<i>VSG</i> Switching Hierarchy in T. brucei

<p>The graph is adapted from [<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0060185#pbio-0060185-b009" target="_blank">9</a>] and shows the numbers of T. brucei cells (parasitaemia) measured in a cow for up to 70 days post-infection (this measurement is depicted by inversely plotting the prepatent period, in days, that a 0.2-ml inoculum of cattle blood achieves a parasitaemia of 1 × 10^8.1 trypanosomes ml⁻¹ units in an immunosuppressed mouse). Below the graph is a depiction of <i>VSG</i> gene activation timing (see <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0060185#pbio-0060185-g002" target="_blank">Figure 2</a> for details of the switch mechanisms). During <i>VSG</i> switches driven by recombination, silent <i>VSG</i>s at a telomere are, in general, activated more frequently that subtelomeric array <i>VSG</i>s, which are activated more frequently than <i>VSG</i> pseudogenes (pseudo). It is unclear (indicated by a question mark) if transcriptional switches between <i>VSG</i> bloodstream expression sites (BES) occur predominantly at the start of an infection or continue throughout.</p

Mechanisms of <i>VSG</i> Switching during Antigenic Variation in T. brucei

<p>The <i>VSG</i> gene expressed prior to a switch (indicated by a blue box) is transcribed from an expression site (ES) that is found at the telomere (vertical black line) of a chromosome (horizontal black line); active transcription of the ES is indicated by a dotted arrow, <i>ESAG</i>s are depicted by black boxes, and 70-bp repeat sequence is shown as a hatched box. Gene conversion to generate a <i>VSG</i> switch can occur by copying a silent <i>VSG</i> (red box) from a subtelomeric array into the ES, replacing the resident <i>VSG</i>; the amount of sequence copied during gene conversion is illustrated, and normally encompasses the <i>VSG</i> ORF and extends upstream to the 70-bp repeats. The silent <i>VSG</i> donor can also be telomeric (either in a mini chromosome or in an inactive ES); here, the downstream limit of conversion can extend to the telomere repeats, while the upstream limit can either be in the 70-bp repeats or the <i>ESAG</i>s (if the donor is in an ES). Segmental <i>VSG</i> conversion involves the copying of sequence from multiple, normally nonfunctional <i>VSG</i>s (pink, red, or green boxes) to generate a novel mosaic <i>VSG</i> in the ES. In transcriptional <i>VSG</i> switching, recombination appears not to be involved; instead, limited transcription at a silent <i>VSG</i> ES (indicated by a small arrow) becomes activated to generate fully active transcription, while the previously active ES is silenced.</p

Functions of Dot1-Mediated Histone H3 Methylation

<p>Two models for the role of Dot1-mediated methylation of histone H3 are diagrammed, comparing a Dot1 mutant (ΔDot1) and a wild-type cell. The repulsion model is derived from [<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0060185#pbio-0060185-b021" target="_blank">21</a>]. Methylation of histone H3 is indicated by “me”, and the level of transcription of a chromosome (black line) is indicated by a shaded gray bar (a thick bar indicates active transcription, a thin bar indicates silenced transcription). Silencing factors (such as Sir proteins in yeast; light blue circles) are indicated localised to the telomere (vertical line) in wild-type cells, being excluded from elsewhere by H3K79 methylation. Mutation of Dot1 removes H3K79 methylation, de-repressing transcription of the telomeric region. The recruitment model is based on [<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0060185#pbio-0060185-b032" target="_blank">32</a>], and shows the same region of chromosome after suffering a DNA double-strand break (gap in the line). Here, histone H3K79 methylation recruits a checkpoint signalling factor (Rad9 in yeast; dark blue circle), and in the absence of histone H3K79 methylation processing of the DNA break to yield single stranded DNA is increased, amplifying the DNA damage signalling cascade.</p

A transit timing variation observed for the long-period extremely low density exoplanet HIP 41378f

