1,427 research outputs found
Development of a micrometeoroid accelerator Final report
Design and performance of arc plasma micrometeoroid accelerator system
Characterization of the ZFX family of transcription factors that bind downstream of the start site of CpG island promoters
Our study focuses on a family of ubiquitously expressed human C₂H₂ zinc finger proteins comprised of ZFX, ZFY and ZNF711. Although their protein structure suggests that ZFX, ZFY and ZNF711 are transcriptional regulators, the mechanisms by which they influence transcription have not yet been elucidated. We used CRISPR-mediated deletion to create bi-allelic knockouts of ZFX and/or ZNF711 in female HEK293T cells (which naturally lack ZFY). We found that loss of either ZFX or ZNF711 reduced cell growth and that the double knockout cells have major defects in proliferation. RNA-seq analysis revealed that thousands of genes showed altered expression in the double knockout clones, suggesting that these TFs are critical regulators of the transcriptome. To gain insight into how these TFs regulate transcription, we created mutant ZFX proteins and analyzed them for DNA binding and transactivation capability. We found that zinc fingers 11–13 are necessary and sufficient for DNA binding and, in combination with the N terminal region, constitute a functional transactivator. Our functional analyses of the ZFX family provides important new insights into transcriptional regulation in human cells by members of the large, but under-studied family of C₂H₂ zinc finger proteins
The Evolving Activity of the Dynamically Young Comet C/2009 P1 (Garradd)
We used the UltraViolet-Optical Telescope on board Swift to observe the
dynamically young comet C/2009 P1 (Garradd) from a heliocentric distance of 3.5
AU pre-perihelion until 4.0 AU outbound. At 3.5 AU pre-perihelion, comet
Garradd had one of the highest dust-to-gas ratios ever observed, matched only
by comet Hale-Bopp. The evolving morphology of the dust in its coma suggests an
outburst that ended around 2.2 AU pre-perihelion. Comparing slit-based
measurements and observations acquired with larger fields of view indicated
that between 3 AU and 2 AU pre-perihelion a significant extended source started
producing water in the coma. We demonstrate that this source, which could be
due to icy grains, disappeared quickly around perihelion. Water production by
the nucleus may be attributed to a constantly active source of at least 75
km, estimated to be more than 20 percent of the surface. Based on our
measurements, the comet lost kg of ice and dust during this
apparition, corresponding to at most a few meters of its surface.Even though
this was likely not Garradd's first passage through the inner solar system, the
activity of the comet was complex and changed significantly during the time it
was observed
A Quarter-Century of Observations of Comet 10P/Tempel 2 at Lowell Observatory: Continued Spin-Down, Coma Morphology, Production Rates, and Numerical Modeling
We report on photometry and imaging of Comet 10P/Tempel 2 obtained at Lowell
Observatory from 1983 through 2011. We measured a nucleus rotation period of
8.950 +/- 0.002 hr from 2010 September to 2011 January. This rotation period is
longer than the period we previously measured in 1999, which was itself longer
than the period measured in 1988. A nearly linear jet was observed which varied
little during a rotation cycle in both R and CN images acquired during the 1999
and 2010 apparitions. We measured the projected direction of this jet
throughout the two apparitions and, under the assumption that the source region
of the jet was near the comet's pole, determined a rotational pole direction of
RA/Dec = 151deg/+59deg from CN measurements and RA/Dec = 173deg/+57deg from
dust measurements (we estimate a circular uncertainty of 3deg for CN and 4deg
for dust). Different combinations of effects likely bias both gas and dust
solutions and we elected to average these solutions for a final pole of RA/Dec
= 162 +/- 11deg/+58 +/- 1deg. Photoelectric photometry was acquired in 1983,
1988, 1999/2000, and 2010/2011. The activity exhibited a steep turn-on ~3
months prior to perihelion (the exact timing of which varies) and a relatively
smooth decline after perihelion. The activity during the 1999 and 2010
apparitions was similar; limited data in 1983 and 1988 were systematically
higher and the difference cannot be explained entirely by the smaller
perihelion distance. We measured a "typical" composition, in agreement with
previous investigators. Monte Carlo numerical modeling with our pole solution
best replicated the observed coma morphology for a source region located near a
comet latitude of +80deg and having a radius of ~10deg. Our model reproduced
the seasonal changes in activity, suggesting that the majority of Tempel 2's
activity originates from a small active region located near the pole.Comment: Accepted by AJ; 29 pages of text (preprint style), 8 tables, 7
figure
The Highly Unusual Outgassing of Comet 103P/Hartley 2 from Narrowband Photometry and Imaging of the Coma
