10 research outputs found
Approche de la Couleur
Engineering school- apprendre à voir/regarder les couleurs de façon consciente : « s'éveiller au monde de la couleur ». Bien que cet aspect ne puisse pas s'enseigner au moyen des livres et des formules, c'est le point le plus important de ce cours : rien ne remplace l'œil d'un bon coloriste, et pour le former, il faut observer autour de soi.- Apprendre à reconnaître, trier et caractériser les couleurs. Ce point vient soutenir le précédent.- Apprendre à mesurer précisément une couleur au moyen d'appareils de mesure dont il faut comprendre le fonctionnement, afin de la contrôler et la reproduire à l'identique.- Dans le domaine de la colorimétrie, la couleur à reproduire s'appelle le « standard » et sa reproduction prend le nom de « contretype ». On parle donc de « contretyper un standard », et la production de contretypes acceptables pour un standard donné est un des problèmes récurent de la colorimétrie
Modelling the effect of ribosome mobility on the rate of protein synthesis
Translation is one of the main steps in the synthesis of proteins. It
consists of ribosomes that translate sequences of nucleotides encoded on mRNA
into polypeptide sequences of amino acids. Ribosomes bound to mRNA move
unidirectionally, while unbound ribosomes diffuse in the cytoplasm. It has been
hypothesized that finite diffusion of ribosomes plays an important role in
ribosome recycling and that mRNA circularization enhances the efficiency of
translation. In order to estimate the effect of cytoplasmic diffusion on the
rate of translation, we consider a Totally Asymmetric Simple Exclusion Process
(TASEP) coupled to a finite diffusive reservoir, which we call the Ribosome
Transport model with Diffusion (RTD). In this model, we derive an analytical
expression for the rate of protein synthesis as a function of the diffusion
constant of ribosomes, which is corroborated with results from continuous-time
Monte Carlo simulations. Using a wide range of biological relevant parameters,
we conclude that diffusion in biological cells is fast enough so that it does
not play a role in controlling the rate of translation initiation.Comment: article, 16 pages, 5 figure
Modeling supercoiled DNA interacting with an anchored cluster of proteins: towards a quantitative estimation of chromosomal DNA supercoiling
We investigate the measurement of DNA supercoiling density () along
chromosomes using interaction frequencies between DNA and DNA-anchored clusters
of proteins. Specifically, we show how the physics of DNA supercoiling leads,
in bacteria, to the quantitative modeling of binding properties of ParB
proteins around their centromere-like site, {\it parS}. Using this framework,
we provide an upper bound for in the {\it Escherichia coli}
chromosome, consistent with plasmid values, and offer a proof of concept for a
high accuracy measurement. To reach these conclusions, we revisit the problem
of the formation of ParB clusters. We predict, in particular, that they result
from a non-equilibrium, stationary balance between an influx of produced
proteins and an outflux of excess proteins, i.e., they behave like liquid-like
protein condensates with unconventional ``leaky'' boundaries.Comment: 5 pages + Supplementary Info (including 3 figures
Signature of (anti)cooperativity in the stochastic fluctuations of small systems: application to the bacterial flagellar motor
The cooperative binding of molecular agents onto a substrate is pervasive in
living systems. To study whether a system shows cooperativity, one can rely on
a fluctuation analysis of quantities such as the number of substrate-bound
units and the residence time in an occupancy state. Since the relative standard
deviation from the statistical mean monotonically decreases with the number of
binding sites, these techniques are only suitable for small enough systems,
such as those implicated in stochastic processes inside cells. Here, we present
a general-purpose grand canonical Hamiltonian description of a small
one-dimensional (1D) lattice gas with either nearest-neighbor or long-range
interactions as prototypical examples of cooperativity-influenced adsorption
processes. First, we elucidate how the strength and sign of the interaction
potential between neighboring bound particles on the lattice determine the
intensity of the fluctuations of the mean occupancy. We then employ this
relationship to compare the theoretical predictions of our model to data from
single molecule experiments on bacterial flagellar motors (BFM) of E. coli. In
this way, we find evidence that cooperativity controls the mechano-sensitive
dynamical assembly of the torque-generating units, the so-called stator units,
onto the BFM. Furthermore, in an attempt to quantify fluctuations and the
adaptability of the BFM, we estimate the stator-stator interaction potential.
