Prolonging assembly through dissociation:A self assembly paradigm in microtubules

We study a one-dimensional model of microtubule assembly/disassembly in which GTP bound to tubulins within the microtubule undergoes stochastic hydrolysis. In contrast to models that only consider a cap of GTP-bound tubulin, stochastic hydrolysis allows GTP-bound tubulin remnants to exist within the microtubule. We find that these buried GTP remnants enable an alternative mechanism of recovery from shrinkage, and enhances fluctuations of filament lengths. Under conditions for which this alternative mechanism dominates, an increasing depolymerization rate leads to a decrease in dissociation rate and thus a net increase in assembly.Comment: accepted for publication in Physical Review

Evolution of nectar spur length in a clade of Linaria reflects changes in cell division rather than in cell expansion.

BACKGROUND AND AIMS: Nectar spurs (tubular outgrowths of a floral organ which contain, or give the appearance of containing, nectar) are hypothesized to be a 'key innovation' which can lead to rapid speciation within a lineage, because they are involved in pollinator specificity. Despite the ecological importance of nectar spurs, relatively little is known about their development. We used a comparative approach to investigate variation in nectar spur length in a clade of eight Iberian toadflaxes. METHODS: Spur growth was measured at the macroscopic level over time in all eight species, and growth rate and growth duration compared. Evolution of growth rate was reconstructed across the phylogeny. Within the clade we then focused on Linaria becerrae and Linaria clementei, a pair of sister species which have extremely long and short spurs, respectively. Characterization at a micromorphological level was performed across a range of key developmental stages to determine whether the difference in spur length is due to differential cell expansion or cell division. KEY RESULTS: We detected a significant difference in the evolved growth rates, while developmental timing of both the initiation and the end of spur growth remained similar. Cell number is three times higher in the long spurred L. becerrae compared with L. clementei, whereas cell length is only 1.3 times greater. In addition, overall anisotropy of mature cells is not significantly different between the two species. CONCLUSIONS: We found that changes in cell number and therefore in cell division largely explain evolution of spur length. This contrasts with previous studies in Aquilegia which have found that variation in nectar spur length is due to directed cell expansion (anisotropy) over variable time frames. Our study adds to knowledge about nectar spur development in a comparative context and indicates that different systems may have evolved nectar spurs using disparate mechanisms

Four jet event shapes in electron-positron annihilation

We report next-to-leading order perturbative QCD predictions of 4 jet event shape variables for the process e+e- \to 4 jets obtained using the general purpose Monte Carlo EERAD2. This program is based on the known `squared' one loop matrix elements for the virtual \gamma^* \to 4 parton contribution and squared matrix elements for 5 parton production. To combine the two distinct final states numerically we present a hybrid of the commonly used subtraction and slicing schemes based on the colour antenna structure of the final state which can be readily applied to other processes. We have checked that the numerical results obtained with EERAD2 are consistent with next-to-leading order estimates of the distributions of previously determined four jet-like event variables. We also report the next-to-leading order scale independent coefficients for some previously uncalculated observables; the light hemisphere mass, narrow jet broadening, Aplanarity and the 4 jet transition variables with respect to the JADE and Geneva jet finding algorithms. For each of these observables, the next-to-leading order corrections calculated at the physical scale significantly increase the rate compared to leading order (the K factor is approximately 1.5 -- 2). With the exception of the 4 jet transition variables, the published DELPHI data lies well above the O(\alpha_s^3) predictions. The renormalisation scale uncertainty is still large and in most cases the data prefers a scale significantly smaller than the physical scale. This situation is reminiscent of that for three jet shape variables, and should be improved by the inclusion of power corrections and resummation of large infrared logarithms.Comment: 41 pages, LaTeX, 16 figures, discussion of Aplanarity adde

Natural Populations of Woodchuck Hepatitis Virus Contain Variant Precore and Core Sequences Including a Premature Stop Codon in the Epsilon Motif

