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  • 2015-2019  (49)
  • 1985-1989  (3)
  • 1940-1944  (1)
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  • 1
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    Airborne Characterization of Sub-saturated Aerosol Hygroscopicity and Dry Refractive Index from the Surface to 6.5 km during the SEAC4RS Campaign (2016)
    Wiley
    Details
    Publication Date: 2016-04-07
    Description: In-situ aerosol particle measurements were conducted during 21 NASA DC-8 flights in the SEAC 4 RS field campaign over the United States, Canada, Pacific Ocean, and Gulf of Mexico. For the first time, this study reports rapid, size-resolved hygroscopic growth and real refractive index ( RI at 532 nm) data between the surface and upper troposphere in a variety of air masses including wildfires, agricultural fires, biogenic, marine, and urban outflow. The Differential Aerosol Sizing and Hygroscopicity Spectrometer Probe (DASH-SP) quantified size-resolved diameter growth factors ( GF  =  D p,wet / D p,dry ), that are used to infer the hygroscopicity parameter κ. Thermokinetic simulations were conducted to estimate the impact of partial particle volatilization within the DASH-SP across a range of sampling conditions. Analyses of GF and RI data as a function of air mass origin, dry size, and altitude are reported, in addition to κ values for the inorganic and organic fractions of aerosol. Average RI values are found to be fairly constant (1.52–1.54) for all air mass categories. An algorithm is used to compare size-resolved DASH-SP GF with bulk scattering f (RH = 80%) data obtained from a pair of nephelometers and the results show that the two can only be reconciled if GF is assumed to decrease with increasing dry size above 400 nm (i.e., beyond the upper bound of DASH-SP measurements). Individual case studies illustrate variations of hygroscopicity as a function of dry size, environmental conditions, altitude, and composition.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Negative allosteric regulation of Enterococcus faecalis small alarmone synthetase RelQ by single-stranded RNA (2017)
    National Academy of Sciences
    Details
    Publication Date: 2017-03-20
    Description: The alarmone nucleotides guanosine pentaphosphate (pppGpp) and tetraphosphate (ppGpp), collectively referred to as (p)ppGpp, are key regulators of bacterial growth, stress adaptation, pathogenicity, and antibiotic tolerance. We show that the tetrameric small alarmone synthetase (SAS) RelQ from the Gram-positive pathogen Enterococcus faecalis is a sequence-specific RNA-binding protein. RelQ’s enzymatic and RNA binding activities are subject to intricate allosteric regulation. (p)ppGpp synthesis is potently inhibited by the binding of single-stranded RNA. Conversely, RelQ’s enzymatic activity destabilizes the RelQ:RNA complex. pppGpp, an allosteric activator of the enzyme, counteracts the effect of RNA. Tetramerization of RelQ is essential for this regulatory mechanism, because both RNA binding and enzymatic activity are abolished by deletion of the SAS-specific C-terminal helix 5α. The interplay of pppGpp binding, (p)ppGpp synthesis, and RNA binding unites two archetypal regulatory paradigms within a single protein. The mechanism is likely a prevalent but previously unappreciated regulatory switch used by the widely distributed bacterial SAS enzymes.
    Print ISSN: 0027-8424
    Electronic ISSN: 1091-6490
    Topics: Biology , Medicine , Natural Sciences in General
    Published by National Academy of Sciences
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  • 3
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    The novel asymmetric entry intermediate of a picornavirus captured with nanodiscs (2016)
    American Association for the Advancement of Science (AAAS)
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    Publication Date: 2016-08-26
    Description: Many nonenveloped viruses engage host receptors that initiate capsid conformational changes necessary for genome release. Structural studies on the mechanisms of picornavirus entry have relied on in vitro approaches of virus incubated at high temperatures or with excess receptor molecules to trigger the entry intermediate or A-particle. We have induced the coxsackievirus B3 entry intermediate by triggering the virus with full-length receptors embedded in lipid bilayer nanodiscs. These asymmetrically formed A-particles were reconstructed using cryo-electron microscopy and a direct electron detector. These first high-resolution structures of a picornavirus entry intermediate captured at a membrane with and without imposing icosahedral symmetry (3.9 and 7.8 Å, respectively) revealed a novel A-particle that is markedly different from the classical A-particles. The asymmetric receptor binding triggers minimal global capsid expansion but marked local conformational changes at the site of receptor interaction. In addition, viral proteins extrude from the capsid only at the site of extensive protein remodeling adjacent to the nanodisc. Thus, the binding of the receptor triggers formation of a unique site in preparation for genome release.
