368 research outputs found
Spectroscopic and photometric studies of white dwarfs in the Hyades
The Hyades cluster is known to harbour ten so-called classical white dwarf
members. Numerous studies through the years have predicted that more than twice
this amount of degenerate stars should be associated with the cluster. Using
the PPMXL catalog of proper motions and positions, a recent study proposed 17
new white dwarf candidates. We review the membership of these candidates by
using published spectroscopic and photometric observations, as well as by
simulating the contamination from field white dwarfs. In addition to the ten
classical Hyades white dwarfs, we find six white dwarfs that may be of Hyades
origin and three more objects that have an uncertain membership status due to
their unknown or imprecise atmospheric parameters. Among those, two to three
are expected as field stars contamination. Accurate radial velocity
measurements will confirm or reject the candidates. One consequence is that the
longstanding problem that no white dwarf older than ~340 Myr appears to be
associated with the cluster remains unsolved.Comment: 14 pages, 9 figures, accepted for publication in the Astronomy and
Astrophysics journa
Structures of mammalian RNA polymerase II pre-initiation complexes
The initiation of transcription is a focal point for the regulation of gene activity during mammalian cell differentiation and development. To initiate transcription, RNA polymerase II (Pol II) assembles with general transcription factors into a pre-initiation complex (PIC) that opens promoter DNA. Previous work provided the molecular architecture of the yeast1,2,3,4,5,6,7,8,9 and human10,11 PIC and a topological model for DNA opening by the general transcription factor TFIIH12,13,14. Here we report the high-resolution cryo-electron microscopy structure of PIC comprising human general factors and Sus scrofa domesticus Pol II, which is 99.9% identical to human Pol II. We determine the structures of PIC with closed and opened promoter DNA at 2.5–2.8 Å resolution, and resolve the structure of TFIIH at 2.9–4.0 Å resolution. We capture the TFIIH translocase XPB in the pre- and post-translocation states, and show that XPB induces and propagates a DNA twist to initiate the opening of DNA approximately 30 base pairs downstream of the TATA box. We also provide evidence that DNA opening occurs in two steps and leads to the detachment of TFIIH from the core PIC, which may stop DNA twisting and enable RNA chain initiation
Global survey of star clusters in the Milky Way IV. 63 new open clusters detected by proper motions
AIMS: In their 1st extension to the Milky Way Star Clusters (MWSC) survey,
Schmeja et al. applied photometric filters to the 2MASS to find new cluster
candidates that were subsequently confirmed or rejected by the MWSC pipeline.
To further extend the MWSC census, we aimed at discovering new clusters by
conducting an almost global search in proper motion catalogues as a starting
point. METHODS: We first selected high-quality samples from the PPMXL and UCAC4
for comparison and verification of the proper motions. For 441 circular proper
motion bins (radius 15 mas/yr) within 50 mas/yr, the sky outside a thin
Galactic plane zone (5) was binned in small areas ('sky
pixels') of 0.250.25 deg. Sky pixels with enhanced numbers of stars
with a certain common proper motion in both catalogues were considered as
cluster candidates. After visual inspection of the sky images, we built an
automated procedure that combined these representations of the sky for
neighbouring proper motion subsamples after a background correction. RESULTS:
About half of our 692 candidates overlapped with known clusters (46 globular
and 68 open clusters in the Galaxy, about 150 known clusters of galaxies) or
the Magellanic Clouds. About 10% of our candidates turned out to be 63 new open
clusters confirmed by the MWSC pipeline. They occupy predominantly the two
inner Galactic quadrants and have apparent sizes and numbers of high-probable
members slightly larger than those of the typically small MWSC clusters,
whereas their other parameters (ages, distances, tidal radii) fall in the
typical ranges. As our search aimed at finding compact clusters, we did not
find new very nearby (extended) clusters. (abridged)Comment: 14 pages, 14 figures, accepted for publication in Astronomy and
Astrophysic
Global survey of star clusters in the Milky Way II. The catalogue of basic parameters
Although they are the main constituents of the Galactic disk population, for
half of the open clusters in the Milky Way reported in the literature nothing
is known except the raw position and an approximate size. The main goal of this
study is to determine a full set of uniform spatial, structural, kinematic, and
astrophysical parameters for as many known open clusters as possible. On the
basis of stellar data from PPMXL and 2MASS, we used a dedicated data-processing
pipeline to determine kinematic and photometric membership probabilities for
stars in a cluster region. For an input list of 3784 targets from the
literature, we confirm that 3006 are real objects, the vast majority of them
are open clusters, but associations and globular clusters are also present. For
each confirmed object we determined the exact position of the cluster centre,
the apparent size, proper motion, distance, colour excess, and age. For about
1500 clusters, these basic astrophysical parameters have been determined for
the first time. For the bulk of the clusters we also derived the tidal radius.
