Ihor S. Diakunchak

Siemens Westinghouse Advanced Turbine Systems Program Final Summar

High-precision half-life determination for Na-21 using a 4 pi gas-proportional counter

A high-precision half-life measurement for the superallowed β + transition between the isospin T = 1/2 mirror nuclei 21Na and 21Ne has been performed at the TRIUMF-ISAC radioactive ion beam facility yielding T1/2 = 22.4506(33) s, a result that is a factor of 4 more precise than the previous world-average half-life for 21Na and represents the single most precisely determined half-life for a transition between mirror nuclei to date. The contribution to the uncertainty in the 21Na Ft mirror value due to the half-life is now reduced to the level of the nuclear structure-dependent theoretical corrections, leaving the branching ratio as the dominant experimental uncertainty.status: publishe

Structural basis for the requirement of additional factors for MLL1 SET domain activity and recognition of epigenetic marks

The mixed-lineage leukemia protein MLL1 is a transcriptional regulator with an essential role in early development and hematopoiesis. The biological function of MLL1 is mediated by the histone H3K4 methyltransferase activity of the carboxyl-terminal SET domain. We have determined the crystal structure of the MLL1 SET domain in complex with cofactor product AdoHcy and a histone H3 peptide. This structure indicates that, in order to form a well-ordered active site, a highly variable but essential component of the SET domain must be repositioned. To test this idea, we compared the effect of the addition of MLL complex members on methyltransferase activity and show that both RbBP5 and Ash2L but not Wdr5 stimulate activity. Additionally, we have determined the effect of posttranslational modifications on histone H3 residues downstream and upstream from the target lysine and provide a structural explanation for why H3T3 phosphorylation and H3K9 acetylation regulate activity

Environmental context explains Lévy and Brownian movement patterns of marine predators

An optimal search theory, the so-called Lévy-flight foraging hypothesis, predicts that predators should adopt search strategies known as Lévy flights where prey is sparse and distributed unpredictably, but that Brownian movement is sufficiently efficient for locating abundant prey. Empirical studies have generated controversy because the accuracy of statistical methods that have been used to identify Lévy behaviour has recently been questioned. Consequently, whether foragers exhibit Lévy flights in the wild remains unclear. Crucially, moreover, it has not been tested whether observed movement patterns across natural landscapes having different expected resource distributions conform to the theory’s central predictions. Here we use maximum-likelihood methods to test for Lévy patterns in relation to environmental gradients in the largest animal movement data set assembled for this purpose. Strong support was found for Lévy search patterns across 14 species of open-ocean predatory fish (sharks, tuna, billfish and ocean sunfish), with some individuals switching between Lévy and Brownian movement as they traversed different habitat types. We tested the spatial occurrence of these two principal patterns and found Lévy behaviour to be associated with less productive waters (sparser prey) and Brownian movements to be associated with productive shelf or convergence-front habitats (abundant prey). These results are consistent with the Lévy-flight foraging hypothesis1, supporting the contention that organism search strategies naturally evolved in such a way that they exploit optimal Lévy patterns.<br/