9,886 research outputs found
DNA unwinding heterogeneity by RecBCD results from static molecules able to equilibrate.
Single-molecule studies can overcome the complications of asynchrony and ensemble-averaging in bulk-phase measurements, provide mechanistic insights into molecular activities, and reveal interesting variations between individual molecules. The application of these techniques to the RecBCD helicase of Escherichia coli has resolved some long-standing discrepancies, and has provided otherwise unattainable mechanistic insights into its enzymatic behaviour. Enigmatically, the DNA unwinding rates of individual enzyme molecules are seen to vary considerably, but the origin of this heterogeneity remains unknown. Here we investigate the physical basis for this behaviour. Although any individual RecBCD molecule unwound DNA at a constant rate for an average of approximately 30,000 steps, we discover that transiently halting a single enzyme-DNA complex by depleting Mg(2+)-ATP could change the subsequent rates of DNA unwinding by that enzyme after reintroduction to ligand. The proportion of molecules that changed rate increased exponentially with the duration of the interruption, with a half-life of approximately 1 second, suggesting that a conformational change occurred during the time that the molecule was arrested. The velocity after pausing an individual molecule was any velocity found in the starting distribution of the ensemble. We suggest that substrate binding stabilizes the enzyme in one of many equilibrium conformational sub-states that determine the rate-limiting translocation behaviour of each RecBCD molecule. Each stabilized sub-state can persist for the duration (approximately 1 minute) of processive unwinding of a DNA molecule, comprising tens of thousands of catalytic steps, each of which is much faster than the time needed for the conformational change required to alter kinetic behaviour. This ligand-dependent stabilization of rate-defining conformational sub-states results in seemingly static molecule-to-molecule variation in RecBCD helicase activity, but in fact reflects one microstate from the equilibrium ensemble that a single molecule manifests during an individual processive translocation event
The Host Galaxy and Central Engine of the Dwarf AGN POX 52
We present new multi-wavelength observations of the dwarf Seyfert 1 galaxy
POX 52 in order to investigate the properties of the host galaxy and the active
nucleus, and to examine the mass of its black hole, previously estimated to be
~ 10^5 M_sun. Hubble Space Telescope ACS/HRC images show that the host galaxy
has a dwarf elliptical morphology (M_I = -18.4 mag, Sersic index n = 4.3) with
no detected disk component or spiral structure, confirming previous results
from ground-based imaging. X-ray observations from both Chandra and XMM show
strong (factor of 2) variability over timescales as short as 500 s, as well as
a dramatic decrease in the absorbing column density over a 9 month period. We
attribute this change to a partial covering absorber, with a 94% covering
fraction and N_H = 58^{+8.4}_{-9.2} * 10^21 cm^-2, that moved out of the line
of sight in between the XMM and Chandra observations. Combining these data with
observations from the VLA, Spitzer, and archival data from 2MASS and GALEX, we
examine the spectral energy distribution (SED) of the active nucleus. Its shape
is broadly similar to typical radio-quiet quasar SEDs, despite the very low
bolometric luminosity of L_bol = 1.3 * 10^43 ergs/s. Finally, we compare black
hole mass estimators including methods based on X-ray variability, and optical
scaling relations using the broad H-beta line width and AGN continuum
luminosity, finding a range of black hole mass from all methods to be M_bh =
(2.2-4.2) * 10^5 M_sun, with an Eddington ratio of L_bol/L_edd = 0.2-0.5.Comment: 19 pages, 16 figures, accepted for publication in Ap
SDSS J143030.22-001115.1: A misclassified narrow-line Seyfert 1 galaxy with flat X-ray spectrum
We used multi-component profiles to model H and [O III]4959,5007 lines for SDSS J143030.22-001115.1, a narrow-line Seyfert 1
galaxy (NLS1) in a sample of 150 NLS1s candidates selected from the Sloan
Digital Sky Survey (SDSS) Early Data Release (EDR). After subtracting the
H contribution from narrow line regions (NLRs), we found that its full
width half maximum (FWHM) of broad H line is nearly 2900 \kms,
significantly larger than the customarily adopted criterion of 2000 \kms. With
its weak Fe II multiples, we think that SDSS J143030.22-001115.1 can't be
classified as a genuine NLS1. When we calculate the virial black hole masses of
NLS1s, we should use the H linewidth after subtracting the H
contribution from NLRs.Comment: 7 pages, 1 table, accepted by ChJA
A novel method for deriving the aerosol hygroscopicity parameter based only on measurements from a humidified nephelometer system
Aerosol hygroscopicity is crucial for understanding roles
of aerosol particles in atmospheric chemistry and aerosol climate effects.
Light-scattering enhancement factor f(RH, λ) is one of the
parameters describing aerosol hygroscopicity, which is defined as
f(RH, λ) = σsp(RH, λ)∕σsp(dry, λ), where
σsp(RH, λ) or σsp(dry, λ) represents σsp at
wavelength λ under certain relative humidity (RH) or dry conditions.
