Physics Department, Princeton University

Doug Weibel (U. Wisconsin) "Regulation of bacterial biochemistry at strained bacterial membranes"
We are exploring an evolutionarily conserved mechanism for the localization and regulation of biochemistry at strained membranes in bacteria and mitochondria. In this talk, I highlight this mechanism by discussing the regulation of the universal and widely conserved DNA-repair protein, RecA. An emerging picture of bacterial organization is that membrane curvature causes strain in the bilayer that leads to the accumulation of intrinsically curved, anionic phospholipids at the bacterial poles. We have discovered that a large fraction of RecA in Escherichia coli cells is not associated with DNA during normal growth—it is temporally localized at the cell poles through its interaction with anionic phospholipids. Fascinatingly, this interaction inhibits the ATPase activity of RecA, which turns the DNA repair activity of the protein off. When cells are stressed and DNA repair is required, RecA dissociates from the membrane, rapidly diffuses to the mid-cell where DNA is concentrated, and forms active nucleoprotein filaments. We have traced the regulation of RecA function back to the C-terminal region RecA, which forms a known autoregulatory domain.

Early studies in this field demonstrated that large amounts of RecA associate with membrane fractions during DNA damage (Garvey et al. 1985, J. Biol. Chem. 285, 18984). A fundamental unanswered question is why does RecA bind the membrane. We have pursued this question using an approach that fuses biophysics, biochemistry, and engineering. Our data supports a model in which the cell prevents unproductive RecA-DNA interactions by storing protein at a DNA-free region of the membrane when its enzymatic function is not required. These and other studies in our lab are enabling us to decode spatial and temporal control of biochemistry in bacteria and mitochondria.

Joseph Henry Room  ·  12:00 p.m.– 1:00 p.m.

Condensed Matter Seminar- Johnpierre Paglione, Univ of Maryland-Separation of magnetism and superconductivity in rare earth-doped CaFe2As2 by pressure
Traces of a high-Tc superconducting phase with transition temperatures approaching 50 K have been stabilized by the substitution of light rare earths for Ca atoms in the iron-pnictide material CaFe2As2 [*]. I will present our recent studies of the evolution of superconductivity and magnetism in single crystals of La-doped CaFe2As2 under both quasi-hydrostatic and hydrostatic applied pressures by means of transport, magnetic, and neutron scattering measurements. Upon pressure increase, we find evidence that the "50 K" superconducting phase is an intrinsic property of the materials. However, unlike transition metal-doped 122 iron-superconductors where superconductivity happily coexists with AFM, the little coexistence of SC and AFM appears to mimic that found in 1111 iron-superconductors, suggesting a similar phase diagram. I will discuss this unusual dichotomy between lower-Tc systems that happily coexist with AFM and the tendency for the highest-Tc systems to show phase separation.

[*] S.R. Saha et al, Phys. Rev. B 85, 024525 (2012).
PCTS Seminar Room  ·  1:15 p.m.– 2:30 p.m.

High Energy Theory Seminar - Tim Cohen, SLAC Stanford - "Jet Substructure by Accident"
abstract:  We propose a new search strategy for high-multiplicity hadronic final states. When new particles are produced at threshold, the distribution of their decay products is approximately isotropic. If there are many partons in the final state, it is likely that several will be clustered into the same large-radius jet. The resulting jet exhibits substructure, even though the parent states are not boosted. This "accidental" substructure is a powerful discriminant against background because it is more pronounced for high-multiplicity signals than for QCD multijets. We demonstrate how to take advantage of accidental substructure to reduce backgrounds without relying on the presence of missing energy. As an example, we present the expected limits for several R-parity violating gluino decay topologies. This approach allows for the determination of QCD backgrounds using data-driven methods, which is crucial for the feasibility of any search that targets signatures with many jets and suppressed missing energy.

PCTS Seminar Room  ·  2:30 p.m.– 4:00 p.m.