of biological systems performing search processes. Assessing the search reliability of ever locating the target and the search efficiency of doing so economically of such dynamics thus poses an important problem. Here we model this dynamics by a one-dimensional fractional Fokker-Planck equation combining unbiased Brownian motion and Levy flights. By solving this equation both analytically and numerically we show that the superposition of recurrent Brownian motion and Levy flights with stable exponent α<1, by itself implying zero probability of hitting a point on a line, lead to transient motion with finite probability of hitting any point on the line. We present results for the exact dependence of the values of both the search reliability and the search efficiency on the distance between the starting and target positions as well as the choice of the scaling exponent α of the Levy flight component
An analytic expression for the HBT extrinsic base-collector capacitance derived from S-parameter measurements
Direct extraction is the most accurate method for the determination of equivalent-circuits of heterojunction bipolar transistors (HBTs). However, previous work lacks an exact expression for the extrinsic base-collector capacitance, which models the distributed nature of the base. This paper gives the derivation of an exact expression for this capacitance. As a result, each intrinsic equivalent-circuit parameter is determined using a simple exact expression at each measured frequency. The expression is valid for both the hybrid- /spl pi/ and the physics-based T-topology equivalent circuits. Extraction results for InP- and GaAs-HBTs are given
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