We estimated additionally average radial velocities for more than 30% of the
confirmed clusters. The present sample (called MWSC) reaches both the central
parts of the Milky Way and its outer regions. It is almost complete up to 1.8
kpc from the Sun and also covers neighbouring spiral arms. However, for a small
subset of the oldest open clusters () we found some evidence
of incompleteness within about 1 kpc from the Sun.Comment: 8 pages, 5 figures, accepted for publication in Astronomy and
Astrophysic
Why Simple Stellar Population models do not reproduce the colours of Galactic open clusters
(...) We search for an explanation of the disagreement between the observed
integrated colours of 650 local Galactic clusters and the theoretical colours
of present-day SSP models. We check the hypothesis that the systematic offsets
between observed and theoretical colours, which are and
, are caused by neglecting the discrete nature of the
underlying mass function. Using Monte Carlo simulations, we construct
artificial clusters of coeval stars taken from a mass distribution defined by
an Salpeter initial mass function (IMF) and compare them with corresponding
"continuous-IMF" SSP models. If the discreteness of the IMF is taken into
account, the model fits the observations perfectly and is able to explain
naturally a number of red "outliers" observed in the empirical colour-age
relation. We find that the \textit{systematic} offset between the continuous-
and discrete-IMF colours reaches its maximum of about 0.5 in for a
cluster mass at ages , and diminishes
substantially but not completely to about one hundredth of a magnitude at at cluster masses . At younger ages, it is still
present even in massive clusters, and for it is
larger than 0.1 mag in . Only for very massive clusters () with ages is the offset small (of the order of 0.04
mag) and smaller than the typical observational error of colours of
extragalactic clusters.Comment: 4 pages, 3 figures, accepted for publication in Astronomy and
Astrophysics Letters, revised version after language editing and with an
additional reference to Cervino and Luridiana (2004
PPM-Extended (PPMX) - a catalogue of positions and proper motions
Aims: We build a catalogue PPM-Extended (PPMX) on the ICRS system which is
complete down to a well-defined limiting magnitude and contains the best
presently available proper motions to be suited for kinematical studies in the
Galaxy.
Methods: We perform a rigorous weighted least-squares adjustment of
individual observations, spread over more than a century, to determine mean
positions and proper motions. The stellar content of PPMX is taken from GSC 1.2
supplemented by catalogues like ARIHIP, PPM and Tycho-2 at the bright end. All
observations have been weighted according to their individual accuracy. The
catalogue has been screened towards rejecting false entries in the various
source catalogues.
Results: PPM-Extended (PPMX) is a catalogue of 18,088,920 stars containing
astrometric and photometric information. Its limiting magnitude is about 15.2
in the GSC photometric system. PPMX consists of three parts: a) a survey
complete down to R_U = 12.8 in the magnitude system of UCAC2; b) additional
stars of high-precision proper motions, and c) all other stars from GSC 1.2
identified in 2MASS. The typical accuracy of the proper motions is 2mas/y for
66 percent of the survey stars (a) and the high-precision stars (b), and about
10 mas/y for all other stars. PPMX contains photometric information from
ASCC-2.5 and 2MASS.Comment: 9 pages, 8 figures, accepted for publication in Astronomy and
Astrophysic
Structure of the human Mediator-RNA polymerase II pre-initiation complex
Mediator is a conserved coactivator that enables regulated transcription initiation at eukaryotic genes1–3. Mediator is recruited by transcriptional activators and binds the pre-initiation complex (PIC) to stimulate RNA polymerase II (Pol II) phosphorylation and promoter escape1–6. Here we prepare a recombinant human Mediator, reconstitute a 50-subunit Mediator-PIC complex, and determine the structure of the complex by cryo-EM. Mediator uses its head module to contact the Pol II stalk and the general transcription factors TFIIB and TFIIE, resembling the Mediator-PIC interactions in the corresponding yeast complex7–9. The metazoan subunits MED27-MED30 associate with exposed regions in MED14 and MED17 to form the proximal part of the Mediator tail module that binds activators. Mediator positions the flexibly linked CDK-activating kinase (CAK) of the general transcription factor TFIIH near the linker to the C-terminal repeat domain (CTD) of Pol II. The Mediator shoulder domain holds the CAK subunit CDK7, whereas the hook domain contacts a CDK7 element that flanks the kinase active site. The shoulder and hook reside in the Mediator head and middle modules, respectively, which can move relative to each other and may induce an active conformation of the CDK7 kinase to allosterically stimulate CTD phosphorylation
Cryo-EM structure of mammalian RNA polymerase II in complex with human RPAP2
Nuclear import of RNA polymerase II (Pol II) involves the conserved factor RPAP2. Here we report the cryo-electron microscopy (cryo-EM) structure of mammalian Pol II in complex with human RPAP2 at 2.8 Å resolution. The structure shows that RPAP2 binds between the jaw domains of the polymerase subunits RPB1 and RPB5. RPAP2 is incompatible with binding of downstream DNA during transcription and is displaced upon formation of a transcription pre-initiation complex
Structures of transcription preinitiation complex engaged with the +1 nucleosome
The preinitiation complex (PIC) assembles on promoters of protein-coding genes to position RNA polymerase II (Pol II) for transcription initiation. Previous structural studies revealed the PIC on different promoters, but did not address how the PIC assembles within chromatin. In the yeast Saccharomyces cerevisiae, PIC assembly occurs adjacent to the +1 nucleosome that is located downstream of the core promoter. Here we present cryo-EM structures of the yeast PIC bound to promoter DNA and the +1 nucleosome located at three different positions. The general transcription factor TFIIH engages with the incoming downstream nucleosome and its translocase subunit Ssl2 (XPB in human TFIIH) drives the rotation of the +1 nucleosome leading to partial detachment of nucleosomal DNA and intimate interactions between TFIIH and the nucleosome. The structures provide insights into how transcription initiation can be influenced by the +1 nucleosome and may explain why the transcription start site is often located roughly 60 base pairs upstream of the dyad of the +1 nucleosome in yeast
- …