Traditionally, an overall hygroscopicity parameter κ can be
retrieved from measured f(RH, λ), hereinafter referred to as
κf(RH), by combining concurrently measured particle number
size distribution (PNSD) and mass concentration of black carbon. In this
paper, a new method is proposed to directly derive κf(RH)
based only on measurements from a three-wavelength humidified nephelometer
system. The advantage of this newly proposed approach is that
κf(RH) can be estimated without any additional information
about PNSD and black carbon. This method is verified with measurements from
two different field campaigns. Values of κf(RH) estimated
from this new method agree very well with those retrieved by using the
traditional method: all points lie near the 1 : 1 line and the square of
correlation coefficient between them is 0.99. The verification results
demonstrate that this newly proposed method of deriving
κf(RH) is applicable at different sites and in seasons of
the North China Plain and might also be applicable in other regions around
the world
Parallel electric field generation by Alfven wave turbulence
{This work aims to investigate the spectral structure of the parallel
electric field generated by strong anisotropic and balanced Alfvenic turbulence
in relation with the problem of electron acceleration from the thermal
population in solar flare plasma conditions.} {We consider anisotropic Alfvenic
fluctuations in the presence of a strong background magnetic field. Exploiting
this anisotropy, a set of reduced equations governing non-linear, two-fluid
plasma dynamics is derived. The low- limit of this model is used to
follow the turbulent cascade of the energy resulting from the non-linear
interaction between kinetic Alfven waves, from the large magnetohydrodynamics
(MHD) scales with down to the small "kinetic" scales
with , being the ion sound gyroradius.}
{Scaling relations are obtained for the magnitude of the turbulent
electromagnetic fluctuations, as a function of and ,
showing that the electric field develops a component parallel to the magnetic
field at large MHD scales.} {The spectrum we derive for the parallel electric
field fluctuations can be effectively used to model stochastic resonant
acceleration and heating of electrons by Alfven waves in solar flare plasma
conditions
Understanding AGN-Host Connection in Partially Obscured Active Galactic Nuclei. Part I: The Nature of AGN+HII Composites
The goal of our serial papers is to examine the evolutionary connection
between AGN and star formation in its host galaxy in the partially obscured
AGNs (i.e., Seyfert 1.8 and 1.9 galaxies). Taking advantage of these galaxies,
the properties of both components can be studied together by direct
measurements. In this paper, we focus on the broad-line composite galaxies
(composite AGNs) which are located between the theoretical and empirical
separation lines in the [NII]/Ha vs. [OIII]/Hb diagram. These galaxies are
searched for from the composite galaxies provided by the SDSS DR4 MPA/JHU
catalogs. After re-analyze the spectra, we perform a fine classification for
the 85 composite AGNs in terms of the BPT diagrams. All the objects located
below the three theoretical separation lines are associated with a young
stellar population (<1Gyrs), while either a young or old stellar population is
identified in the individual multiply-classified object. The
multiply-classified objects with a very old stellar population are located in
the LINER region in the [OI]/Ha vs. [OIII]/Hb diagram. We then consider the
connection between AGN and star formation to derive the key results. The
Eddington ratio inferred from the broad Ha emission, the age of the stellar
population of AGN's host as assessed by D_n(4000), and the line ratio [OI]/Ha
are found to be related with each other. These relations strongly support the
evolutionary scenario in which AGNs evolve from high L/L_Edd state with soft
spectrum to low L/L_Edd state with hard spectrum as young stellar population
ages and fades. The significant correlation between the line ratio [OI]/Ha and
D_n(4000) leads us to suggest that the line ratio could be used to trace the
age of stellar population in type I AGNs.Comment: 39 pages, 11 figures, 1 table, accepted by Ap
A study of aerosol liquid water content based on hygroscopicity measurements at high relative humidity in the North China Plain
Water can be a major component of aerosol particles, also serving as a
medium for aqueous-phase reactions. In this study, a novel method is
presented to calculate the aerosol liquid water content at high relative
humidity based on measurements of aerosol hygroscopic growth factor,
particle number size distribution and relative humidity in the Haze in China
(HaChi) summer field campaign (July–August 2009) in the North China Plain.
The aerosol liquid water content calculated using this method agreed well
with that calculated using a thermodynamic equilibrium model (ISORROPIA II)
at high relative humidity (>60%) with a correlation
coefficient of 0.96. At low relative humidity (<60%), an
underestimation was found in the calculated aerosol liquid water content by
the thermodynamic equilibrium model. This discrepancy mainly resulted from
the ISORROPIA II model, which only considered limited aerosol chemical
compositions. The mean and maximum values of aerosol liquid water content
during the HaChi campaign reached 1.69 × 10−4 g m−3
and 9.71 × 10−4 g m−3, respectively. A
distinct diurnal variation of the aerosol liquid water content was found,
with lower values during daytime and higher ones at night. The aerosol
liquid water content depended strongly on the relative humidity. The aerosol
liquid water content in the accumulation mode dominated the total aerosol
liquid water content
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