Gene amplification as a form of population-level gene expression regulation.

Organisms cope with change by taking advantage of transcriptional regulators. However, when faced with rare environments, the evolution of transcriptional regulators and their promoters may be too slow. Here, we investigate whether the intrinsic instability of gene duplication and amplification provides a generic alternative to canonical gene regulation. Using real-time monitoring of gene-copy-number mutations in Escherichia coli, we show that gene duplications and amplifications enable adaptation to fluctuating environments by rapidly generating copy-number and, therefore, expression-level polymorphisms. This amplification-mediated gene expression tuning (AMGET) occurs on timescales that are similar to canonical gene regulation and can respond to rapid environmental changes. Mathematical modelling shows that amplifications also tune gene expression in stochastic environments in which transcription-factor-based schemes are hard to evolve or maintain. The fleeting nature of gene amplifications gives rise to a generic population-level mechanism that relies on genetic heterogeneity to rapidly tune the expression of any gene, without leaving any genomic signature.

Influence of M-phase chromatin on the anisotropy of microtubule asters.

In many eukaryotic cells going through M-phase, a bipolar spindle is formed by microtubules nucleated from centrosomes. These microtubules, in addition to being "captured" by kinetochores, may be stabilized by chromatin in two different ways: short-range stabilization effects may affect microtubules in close contact with the chromatin, while long-range stabilization effects may "guide" microtubule growth towards the chromatin (e.g., by introducing a diffusive gradient of an enzymatic activity that affects microtubule assembly). Here, we use both meiotic and mitotic extracts from Xenopus laevis eggs to study microtubule aster formation and microtubule dynamics in the presence of chromatin. In "low-speed" meiotic extracts, in the presence of salmon sperm chromatin, we find that short-range stabilization effects lead to a strong anisotropy of the microtubule asters. Analysis of the dynamic parameters of microtubule growth show that this anisotropy arises from a decrease in the catastrophe frequency, an increase in the rescue frequency and a decrease in the growth velocity. In this system we also find evidence for long-range "guidance" effects, which lead to a weak anisotropy of the asters. Statistically relevant results on these long-range effects are obtained in "high-speed" mitotic extracts in the presence of artificially constructed chromatin stripes. We find that aster anisotropy is biased in the direction of the chromatin and that the catastrophe frequency is reduced in its vicinity. In this system we also find a surprising dependence of the catastrophe and the rescue frequencies on the length of microtubules nucleated from centrosomes: the catastrophe frequency increase and the rescue frequency decreases with microtubule length.

Shape deformations of surface-charged microdroplets.

We present the deformation pathway of critically charged glycol and water droplets from the onset of the Rayleigh instability and compare it to numerical results, obtained for perfectly conducting inviscid droplets. In this simple model presented here, the time evolution of the droplet shape is given by the velocity potential equation. The Laplace equation for the velocity potential is solved by expanding the potential onto harmonic functions. For the part of the pathway dominated by electrostatic pressure, the calculations reproduce the experimental data nicely, obtained for both, glycol and water microdroplets. We find that the droplet shape and in particular the tips, just before charge emission, are well fitted by a lemon shape. We stress that the tip is tangent to a cone of 39 degrees and thus significantly narrower than a Taylor cone.

Temperature dependence of the polarizability of sodium clusters

We calculate the static dipole polarizability at finite temperature of sodium clusters of size 8, 20, 40, 55, 93, and 139 using an extended Thomas-Fermi description of the valence electrons. We find polarizabilities at 300 K that are roughly 15% greater than at 0 K, consistent with discrepancies between theoretical polarizabilities at 0 K and measured polarizabilities. We predict that a sharp rise in the polarizability, of about 5%, occurs for sizes of 55 and 139 when the cluster melts, offering the possibility of an alternative method for measuring cluster melting points.

Charge-induced fragmentation of sodium clusters.

The fission of highly charged sodium clusters with fissilities X>1 is studied by ab initio molecular dynamics. Na4+24 is found to undergo predominantly sequential Na+3 emission on a time scale of 1 ps, while Na(Q+)(24) ( 5< or =Q< or =8) undergoes multifragmentation on a time scale > or =0.1 ps, with Na+ increasingly the dominant fragment as Q increases. All singly charged fragments Na(+)(n) up to size n = 6 are observed. The observed fragment spectrum is, within statistical error, independent of the temperature T of the parent cluster for T< or =1500 K. These findings are consistent with and explain recent trends observed experimentally.

Shape oscillations and stability of charged microdroplets.

Classical Rayleigh theory predicts an instability of a surface charged liquid sphere, when the Coulomb energy E(C) exceeds twice the surface energy E(S). Previously, electrified liquid droplets have been found to disintegrate at a fissility X=E(C)/2E(S) well below one, however. We determine the stability of charged droplets in an electrodynamic levitator by observing the amplitude and phase of their quadrupolar shape oscillations as a function of the fissility. With this novel approach, which does not rely on an independent determination of the charge and surface tension of the droplets, we are able to confirm for the first time the Rayleigh limit of stability at X=1 for micrometer sized droplets of ethylene glycol.

Rayleigh instabilities in multiply charged sodium clusters.

The stability of multiply charged sodium clusters Na(q+)(n) (q< or =10) produced in collisions between neutral clusters and multiply charged ions A(z+) ( z = 1 to 28) is experimentally investigated. Multiply charged clusters are formed within a large range of temperatures and fissilities. They are identified by means of a high-resolution reflectron-type time-of-flight mass spectrometer (m/deltam approximately 14 000). The maximum fissility of stable clusters is obtained for z = 28 and is X approximately 0.85+/-0.07, slightly below the Rayleigh limit (X = 1). It is mainly limited by the initial cluster temperature (T approximately 100 K).