    Electronic ISSN: 2375-2548
    Topics: Natural Sciences in General
    Published by American Association for the Advancement of Science (AAAS)
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  • 4
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    The Y233 gatekeeper of DmpR modulates effector‐responsive transcriptional control of σ54‐RNA polymerase (2019)
    Details
    Publication Date: 2019
    Description: Abstract DmpR is the obligate transcriptional activator of genes involved in (methyl)phenol catabolism by Pseudomonas putida. DmpR belongs to the AAA+ class of mechano‐transcriptional regulators that employ ATP‐hydrolysis to engage and remodel σ54‐RNA polymerase to allow transcriptional initiation. Previous work has established that binding of phenolic effectors by DmpR is a prerequisite to relieve inter‐domain repression and allow ATP‐binding to trigger transition to its active multimeric conformation, and further that a structured inter‐domain linker between the effector‐ and ATP‐binding domains is involved in coupling these processes. Here we present evidence from ATPase and in vivo and in vitro transcription assays that a tyrosine residue of the inter‐domain linker (Y233) serves as a gatekeeper to constrain ATP‐hydrolysis and aromatic effector‐responsive transcriptional activation by DmpR. An alanine substitution of Y233A results in both increased ATPase activity and enhanced sensitivity to aromatic effectors. We propose a model in which effector‐binding relocates Y233 to synchronise signal‐reception with multimerisation to provide physiologically appropriate sensitivity of the transcriptional response. Given that Y233 counterparts are present in many ligand‐responsive mechano‐transcriptional regulators, the model is likely to be pertinent for numerous members of this family and has implications for development of enhanced sensitivity of biosensor used to detect pollutants. This article is protected by copyright. All rights reserved.
    Print ISSN: 1462-2912
    Electronic ISSN: 1462-2920
    Topics: Biology
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  • 5
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    Ambient observations of hygroscopic growth factor and f(RH) below 1: Case studies from surface and airborne measurements (2016)
    Wiley
    Details
    Publication Date: 2016-11-08
    Description: This study reports a detailed set of ambient observations of optical/physical shrinking of particles from exposure to water vapor with consistency across different instruments and regions. Data has been utilized from (i) a shipboard humidified tandem differential mobility analyzer (HTDMA) during the Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE) in 2011, (ii) multiple instruments on the NASA DC-8 research aircraft during the Studies of Emissions, Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC 4 RS) in 2013, as well as (iii) the Differential Aerosol Sizing and Hygroscopicity Spectrometer Probe (DASH-SP) during ambient measurements in Tucson, Arizona during Summer 2014 and Winter 2015. Hygroscopic growth factor (ratio of humidified-to-dry diameter, GF  =  D p,wet /D p,dry ) and f (RH) (ratio of humidified-to-dry scattering coefficients) values below 1 were observed across the range of relative humidity (RH) investigated (75-95%). A commonality of observations of GF and f (RH) below 1 in these experiments was the presence of particles enriched with carbonaceous matter, especially from biomass burning. Evidence of externally mixed aerosol, and thus multiple GF s with at least one GF  〈 1, was observed concurrently with f (RH) 〈 1 during smoke periods. Possible mechanisms responsible for observed shrinkage are discussed and include particle restructuring, volatilization effects, and refractive index modifications due to aqueous processing resulting in optical size modification. To further investigate ambient observations of GF s and f (RH) values less than 1, it is recommended to add an optional pre-humidification bypass module to hygroscopicity instruments, to preemptively collapse particles prior to controlled RH measurements.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 6
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    Analysis of Remotely-Sensed and Surface Data of Aerosols and Meteorology for the Mexico Megalopolis Area Between 2003-2015 (2017)
    Wiley
    Details
    Publication Date: 2017-08-04
    Description: This paper presents an aerosol characterization study from 2003-2015 for the Mexico City Metropolitan Area (MCMA) using remotely sensed aerosol data, ground-based measurements, air mass trajectory modeling, aerosol chemical composition modeling, and reanalysis data for the broader Megalopolis of Central Mexico region. The most extensive biomass burning emissions occur between March and May concurrent with the highest aerosol optical depth, ultraviolet aerosol index, and surface particulate matter (PM) mass concentration values. A notable enhancement in coarse PM levels is observed during vehicular rush hour periods on weekdays versus weekends owing to non-engine related emissions such as re-suspended dust. Among wet deposition species measured, PM 2.5 , PM 10 , and PM coarse (PM 10 – PM 2.5 ) were best correlated with NH 4 + , SO 4 2- , and Ca 2+ , suggesting that the latter three constituents are important components of the aerosol seeding rain drops that eventually deposit to the surface in the study region. Reductions in surface PM mass concentrations were observed in 2014-2015 owing to reduced regional biomass burning as compared to 2003-2013.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 7
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    Inter-sigmulon communication through topological promoter coupling (2016)
    Oxford University Press
    Details
    Publication Date: 2016-12-01
    Description: Divergent transcription from within bacterial intergenic regions frequently involves promoters dependent on alternative -factors. This is the case for the non-overlapping 70 - and 54 -dependent promoters that control production of the substrate-responsive regulator and enzymes for (methyl)phenol catabolism. Here, using an array of in vivo and in vitro assays, we identify transcription-driven supercoiling arising from the 54 -promoter as the mechanism underlying inter-promoter communication that results in stimulation of the activity of the 70 -promoter. The non-overlapping ‘back-to-back’ configuration of a powerful 54 -promoter and weak 70 -promoter within this system offers a previously unknown means of inter-sigmulon communication that renders the 70 -promoter subservient to signals that elicit 54 -dependent transcription without it possessing a cognate binding site for the 54 -RNA polymerase holoenzyme. This mode of control has the potential to be a prevalent, but hitherto unappreciated, mechanism by which bacteria adjust promoter activity to gain appropriate transcriptional control.