HIP 41378 f is a temperate 9.2 ± 0.1 R⊕ planet with period of 542.08 d and an extremely low density of 0.09 ± 0.02 g cm−3. It transits the bright star HIP 41378 (V = 8.93), making it an exciting target for atmospheric characterization including transmission spectroscopy. HIP 41378 was monitored photometrically between the dates of 2019 November 19 and 28. We detected a transit of HIP 41378 f with NGTS, just the third transit ever detected for this planet, which confirms the orbital period. This is also the first ground-based detection of a transit of HIP 41378 f. Additional ground-based photometry was also obtained and used to constrain the time of the transit. The transit was measured to occur 1.50 h earlier than predicted. We use an analytic transit timing variation (TTV) model to show the observed TTV can be explained by interactions between HIP 41378 e and HIP 41378 f. Using our TTV model, we predict the epochs of future transits of HIP 41378 f, with derived transit centres of TC, 4 = 2459355.087+0.031−0.022 (2021 May) and TC, 5 = 2459897.078+0.114−0.060 (2022 November)

Populating the brown dwarf and stellar boundary: Five stars with transiting companions near the hydrogen-burning mass limit

We report the discovery of five transiting companions near the hydrogen-burning mass limit in close orbits around main sequence stars originally identified by the Transiting Exoplanet Survey Satellite (TESS) as TESS objects of interest (TOIs): TOI-148, TOI-587, TOI-681, TOI-746, and TOI-1213. Using TESS and ground-based photometry as well as radial velocities from the CORALIE, CHIRON, TRES, and FEROS spectrographs, we found the companions have orbital periods between 4.8 and 27.2 days, masses between 77 and 98 MJup, and radii between 0.81 and 1.66 RJup. These targets have masses near the uncertain lower limit of hydrogen core fusion (~73-96 MJup), which separates brown dwarfs and low-mass stars. We constrained young ages for TOI-587 (0.2 ± 0.1 Gyr) and TOI-681 (0.17 ± 0.03 Gyr) and found them to have relatively larger radii compared to other transiting companions of a similar mass. Conversely we estimated older ages for TOI-148 and TOI-746 and found them to have relatively smaller companion radii. With an effective temperature of 9800 ± 200 K, TOI-587 is the hottest known main-sequence star to host a transiting brown dwarf or very low-mass star. We found evidence of spin-orbit synchronization for TOI-148 and TOI-746 as well as tidal circularization for TOI-148. These companions add to the population of brown dwarfs and very low-mass stars with well measured parameters ideal to test formation models of these rare objects, the origin of the brown dwarf desert, and the distinction between brown dwarfs and hydrogen-burning main sequence stars

TOI-431/HIP 26013: a super-Earth and a sub-Neptune transiting a bright, early K dwarf, with a third RV planet

We present the bright (Vmag = 9.12), multiplanet system TOI-431, characterized with photometry and radial velocities (RVs). We estimate the stellar rotation period to be 30.5 ± 0.7 d using archival photometry and RVs. Transiting Exoplanet Survey Satellite (TESS) objects of Interest (TOI)-431 b is a super-Earth with a period of 0.49 d, a radius of 1.28 ± 0.04 R, a mass of 3.07 ± 0.35 M, and a density of 8.0 ± 1.0 g cm-3; TOI-431 d is a sub-Neptune with a period of 12.46 d, a radius of 3.29 ± 0.09 R, a mass of 9.90+1.53-1.49 M, and a density of 1.36 ± 0.25 g cm-3. We find a third planet, TOI-431 c, in the High Accuracy Radial velocity Planet Searcher RV data, but it is not seen to transit in the TESS light curves. It has an Msin i of 2.83+0.41-0.34 M, and a period of 4.85 d. TOI-431 d likely has an extended atmosphere and is one of the most well-suited TESS discoveries for atmospheric characterization, while the super-Earth TOI-431 b may be a stripped core. These planets straddle the radius gap, presenting an interesting case-study for atmospheric evolution, and TOI-431 b is a prime TESS discovery for the study of rocky planet phase curves