342,132 research outputs found
Plasmid copy number underlies adaptive mutability in bacteria.
The origin of mutations under selection has been intensively studied using the Cairns-Foster system, in which cells of an Escherichia coli lac mutant are plated on lactose and give rise to 100 Lac+ revertants over several days. These revertants have been attributed variously to stress-induced mutagenesis of nongrowing cells or to selective improvement of preexisting weakly Lac+ cells with no mutagenesis. Most revertant colonies (90%) contain stably Lac+ cells, while others (10%) contain cells with an unstable amplification of the leaky mutant lac allele. Evidence is presented that both stable and unstable Lac+ revertant colonies are initiated by preexisting cells with multiple copies of the F'lac plasmid, which carries the mutant lac allele. The tetracycline analog anhydrotetracycline (AnTc) inhibits growth of cells with multiple copies of the tetA gene. Populations with tetA on their F'lac plasmid include rare cells with an elevated plasmid copy number and multiple copies of both the tetA and lac genes. Pregrowth of such populations with AnTc reduces the number of cells with multiple F'lac copies and consequently the number of Lac+ colonies appearing under selection. Revertant yield is restored rapidly by a few generations of growth without AnTc. We suggest that preexisting cells with multiple F'lac copies divide very little under selection but have enough energy to replicate their F'lac plasmids repeatedly until reversion initiates a stable Lac+ colony. Preexisting cells whose high-copy plasmid includes an internal lac duplication grow under selection and produce an unstable Lac+ colony. In this model, all revertant colonies are initiated by preexisting cells and cannot be stress induced
BL Lacertae Objects and the Extragalactic Gamma-Ray Background
A tight correlation between gamma-ray and radio emission is found for a
sample of BL Lacertae (BL Lac) objects detected by Fermi Gamma-ray Space
Telescope (Fermi) and the Energetic Gamma-Ray Experiment Telescope (EGRET). The
gamma-ray emission of BL Lac objects exhibits strong variability, and the
detection rate of gamma-ray BL Lac objects is low, which may be related to the
gamma-ray duty cycle of BL Lac objects. We estimate the gamma-ray duty cycle ~
0.11, for BL Lac objects detected by EGRET and Fermi. Using the empirical
relation of gamma-ray emission with radio emission and the estimated gamma-ray
duty cycle, we derive the gamma-ray luminosity function (LF) of BL Lac objects
from their radio LF. Our derived gamma-ray LF of BL Lac objects can almost
reproduce that calculated with the recently released Fermi bright active
galactic nuclei (AGN) sample. We find that about 45% of the extragalactic
diffuse gamma-ray background (EGRB) is contributed by BL Lac objects. Combining
the estimate of the quasar contribution to the EGRB in the previous work, we
find that about 77% of the EGRB is contributed by BL Lac objects and radio
quasars.Comment: 12 pages, 4 figures, submitte
Intranight optical variability of radio-quiet BL Lacertae objects
Aims: Intranight variation (or microvariation) is a common phenomenon of
radio-loud BL Lac objects. However, it is not clear whether the recently found
radio-quiet BL Lac objects have the same properties. The occurrence rate of
intranight variation is helpful in distinguishing the mechanism of the
continuum of radio-quiet BL Lac objects.
Methods: We conducted a photometric monitoring of 8 radio-quiet BL Lac
objects by the Xinglong 2.16m and Lijiang 2.4m telescopes. The differential
light curves are calculated between each target and two comparison stars. To
quantify the variation, the significance of variation is examined by a scaled
-test.
Results: No significant variation is found in the 11 sessions of light curves
of 8 radio-quiet BL Lac objects (one galactic source is excluded). The lack of
microvariation in radio-quiet BL Lac objects is consistent with the detection
rate of microvariation in normal radio-quiet AGNs, but much lower than for
radio-loud AGNs. This result indicates that the continua of the radio-quiet BL
Lac objects are not dominated by jets that will induce frequent
microvariations.Comment: Accepted for publication in Astronomy and Astrophysics; 7 pages, 1
figure, 3 table
Quasi-periodicities of BL Lac Objects
We review the reports of possible year-long quasi-periodicities of BL Lac
objects in the -ray and optical bands, and present a homogeneous time
analysis of the light curves of PKS2155304, PG1553+113, and BL Lac. Based on
results from a survey covering the entire Fermi -ray sky we have
estimated the fraction of possible quasi-periodic BL Lac objects. We compared
the cyclical behaviour in BL Lac objects with that derived from the search of
possible optical periodicities in quasars, and find that at z1 the
cosmic density of quasi-periodic BL Lac objects is larger than that of
quasi-periodic quasars. If the BL Lac quasi-periodicities were due to a
supermassive binary black hole (SBBH) scenario, there could be a tension with
the upper limits on the gravitational wave background measured by the pulsar
timing array. The argument clearly indicates the difficulties of generally
associating quasi-periodicities of BL Lac objects with SBBHs.Comment: In publication on A&A, 6 pages, 4 figure (11 plots). Minor
corrections adde
Polarimetry of optically selected BL Lac candidates from the SDSS
We present and discuss polarimetric observations of 182 targets drawn from an
optically selected sample of 240 probable BL Lac candidates out of the SDSS
compiled by Collinge et al. (2005). In contrast to most other BL Lac candidate
samples extracted from the SDSS, its radio- and/or X-ray properties have not
been taken into account for its derivation. Thus, because its selection is
based on optical properties alone, it may be less prone to selection effects
inherent in other samples derived at different frequencies, so it offers a
unique opportunity to extract the first unbiased BL Lac luminosity function
that is suitably large in size.