We report on photometry and imaging of Comet 103P/Hartley 2 obtained at
Lowell Observatory from 1991 through 2011. We acquired photoelectric photometry
on two nights in 1991, four nights in 1997/98, and 13 nights in 2010/11. We
observed a strong secular decrease in water and all other observed species
production in 2010/11 from the 1991 and 1997/98 levels. We see evidence for a
strong asymmetry with respect to perihelion in the production rates of our
usual bandpasses, with peak production occurring ~10 days post-perihelion and
production rates considerably higher post-perihelion. The composition was
"typical", in agreement with the findings of other investigators. We obtained
imaging on 39 nights from 2010 July until 2011 January. We find that, after
accounting for their varying parentage and lifetimes, the C2 and C3 coma
morphology resemble the CN morphology we reported previously. These species
exhibited an hourglass shape in October and November, and the morphology
changed with rotation and evolved over time. The OH and NH coma morphology
showed hints of an hourglass shape near the nucleus, but was also enhanced in
the anti-sunward hemisphere. This tailward brightness enhancement did not vary
significantly with rotation and evolved with the viewing geometry. We conclude
that all five gas species likely originate from the same source regions on the
nucleus, but that OH and NH were derived from small grains of water and ammonia
ice that survived long enough to be affected by radiation pressure and driven
in the anti-sunward direction. We detected the faint, sunward facing dust jet
reported by other authors, and did not detect a corresponding gas feature. This
jet varied little during a night but exhibited some variations from night to
night, suggesting it is located near the total angular momentum vector.Comment: Accepted by Icarus; 20 pages of text (preprint style), 5 tables, 7
figure
The Increasing Rotation Period of Comet 10P/Tempel 2
We imaged comet 10P/Tempel 2 on 32 nights from 1999 April through 2000 March.
R-band lightcurves were obtained on 11 of these nights from 1999 April through
1999 June, prior to both the onset of significant coma activity and perihelion.
Phasing of the data yields a double-peaked lightcurve and indicates a nucleus
rotational period of 8.941 +/- 0.002 hr with a peak-to-peak amplitude of ~0.75
mag. Our data are sufficient to rule out all other possible double-peaked
solutions as well as the single- and triple- peaked solutions. This rotation
period agrees with one of five possible solutions found in post-perihelion data
from 1994 by Mueller and Ferrin (1996, Icarus, 123, 463-477), and unambiguously
eliminates their remaining four solutions. We applied our same techniques to
published lightcurves from 1988 which were obtained at an equivalent orbital
position and viewing geometry as in 1999. We found a rotation period of 8.932
+/- 0.001 hr in 1988, consistent with the findings of previous authors and
incompatible with our 1999 solution. This reveals that Tempel 2 spun-down by
~32 s between 1988 and 1999 (two intervening perihelion passages). If the
spin-down is due to a systematic torque, then the rotation period prior to
perihelion during the 2010 apparition is expected to be an additional 32 s
longer than in 1999.Comment: Accepted by The Astronomical Journal; 22 pages of text, 3 tables, 6
figure
dCas9-based epigenome editing suggests acquisition of histone methylation is not sufficient for target gene repression.
Distinct epigenomic profiles of histone marks have been associated with gene expression, but questions regarding the causal relationship remain. Here we investigated the activity of a broad collection of genomically targeted epigenetic regulators that could write epigenetic marks associated with a repressed chromatin state (G9A, SUV39H1, KrĂĽppel-associated box (KRAB), DNMT3A as well as the first targetable versions of Ezh2 and Friend of GATA-1 (FOG1)). dCas9 fusions produced target gene repression over a range of 0- to 10-fold that varied by locus and cell type. dCpf1 fusions were unable to repress gene expression. The most persistent gene repression required the action of several effector domains; however, KRAB-dCas9 did not contribute to persistence in contrast to previous reports. A 'direct tethering' strategy attaching the Ezh2 methyltransferase enzyme to dCas9, as well as a 'recruitment' strategy attaching the N-terminal 45 residues of FOG1 to dCas9 to recruit the endogenous nucleosome remodeling and deacetylase complex, were both successful in targeted deposition of H3K27me3. Surprisingly, however, repression was not correlated with deposition of either H3K9me3 or H3K27me3. Our results suggest that so-called repressive histone modifications are not sufficient for gene repression. The easily programmable dCas9 toolkit allowed precise control of epigenetic information and dissection of the relationship between the epigenome and gene regulation
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