November 27, 2011 Min Oscillations In Vitro presented by Lindsay

When: November 27, 2011 12:00
Where: Physics 377
Presenter: Lindsay Moore (Erez Braun's group, physics)
Link to paper: http://www.sciencemag.org/content/320/5877/789.full.pdf

Spatial Regulators for Bacterial Cell Division Self-Organize into Surface Waves in Vitro

1. Martin Loose, 
2. Elisabeth Fischer-Friedrich, 
3. Jonas Ries, 
4. Karsten Kruse, and 
5. Petra Schwille

 Biotechnologisches Zentrum der Technischen Universität Dresden, Tatzberg 47-51, 01307 Dresden, Germany.

 Max-Planck-Institute for Molecular Cell Biology and Genetics, Pfotenhauerstrasse 108, 01307 Dresden, Germany.

 Max-Planck-Institut für Physik komplexer Systeme, Nöthnitzer Strasse 38, 01187 Dresden, Germany.

 Theoretische Physik, Universität des Saarlandes, Postfach 151150, 66041 Saarbrücken, Germany.

In the bacterium Escherichia coli, the Min proteins oscillate between the cell poles to select the cell center as division site. This dynamic pattern has been proposed to arise by self-organization of these proteins, and several models have suggested a reaction-diffusion type mechanism. Here, we found that the Min proteins spontaneously formed planar surface waves on a flat membrane in vitro. The formation and maintenance of these patterns, which extended for hundreds of micrometers, required adenosine 5′-triphosphate (ATP), and they persisted for hours. We present a reaction-diffusion model of the MinD and MinE dynamics that accounts for our experimental observations and also captures the in vivo oscillations.

November 13, 2011 - Intrinsic vs. Extrinsic Fluctuations in Protein Number presented by Yuval

When: November 13, 2011 12:00
Where: Physics 377
Presenter: Yuval Elhanati (Naama Brenner's group, physics)
Link to paper: http://www.pnas.org/content/early/2011/06/30/1018832108.full.pdf+html

Separating intrinsic from extrinsic fluctuations in dynamic biological systems

Andreas Hilfinger and Johan Paulsson

Department of Systems Biology, Harvard University, 200 Longwood Avenue, Boston, MA 02115

From molecules in cells to organisms in ecosystems, biological populations fluctuate due to the intrinsic randomness of individual events and the extrinsic influence of changing environments. The combined effect is often too complex for effective analysis, and many studies therefore make simplifying assumptions, for example ignoring either intrinsic or extrinsic effects to reduce the number of model assumptions. Here we mathematically demonstrate how two identical and independent reporters embedded in a shared fluctuating environment can be used to identify intrinsic and extrinsic noise terms, but also how these contributions are qualitatively and quantitatively different from what has been previously reported. Furthermore, we show for which classes of biological systems the noise contributions identified by dual-reporter methods correspond to the noise contributions predicted by correct stochastic models of either intrinsic or extrinsic mechanisms. We find that for broad classes of systems, the extrinsic noise from the dual-reporter method can be rigorously analyzed using models that ignore intrinsic stochasticity. In contrast, the intrinsic noise can be rigorously analyzed using models that ignore extrinsic stochasticity only under very special conditions that rarely hold in biology. Testing whether the conditions are met is rarely possible and the dual-reporter method may thus produce flawed conclusions about the properties of the system, particularly about the intrinsic noise. Our results contribute toward establishing a rigorous framework to analyze dynamically fluctuating biological systems.