We found 124 out of 182 targets (68%) to be polarized, 95 of the polarized
targets (77%) to be highly polarized (> 4%). The low-frequency peaked BL Lac
candidates in the sample are on average only slightly more polarized than the
high-frequency peaked ones. Compared to earlier studies, we found a high duty
cycle in high polarization (~66 +2/-14% to be > 4% polarized) in high-frequency
peaked BL Lac candidates. This may come from our polarization analysis, which
minimizes the contamination by host galaxy light.
No evidence of radio-quiet BL Lac objects in the sample was found.
Our observations show that the probable sample of BL Lac candidates of
Collinge et al. (2005) indeed contains a large number of bona fide BL Lac
objects. High S/N spectroscopy and deep X-ray observations are required to
construct the first luminosity function of optically selected BL Lac objects
and to test more stringently for any radio-quiet BL Lac objects in the sample.Comment: Accepted for publication in A&A. 14 pages, 9 figures, table 1 in
electronic form onl
Kinetic approaches to lactose operon induction and bimodality
The quasi-equilibrium approximation is acceptable when molecular interactions
are fast enough compared to circuit dynamics, but is no longer allowed when
cellular activities are governed by rare events. A typical example is the
lactose operon (lac), one of the most famous paradigms of transcription
regulation, for which several theories still coexist to describe its behaviors.
The lac system is generally analyzed by using equilibrium constants,
contradicting single-event hypotheses long suggested by Novick and Weiner
(1957). Enzyme induction as an all-or-none phenomenon. Proc. Natl. Acad. Sci.
USA 43, 553-566) and recently refined in the study of (Choi et al., 2008. A
stochastic single-molecule event triggers phenotype switching of a bacterial
cell. Science 322, 442-446). In the present report, a lac repressor
(LacI)-mediated DNA immunoprecipitation experiment reveals that the natural
LacI-lac DNA complex built in vivo is extremely tight and long-lived compared
to the time scale of lac expression dynamics, which could functionally
disconnect the abortive expression bursts and forbid using the standard modes
of lac bistability. As alternatives, purely kinetic mechanisms are examined for
their capacity to restrict induction through: (i) widely scattered derepression
related to the arrival time variance of a predominantly backward asymmetric
random walk and (ii) an induction threshold arising in a single window of
derepression without recourse to nonlinear multimeric binding and Hill
functions. Considering the complete disengagement of the lac repressor from the
lac promoter as the probabilistic consequence of a transient stepwise
mechanism, is sufficient to explain the sigmoidal lac responses as functions of
time and of inducer concentration. This sigmoidal shape can be misleadingly
interpreted as a phenomenon of equilibrium cooperativity classically used to
explain bistability, but which has been reported to be weak in this system
Laboratory Detection of the Antiphospholipid Syndrome via Calibrated Automated Thrombography
Lupus anticoagulants (LAC) consist of anti phospholipid antibodies, detected via their anti coagulant properties in vitro. Strong LAC relate to thromboembolic events, a hallmark of the anti-phospholipid syndrome. We have analyzed whether detection of this syndrome would benefit from thrombin generation measurements. Therefore, calibrated automated thrombography was done in normal plasma (n=30) and LAC patient plasma (n=48 non-anticoagulated, n=12 on oral anti coagulants), diluted 1: 1 with a normal plasma pool. The anti-beta(2)-glycoprotein I monoclonal antibody 23H9, with known LAC properties, delayed the lag time and reduced the peak height during thrombin generation induction in normal plasma dose-dependently (0-150 mu g/ml). At variance, LAC patient 1: 1 plasma mixtures manifested variable lag time prolongations and/or peak height reductions. Coupling these two most informative thrombin generation parameters in a peak height/lag time ratio,and upon normalization versus the normal plasma pool, this ratio distributed normally and was reduced in the plasma mixtures, for 59/60 known LAC plasmas. The normalized peak height/lag time ratio correlated well with the normalized dilute prothrombin time,diluted Russell's viper venom time and silica clotting time, measured in 1: 1 plasma mixtures (correlation coefficients 0.59-0.72). The anticoagulant effects of activated protein C (0-7.5 nM) or 23H9 (0-150 mu g/ml), spiked in the 1: 1 LAC plasma mixtures were reduced for the majority of patients, compatible with functional competition between patient LAC and activated protein C and LAC and 23H9, respectively. Hence,the normalized thrombin gene ration-derived peak height/lag time ratio identifies LAC in plasma with high sensitivity in a single assay, irrespective of the patient's treatment with oral anticoagulants
The fraction of BL Lac objects in groups of galaxies
Context: BL Lac objects are a rare class of active galactic nuclei that
typically show featureless optical spectra that make it difficult to estimate
the redshift. A novel method for estimating the redshift of BL Lac objects has
recently been proposed which assumes that these objects typically reside in
groups of galaxies.
Aims: The aim of this work is to estimate the fraction of BL Lac objects that
reside in groups of galaxies.
Methods: We use a sample of groups (M12 catalogue) selected by applying a
friends-of-friends algorithm in the SDSS DR12. Galaxies in the M12 sample were
cross-correlated with the sample of blazars in the BZCAT.
Results: We found that 121 galaxies in the M12 catalogue are blazars in the
BZCAT sample, all but one are BL Lac objects, and a large fraction are
classified as BL Lac-galaxy dominated. Analysing the fraction of galaxies in
groups as a function of redshift we have estimated a correction factor that
takes into account the typical incompleteness of the catalogues based on the
friends-of-friends algorithm. Once this factor was applied to the sample of BL
Lac objects with a counterpart in the M12 catalogue, we found that the
percentage of BL Lac objects in groups is .
Conclusions: The high rate of BL Lac objects in groups found in this work
strongly supports a recent method that has successfully estimated the redshift
of BL Lac objects with featureless spectra.Comment: Accepted for publication in Astronomy and Astrophysics (Letter
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