Anharmonic Torsional Stiffness of DNA Revealed under Small External Torques

DNA supercoiling plays an important role in a variety of cellular processes. The torsional stress related with supercoiling may be also involved in gene regulation through the local structure and dynamics of the double helix. To check this possibility steady torsional stress was applied to DNA in the course of all-atom molecular dynamics simulations. It is found that small static untwisting significantly reduces the torsional persistence length ($l_t$) of GC-alternating DNA. For the AT-alternating sequence a smaller effect of the opposite sign is observed. As a result, the measured $l_t$ values are similar under zero stress, but diverge with untwisting. The effect is traced to sequence-specific asymmetry of local torsional fluctuations, and it should be small in long random DNA due to compensation. In contrast, the stiffness of special short sequences can vary significantly, which gives a simple possibility of gene regulation via probabilities of strong fluctuations. These results have important implications for the role of local DNA twisting in complexes with transcription factors.Comment: 8 pages, 5 figures, to appear in Phys. Rev. Let

Dynamical Consequences of Bandpass Feedback Loops in a Bacterial Phosphorelay

Under conditions of nutrient limitation, Bacillus subtilis cells terminally differentiate into a dormant spore state. Progression to sporulation is controlled by a genetic circuit consisting of a phosphorelay embedded in multiple transcriptional feedback loops, which is used to activate the master regulator Spo0A by phosphorylation. These transcriptional regulatory interactions are “bandpass”-like, in the sense that activation occurs within a limited band of Spo0A~P concentrations. Additionally, recent results show that the phosphorelay activation occurs in pulses, in a cell-cycle dependent fashion. However, the impact of these pulsed bandpass interactions on the circuit dynamics preceding sporulation remains unclear. In order to address this question, we measured key features of the bandpass interactions at the single-cell level and analyzed them in the context of a simple mathematical model. The model predicted the emergence of a delayed phase shift between the pulsing activity of the different sporulation genes, as well as the existence of a stable state, with elevated Spo0A activity but no sporulation, embedded within the dynamical structure of the system. To test the model, we used time-lapse fluorescence microscopy to measure dynamics of single cells initiating sporulation. We observed the delayed phase shift emerging during the progression to sporulation, while a re-engineering of the sporulation circuit revealed behavior resembling the predicted additional state. These results show that periodically-driven bandpass feedback loops can give rise to complex dynamics in the progression towards sporulation

Does innovation stimulate employment? A firm-level analysis using comparable micro data on four European countries

This paper studies the impact of process and product innovations introduced by firms on their employment growth. A model that relates employment growth to process innovations and to the growth of sales due to innovative and unchanged products is derived and estimated using a unique source of comparable firm-level data from France, Germany, Spain and the UK. Results for manufacturing show that, although process innovation tends to displace employment, compensation effects are prevalent, and product innovation is associated with employment growth. In the service sector there is less evidence of displacement effects, and growth in sales of new products accounts for a non-negligible proportion of employment growth. Overall the results are similar across countries, with some interesting exceptions

Antimagnets: Controlling magnetic fields with superconductor-metamaterial hybrids

Magnetism is very important in science and technology, from magnetic recording to energy generation to trapping cold atoms. Physicists have managed to master magnetism - to create and manipulate magnetic fields- almost at will. Surprisingly, there is at least one property which until now has been elusive: how to 'switch off' the magnetic interaction of a magnetic material with existing magnetic fields without modifying them. Here we introduce the antimagnet, a design to conceal the magnetic response of a given volume from its exterior, without altering the external magnetic fields, somehow analogous to the recent theoretical proposals for cloaking electromagnetic waves with metamaterials. However, different from these devices requiring extreme material properties, our device is feasible and needs only two kinds of available materials: superconductors and isotropic magnetic materials. Antimagnets may have applications in magnetic-based medical techniques such as MRI or in reducing the magnetic signature of vessels or planes.Comment: 14 pages, 4 figure

Microlensing Events from Measurements of the Deflection Angle

Microlensing events are now regularly being detected by monitoring the flux of a large number of potential sources and measuring the combined magnification of the images. This phenomenon could also be detected directly from the gravitational deflection, by means of high precision astrometry using interferometry. Relative astrometry at the level of 10\muas may become possible in the near future. The gravitational deflection can be measured by astrometric monitoring of a bright star having a background star within a small angular separation. This type of monitoring program will be carried out for the independent reasons of discovering planets from the angular motion they induce on the nearby star around which they are orbiting, and for measuring parallaxes, proper motions and orbits of binary stars. We discuss three applications of the measurement of gravitational deflections by astrometric monitoring: measuring the mass of the bright stars that are monitored, measuring the mass of brown dwarfs or giant planets around the bright stars, and detecting microlensing events by unrelated objects near the line of sight to the two stars. We discuss the number of stars whose mass could be measured by this procedure. We also give expressions for the number of expected microlensing events by unrelated objects, which could be stars, brown dwarfs, or other compact objects accounting for dark matter in the halo or in the disk.Comment: submitted to ApJ Letter

Mycobacterium Tuberculosis Infection and Inflammation: what is Beneficial for the Host and for the Bacterium?

Tuberculosis is still a major health problem in the world. Initial interactions between Mycobacterium tuberculosis and the host mark the pathway of infection and the subsequent host inflammatory response. This inflammatory response is tightly regulated by both the host and the bacterium during different stages of infection. As infection progresses, the initial intense pro-inflammatory response observed is regulated by suppressive mediators balancing inflammation. In this environment, M. tuberculosis battles to survive interfering with the host inflammatory response. In this review we discuss the major effector molecules involved in inflammation in relation to the different stages of M. tuberculosis infection