The switching dynamics of the bacterial flagellar motor

Many swimming bacteria are propelled by flagellar motors that stochastically switch between the clockwise and counterclockwise rotation direction. While the switching dynamics are one of the most important characteristics of flagellar motors, the mechanisms that control switching are poorly understood. We present a statistical-mechanical model of the flagellar rotary motor, which consists of a number of stator proteins that drive the rotation of a ring of rotor proteins, which in turn drives the rotation of a flagellar filament. At the heart of our model is the assumption that the rotor protein complex can exist in two conformational states corresponding to the two respective rotation directions, and that switching between these states depends on interactions with the stator proteins. This naturally couples the switching dynamics to the rotation dynamics, making the switch sensitive to torque and speed. Another key element of our model is that after a switching event, it takes time for the load to build up, due to polymorphic transitions of the filament. Our model predicts that this slow relaxation dynamics of the filament, in combination with the load dependence of the switching frequency, leads to a characteristic switching time, in agreement with recent observations.Comment: 7 pages, 6 figures, RevTeX

Long-time asymptotic of temporal-spatial coherence function for light propagation through time dependent disorder

Long-time asymptotic of field-field correlator for radiation propagated through a medium composed of random point-like scatterers is studied using Bete-Salpeter equation. It is shown that for plane source the fluctuation intensity (zero spatial moment of the correlator) obeys a power-logarithmic stretched exponential decay law, the exponent and preexponent being dependent on the scattering angle. Spatial center of gravity and dispersion of the correlator (normalized first and second spatial moments, respectively) prove to weakly diverge as time tends to infinity. A spin analogy of this problem is discussed.Comment: 12 pages, Latex, no figures, to be publication in Phys. Lett.

Enzyme localization can drastically affect signal amplification in signal transduction pathways

Push-pull networks are ubiquitous in signal transduction pathways in both prokaryotic and eukaryotic cells. They allow cells to strongly amplify signals via the mechanism of zero-order ultrasensitivity. In a push-pull network, two antagonistic enzymes control the activity of a protein by covalent modification. These enzymes are often uniformly distributed in the cytoplasm. They can, however, also be colocalized in space, for instance, near the pole of the cell. Moreover, it is increasingly recognized that these enzymes can also be spatially separated, leading to gradients of the active form of the messenger protein. Here, we investigate the consequences of the spatial distributions of the enzymes for the amplification properties of push-pull networks. Our calculations reveal that enzyme localization by itself can have a dramatic effect on the gain. The gain is maximized when the two enzymes are either uniformly distributed or colocalized in one region in the cell. Depending on the diffusion constants, however, the sharpness of the response can be strongly reduced when the enzymes are spatially separated. We discuss how our predictions could be tested experimentally.Comment: PLoS Comp Biol, in press. 32 pages including 6 figures and supporting informatio

The Universality of Initial Conditions for Globular Cluster Formation

We investigate a simple model for globular cluster (GC) formation. We simulate the violent relaxation of initially homogeneous isothermal stellar spheres and show that it leads to the formation of clusters with radial density profiles that match the observed profiles of GCs. The best match is achieved for dynamically unevolved clusters. In this model, all the observed correlations between global GC parameters are accurately reproduced if one assumes that all the clusters initially had the same value of the stellar density and the velocity dispersion. This suggests that the gas which formed GCs had the same values of density and temperature throughout the universe.Comment: 12 pages, 4 figures, 1 table. Accepted for publication in ApJ Letter

Global HI profiles of spiral galaxies

In this paper we present short HI synthesis observations of 57 galaxies without HI information in the RC3. These are a by-product of a large survey with the WSRT of the neutral hydrogen gas in spiral and irregular galaxies. Global profiles and related quantities are given for the 42 detected galaxies and upper limits for the remaining 15. A number of galaxies have low values of HI mass-to-blue luminosity ratio.Comment: A LATEX file without figures. The postscript version including all the figures can be retrieved from http://www.astro.rug.nl:80/~secr/ Accepted for publication in Astronomy & Astrophysics Suppl. Serie

Bose-Einstein Condensates in Strongly Disordered Traps

A Bose-Einstein condensate in an external potential consisting of a superposition of a harmonic and a random potential is considered theoretically. From a semi-quantitative analysis we find the size, shape and excitation energy as a function of the disorder strength. For positive scattering length and sufficiently strong disorder the condensate decays into fragments each of the size of the Larkin length ${\cal L}$. This state is stable over a large range of particle numbers. The frequency of the breathing mode scales as $1/{\cal L}^2$. For negative scattering length a condensate of size ${\cal L}$ may exist as a metastable state. These finding are generalized to anisotropic traps