26 research outputs found
UV Spectropolarimetry with Polstar: Massive Star Binary Colliding Winds
The winds of massive stars are important for their direct impact on the
interstellar medium, and for their influence on the final state of a star prior
to it exploding as a supernova. However, the dynamics of these winds is
understood primarily via their illumination from a single central source. The
Doppler shift seen in resonance lines is a useful tool for inferring these
dynamics, but the mapping from that Doppler shift to the radial distance from
the source is ambiguous. Binary systems can reduce this ambiguity by providing
a second light source at a known radius in the wind, seen from orbitally
modulated directions. From the nature of the collision between the winds, a
massive companion also provides unique additional information about wind
momentum fluxes. Since massive stars are strong ultraviolet (UV) sources, and
UV resonance line opacity in the wind is strong, UV instruments with a high
resolution spectroscopic capability are essential for extracting this dynamical
information. Polarimetric capability also helps to further resolve ambiguities
in aspects of the wind geometry that are not axisymmetric about the line of
sight, because of its unique access to scattering direction information. We
review how the proposed MIDEX-scale mission Polstar can use UV
spectropolarimetric observations to critically constrain the physics of
colliding winds, and hence radiatively-driven winds in general. We propose a
sample of 20 binary targets, capitalizing on this unique combination of
illumination by companion starlight, and collision with a companion wind, to
probe wind attributes over a range in wind strengths. Of particular interest is
the hypothesis that the radial distribution of the wind acceleration is altered
significantly, when the radiative transfer within the winds becomes optically
thick to resonance scattering in multiple overlapping UV lines.Comment: 26 pages, 12 figures, Review in a topical collection series of
Astrophysics and Space Sciences on the proposed Polstar satellite. arXiv
admin note: substantial text overlap with arXiv:2111.1155
Ultraviolet spectropolarimetry: conservative and nonconservative mass transfer in OB interacting binaries
peer reviewe
Ultraviolet spectropolarimetry: conservative and nonconservative mass transfer in OB interacting binaries
peer reviewe
Noncanonical DNA Motifs as Transactivation Targets by Wild Type and Mutant p53
Sequence-specific binding by the human p53 master regulator is critical to its tumor suppressor activity in response to environmental stresses. p53 binds as a tetramer to two decameric half-sites separated by 0–13 nucleotides (nt), originally defined by the consensus RRRCWWGYYY (n = 0–13) RRRCWWGYYY. To better understand the role of sequence, organization, and level of p53 on transactivation at target response elements (REs) by wild type (WT) and mutant p53, we deconstructed the functional p53 canonical consensus sequence using budding yeast and human cell systems. Contrary to early reports on binding in vitro, small increases in distance between decamer half-sites greatly reduces p53 transactivation, as demonstrated for the natural TIGER RE. This was confirmed with human cell extracts using a newly developed, semi–in vitro microsphere binding assay. These results contrast with the synergistic increase in transactivation from a pair of weak, full-site REs in the MDM2 promoter that are separated by an evolutionary conserved 17 bp spacer. Surprisingly, there can be substantial transactivation at noncanonical ½-(a single decamer) and ¾-sites, some of which were originally classified as biologically relevant canonical consensus sequences including PIDD and Apaf-1. p53 family members p63 and p73 yielded similar results. Efficient transactivation from noncanonical elements requires tetrameric p53, and the presence of the carboxy terminal, non-specific DNA binding domain enhanced transactivation from noncanonical sequences. Our findings demonstrate that RE sequence, organization, and level of p53 can strongly impact p53-mediated transactivation, thereby changing the view of what constitutes a functional p53 target. Importantly, inclusion of ½- and ¾-site REs greatly expands the p53 master regulatory network