    Print ISSN: 0305-1048
    Electronic ISSN: 1362-4962
    Topics: Biology
    Published by Oxford University Press
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  • 8
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    Contrasting aerosol refractive index and hygroscopicity in the inflow and outflow of deep convective storms: Analysis of airborne data from DC3 (2017)
    Wiley
    Details
    Publication Date: 2017-04-19
    Description: We examine three case studies during the Deep Convective Clouds and Chemistry (DC3) field experiment when storm inflow and outflow air were sampled for aerosol sub-saturated hygroscopicity and the real part of refractive index ( n ) with a Differential Aerosol Sizing and Hygroscopicity Probe (DASH-SP) on the NASA DC-8. Relative to inflow aerosol particles, outflow particles were more hygroscopic (by 0.03 based on the estimated κ parameter) in one of the three storms examined. Two of three ‘control’ flights with no storm convection reveal higher κ values, albeit by only 0.02, at high altitude (〉8 km) versus 〈 4 km. Entrainment modeling shows that measured κ values in the outflow of the three storm flights are higher than predicted values (by 0.03-0.11) based on knowledge of κ values from the inflow and clear air adjacent to the storms. This suggests other process(es) contributed to hygroscopicity enhancements such as secondary aerosol formation via aqueous-phase chemistry. Values of n were higher in the outflow of two of the three storm flights, reaching as high as 1.54. More statistically significant differences were observed in control flights (no storms) where n decreased from 1.50-1.52 (〈4 km) to 1.49-1.50 (〉8 km). Chemical data show that enhanced hygroscopicity was coincident with lower organic mass fractions, higher sulfate mass fractions, and higher O:C ratios of organic aerosol. Refractive index did not correlate as well with available chemical data. Deep convection is shown to alter aerosol radiative properties, which has implications for aerosol effects on climate.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 9
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    Surface and Airborne Measurements of Organosulfur and Methanesulfonate Over the Western United States and Coastal Areas (2015)
    Wiley
    Details
    Publication Date: 2015-08-01
    Description: This study reports on ambient measurements of organosulfur (OS) and methanesulfonate (MSA) over the western United States and coastal areas. Particulate OS levels are highest in summertime, and generally increase as a function of sulfate (a precursor) and sodium (a marine tracer) with peak levels at coastal sites. The ratio of OS to total sulfur (TS) is also highest at coastal sites, with increasing values as a function of Normalized Difference Vegetation Index (NDVI) and the ratio of organic carbon to elemental carbon. Correlative analysis points to significant relationships between OS and biogenic emissions from marine and continental sources, factors that coincide with secondary production, and vanadium due to a suspected catalytic role. A major OS species, methanesulfonate (MSA), was examined with intensive field measurements and the resulting data support the case for vanadium's catalytic influence. Mass size distributions reveal a dominant MSA peak between aerodynamic diameters of 0.32—0.56 µm at a desert and coastal site with nearly all MSA mass (≥84%) in sub-micrometer sizes; MSA:non-sea salt sulfate ratios vary widely as a function of particle size and proximity to the ocean. Airborne data indicate that relative to the marine boundary layer, particulate MSA levels are enhanced in urban and agricultural areas, and also the free troposphere when impacted by biomass burning. Some combination of fires and marine-derived emissions leads to higher MSA levels than either source alone. Finally, MSA differences in cloud water and out-of-cloud aerosol are discussed.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 10
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    In-situ measurements of water uptake by black carbon -containing aerosol in wildfire plumes (2016)
    Wiley
    Details
    Publication Date: 2016-12-13
    Description: Water uptake by black carbon (BC)-containing aerosol was quantified in North American wildfire plumes of varying age (1 to ~40 hours old) sampled during the SEAC 4 RS mission (2013). A Humidified Dual SP2 (HD-SP2), is used to optically size BC-containing particles under dry and humid conditions from which we extract the hygroscopicity parameter, κ, of materials internally mixed with BC. Instrumental variability and the uncertainty of the technique are briefly discussed. An ensemble average κ of 0.04 is found for the set of plumes sampled, consistent with previous estimates of bulk aerosol hygroscopicity from biomass burning sources. The temporal evolution of κ in the Yosemite Rim Fire plume is explored to constrain the rate of conversion of BC-containing aerosol from hydrophobic to more hydrophilic modes in these emissions. A BC-specific κ increase of ~0.06 over 40 hours is found, fit well with an exponential curve corresponding to a transition from a κ of 0 to a κ of ~0.09 with an e-folding time of 29 hours. Although only a few percent of wildfire particles contain BC, a similar κ increase is estimated for bulk aerosol and the measured aerosol composition is used to infer that the observed κ change is driven by a combination of incorporation of ammonium sulfate and oxidation of existing organic materials. Finally, a substantial fraction of wildfire-generated BC-containing aerosol is calculated to be CCN active shortly after emission likely indicating efficient wet removal. These results can provide important constraints on model treatment of BC from wildfire sources.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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