We have determined a consensus sequence and the type and the frequency of spontaneous sequence variations in the woodchuck hepatitis virus (WHV) precore gene and the 5' region of the core gene in 101 serum samples from 53 naturally WHV-infected woodchucks by polymerase chain reaction sequencing. Twenty of the 53 woodchucks were found to have variant sequences. Ten patterns of variant sequences were identified in these 20 animals. WHV sequences from 4 woodchucks had 1 nucleotide change, 3 had 2 nucleotide changes and 3 had 3 nucleotide changes. The nucleotide changes were not randomly distributed, but were limited to only 8 sites. Four sites were in the epsilon motif of the precore gene and four were in the 5' region of the core gene. Sixteen of the 53 (30%) woodchucks had precore sequence variants. All altered sites were analogous to previously described mutations in hepatitis B virus. There was a nucleotide change at nucleotide 2016 in codon 29 of the precore region that produced a stop codon in 4 animals. This site is analogous to a common hepatitis B virus e antigen mutation. The sequence from the initial blood samples from 3 of 4 animals with this stop codon producing variant appeared to be the consensus sequence; however, in later samples the variant occurred as a mixed infection with the consensus sequence. The mixed infections were chronic and the proportion of the variant sequence was maintained or increased in the course of infection. In the fourth animal only the variant was found and it persisted for over 14 months of infection. WHV appears to be a valuable model for the study of the structure and function of the hepadnavirus precore region

The “ABC model”: a non-hydrostatic toy model for use in convective-scale data assimilation investigations

In developing methods for convective-scale data assimilation (DA) it is necessary to consider the full range of motions governed by the compressible Navier-Stokes equations (including non-hydrostatic and ageostrophic flow). These equations describe motion on a wide range of time-scales with non-linear coupling. For the purpose of developing new DA techniques that suit the convective-scale problem it is helpful to use so-called ‘toy models’ that are easy to run, and contain the same types of motion as the full equation set. Such a model needs to permit hydrostatic and geostrophic balance at large-scales, but to allow imbalance at small-scales, and in particular, it needs to exhibit intermittent convection-like behaviour. Existing ‘toy models’ are not always sufficient for investigating these issues. A simplified system of intermediate complexity derived from the Euler equations is presented, which supports dispersive gravity and acoustic modes. In this system the separation of time scales can be greatly reduced by changing the physical parameters. Unlike in existing toy models, this allows the acoustic modes to be treated explicitly, and hence inexpensively. In addition, the non-linear coupling induced by the equation of state is simplified. This means that the gravity and acoustic modes are less coupled than in conventional models. A vertical slice formulation is used which contains only dry dynamics. The model is shown to give physically reasonable results, and convective behaviour is generated by localised compressible effects. This model provides an affordable and flexible framework within which some of the complex issues of convective-scale DA can later be investigated. The model is called the “ABC model” after the three tunable parameters introduced: A (the pure gravity wave frequency), B (the modulation of the divergent term in the continuity equation), and C (defining the compressibility)

Sampling Time Effects for Persistence and Survival in Step Structural Fluctuations

The effects of sampling rate and total measurement time have been determined for single-point measurements of step fluctuations within the context of first-passage properties. Time dependent STM has been used to evaluate step fluctuations on Ag(111) films grown on mica as a function of temperature (300-410 K), on screw dislocations on the facets of Pb crystallites at 320K, and on Al-terminated Si(111) over the temperature range 770K - 970K. Although the fundamental time constant for step fluctuations on Ag and Al/Si varies by orders of magnitude over the temperature ranges of measurement, no dependence of the persistence amplitude on temperature is observed. Instead, the persistence probability is found to scale directly with t/Dt where Dt is the time interval used for sampling. Survival probabilities show a more complex scaling dependence which includes both the sampling interval and the total measurement time tm. Scaling with t/Dt occurs only when Dt/tm is a constant. We show that this observation is equivalent to theoretical predictions that the survival probability will scale as Dt/L^z, where L is the effective length of a step. This implies that the survival probability for large systems, when measured with fixed values of tm or Dt should also show little or no temperature dependence.Comment: 27 pages, 10 figure

GAP WORK project report: training for youth practitioners on tackling gender-related violence

This project sought to challenge gender-related violence against (and by) children and young people by developing training for practitioners who have everyday contact with general populations of children and young people (‘youth practitioners’). Through improved knowledge and understanding practitioners can better identify and challenge sexist, sexualising, homophobic or controlling language and behaviour, and know when and how to refer children and young people to the most appropriate support services. This summary outlines the Project and our initial findings about the success of the four training programmes developed and piloted.Co-funded by the DAPHNE III programme of the EU