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Acta Cryst. (2014). A70, C426
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Group A streptococcus (GAS) is a significant human pathogen, renowned for its rapidly and highly desctructive ability to infect a wide variety of tissues. Clinical manifestation of GAS infection ranges from mild pharyngitis to severe life-threating disease such as necrotizing fasciitis. Unlike other gram-positive bacteria, GAS does not produce catalase, but has an ability to resist killing by reactive oxygen species through unknown novel mechanisms. Our previous studies have discovered that the peroxide response regulator (PerR) is crucial for GAS to cope with oxidative stress and it directly regulates the expression of an iron-binding protein Dpr [1,2]. PerR is a member of Fur (ferric uptake regulator) family which is known to be dimeric, metal-binding regulators. Currently, no structural information is available to understand how the similar structures of the Fur family regulators recognize divergent DNA sequences. To study how PerR interacts with dpr promoter DNA, we have conducted a series of mutagenesis, biochemical and structural studies by combining protein crystallography and small-angle X-ray scattering (SAXS). We have determined the PerR crystal structure to 1.6 Å resolution and identified the DNA-binding residues, which suggest PerR binds to the dpr promoter through a winged-helix motif. By performing SAXS studies, we confirmed that the PerR crystal structure reflects its conformation in solution. Furthermore, SAXS analysis allowed us to resolve the molecular architecture of PerR-DNA complex, in which two 30 bp DNA fragments wrap around two PerR homodimers by interacting with the adjacent positively-charged winged-helix motifs. Our results have revealed the PerR-DNA interaction model and illustrated the DNA-binding mode of PerR that is distinct from all other regulators in Fur family [3].

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Acta Cryst. (2014). A70, C745
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Self-assembled metal oxide nanoparticle layers have attracted much attention recently due to potential applications in sensors. Here we report on a UV/ozone-driven re-assembly and oxidation of a self-assembled silver nanoparticle bilayer deposited by a modified Langmuir-Schaefer technique that was probed in-situ by simultaneous measurements of the small- and wide-angle X-ray scattering at grazing incidence (GISAXS, GIWAXS). The experiments were performed at BL23A endstation of NSRRC, Taiwan. Four distinct stages of the system response to the UV/ozone treatment were identified. In the first stage 0-120 s, a gradual extinction of Bragg rods and the nanoparticle short-range order due to a gradual surfactant removal is observed in GISAXS that is connected with an array densification and order improvement obeying the paracrystal model in the shrinking self-assembled regions. In the second stage 120-360 s, the original self-assembly goes to a total extinction while the integral intensity of Ag 111 diffraction in GIWAXS starts to decrease, suggesting the total surfactant removal. No nanoparticle short-range order is observed in stage III 360-730 s where AgO monoclinic phase due to the nanoparticle surface oxidation appears and pure Ag phase disappears. The initial AgO unit cell volume corresponding to the unstrained phase gradually expands by 4.6 % to the end of stage III. In stage IV 730-2000 s (end of measurements), new Bragg rods appear in GISAXS, suggesting a nanoparticle agglomeration with a typical correlation length of approximately 240 nm that was confirmed by ex-situ atomic force microscopy. The agglomeration does not reach saturation even after 2000 s while the AgO phase remains stable. These results have direct implications for tailored preparation of advanced sensors based on metal oxide nanoparticles. The work was supported by the projects APVV-0308-11, VEGA 2/0041/11, SAS-NSC JRP 2011/05, SAS-TUBITAK JRP 2013/6, COST MP1203 and COST CM1101.

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Acta Cryst. (2014). A70, C1747
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The coherent X-ray scattering beamline is one of the phase I beamlines designed for the Taiwan Photon Source, a new 3 GeV ring under construction at the National Synchrotron Radiation Research Center in Taiwan. By using a pair of 2m-long in-vacuum undulators, this beamline will provide a highly coherent beam for X-ray photon correlation spectroscopy principally; moreover, it will share a part of beamtime for small-angle X-ray scattering (SAXS) experiments with similar setup of the beamline. The operating photon energy is designed within the range of 5-20 keV. In vertical direction, the beam spot size at sample position is 1 μm with focusing mirror and by using 1D compound refractive lenses (CRLs) the beam spot size is 10 μm. The horizontal beam spot size is in the range of 1 to 10 μm with a two-stage focusing design. The vertical and horizontal transverse coherence lengths of the 10 μm2 beam spot size at the photon energy of 5 KeV are 212 and 6 μm at sample position respectively. Beside XPCS the beamline configuration can cope with the requirements of most SAXS experiments, including anomalous measurements and micro-beam mapping. In addition, the increasing biological SAXS demand is also considered and the on-line fast performance liquid chromatography (FPLC) will be enclosed for biological users.
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