Finally, we conclude that the system resides in a sweet spot of the parameter
space (phase diagram) suitable for a smoothly adaptive system while minimizing
fluctuations.Comment: 35 pages, 18 figures, 4 table
Introduction au transfert radiatif
DEAUne introduction à la théorie du transfert radiatif et à quelques unes de ses applications est présentée. Ce cours simple et non exhaustif présente la théorie élémentaire, les domaines dans lesquels cette théorie est utile, et les méthodes les plus simples de résolution de l'équation du transfert radiatif (ETR). Des sujets peu abordés par les astrophysiciens y sont traités (sphère intégrante, correction de Giovanelli...). Ce cours correspond à l'enseignement de transfert radiatif du Master Physique Informatique de l'université de Montpellier 2
Erratum to: Modelling the effect of ribosome mobility on the rate of protein synthesis
A Correction to this paper has been published: 10.1140/epje/s10189-021-00019-
Supercoiled DNA and non-equilibrium formation of protein complexes: A quantitative model of the nucleoprotein ParBS partition complex
International audienceParAB S , the most widespread bacterial DNA segregation system, is composed of a centromeric sequence, parS , and two proteins, the ParA ATPase and the ParB DNA binding proteins. Hundreds of ParB proteins assemble dynamically to form nucleoprotein parS -anchored complexes that serve as substrates for ParA molecules to catalyze positioning and segregation events. The exact nature of this ParB S complex has remained elusive, what we address here by revisiting the Stochastic Binding model (SBM) introduced to explain the non-specific binding profile of ParB in the vicinity of parS . In the SBM, DNA loops stochastically bring loci inside a sharp cluster of ParB. However, previous SBM versions did not include the negative supercoiling of bacterial DNA, leading to use unphysically small DNA persistences to explain the ParB binding profiles. In addition, recent super-resolution microscopy experiments have revealed a ParB cluster that is significantly smaller than previous estimations and suggest that it results from a liquid-liquid like phase separation. Here, by simulating the folding of long (≥ 30 kb) supercoiled DNA molecules calibrated with realistic DNA parameters and by considering different possibilities for the physics of the ParB cluster assembly, we show that the SBM can quantitatively explain the ChIP-seq ParB binding profiles without any fitting parameter, aside from the supercoiling density of DNA, which, remarkably, is in accord with independent measurements. We also predict that ParB assembly results from a non-equilibrium, stationary balance between an influx of produced proteins and an outflux of excess proteins, i.e., ParB clusters behave like liquid-like protein condensates with unconventional “leaky” boundaries
A conserved mechanism drives partition complex assembly on bacterial chromosomes and plasmids
Synopsis Chromosome and plasmid segregation in bacteria are mostly driven by ParABS systems. These DNA partitioning machineries rely on large nucleoprotein complexes assembled on centromere sites (parS). However, the mechanism of how a few parS-bound ParB proteins nucleate the formation of highly concentrated ParB clusters remains unclear despite several proposed physico-mathematical models. We discriminated between these different models by varying some key parameters in vivo using the F plasmid partition system. We found that "Nucleation & caging" is the only coherent model recapitulating in vivo data. We also showed that the stochastic self-assembly of partition complexes (i) is a robust mechanism, (ii) does not directly involve ParA ATPase, (iii) results in a dynamic structure of discrete size independent of ParB concentration, and (iv) is not perturbed by active transcription but is by protein complexes. We refined the "Nucleation & caging" model and successfully applied it to the chromosomally encoded Par system of Vibrio cholerae, indicating that this stochastic self-assembly mechanism is widely conserved from plasmids to chromosomes