The μ Switch Region Tandem Repeats Are Important, but Not Required, for Antibody Class Switch Recombination

Class switch DNA recombinations change the constant (C) region of the antibody heavy (H) chain expressed by a B cell and thereby change the antibody effector function. Unusual tandemly repeated sequence elements located upstream of H chain gene exons have long been thought to be important in the targeting and/or mechanism of the switch recombination process. We have deleted the entire switch tandem repeat element (Sμ) from the murine μ H chain gene. We find that the Sμ tandem repeats are not required for class switching in the mouse immunoglobulin H-chain locus, although the efficiency of switching is clearly reduced. Our data demonstrate that sequences outside of the Sμ tandem repeats must be capable of directing the class switch mechanism. The maintenance of the highly repeated Sμ element during evolution appears to reflect selection for a highly efficient switching process rather than selection for a required sequence element

Shifts in targeting of class switch recombination sites in mice that lack mu switch region tandem repeats or Msh2

The mechanisms that target class switch recombination (CSR) to antibody gene switch (S) regions are unknown. Analyses of switch site locations in wild-type mice and in mice that lack the Smu tandem repeats show shifts indicating that a 4-5-kb DNA domain (bounded upstream by the Imu promoter) is accessible for switching independent of Smu sequences. This CSR-accessible domain is reminiscent of the promoter-defined domains that target somatic hypermutation. Within the 4-5-kb CSR domain, the targeting of S site locations also depends on the Msh2 mismatch repair protein because Msh2-deficient mice show an increased focus of sites to the Smu tandem repeat region. We propose that Msh2 affects S site location because sequences with few activation-induced cytidine deaminase targets generate mostly switch DNA cleavages that require Msh2-directed processing to allow CSR joining

Shifts in targeting of class switch recombination sites in mice that lack μ switch region tandem repeats or Msh2

The mechanisms that target class switch recombination (CSR) to antibody gene switch (S) regions are unknown. Analyses of switch site locations in wild-type mice and in mice that lack the Sμ tandem repeats show shifts indicating that a 4–5-kb DNA domain (bounded upstream by the Iμ promoter) is accessible for switching independent of Sμ sequences. This CSR-accessible domain is reminiscent of the promoter-defined domains that target somatic hypermutation. Within the 4–5-kb CSR domain, the targeting of S site locations also depends on the Msh2 mismatch repair protein because Msh2-deficient mice show an increased focus of sites to the Sμ tandem repeat region. We propose that Msh2 affects S site location because sequences with few activation-induced cytidine deaminase targets generate mostly switch DNA cleavages that require Msh2-directed processing to allow CSR joining

The Sμ Tandem Repeat Region Is Critical for Ig Isotype Switching in the Absence of Msh2

AbstractDeficiencies of the Msh2 protein or the Sμ tandem repeat (SμTR) sequences each reduce isotype switching in mice by about 2- to 3-fold. We find that switching in mice deficient for both Msh2 and SμTR is nearly ablated. We propose that the SμTR provides closely spaced cleavage sites that can undergo switch recombination independent of Msh2, whereas cleavages in sequences flanking the SμTR require Msh2 processing to allow recombinational joining. We also find that changes in Sμ sequences alter the focus of switch junctions within Sγ sequences, indicating that sequences of switch regions act together in the choice of switch recombination junctions. These findings help to explain the conservation of tandemly repeated switch regions associated with heavy chain constant genes in species capable of switching

Identification of strontium in the merger of two neutron stars.

Half of all of the elements in the Universe that are heavier than iron were created by rapid neutron capture. The theory underlying this astrophysical r-process was worked out six decades ago, and requires an enormous neutron flux to make the bulk of the elements1. Where this happens is still debated2. A key piece of evidence would be the discovery of freshly synthesized r-process elements in an astrophysical site. Existing models3-5 and circumstantial evidence6 point to neutron-star mergers as a probable r-process site; the optical/infrared transient known as a 'kilonova' that emerges in the days after a merger is a likely place to detect the spectral signatures of newly created neutron-capture elements7-9. The kilonova AT2017gfo-which was found following the discovery of the neutron-star merger GW170817 by gravitational-wave detectors10-was the first kilonova for which detailed spectra were recorded. When these spectra were first reported11,12, it was argued that they were broadly consistent with an outflow of radioactive heavy elements; however, there was no robust identification of any one element. Here we report the identification of the neutron-capture element strontium in a reanalysis of these spectra. The detection of a neutron-capture element associated with the collision of two extreme-density stars establishes the origin of r-process elements in neutron-star mergers, and shows that neutron stars are made of neutron-rich matter13

Gene conversion in human rearranged immunoglobulin genes

Over the past 20 years, many DNA sequences have been published suggesting that all or part of the V<sub>H</sub> segment of a rearranged immunoglobulin gene may be replaced in vivo. Two different mechanisms appear to be operating. One of these is very similar to primary V(D)J recombination, involving the RAG proteins acting upon recombination signal sequences, and this has recently been proven to occur. Other sequences, many of which show partial V<sub>H</sub> replacements with no addition of untemplated nucleotides at the V<sub>H</sub>–V<sub>H</sub> joint, have been proposed to occur by an unusual RAG-mediated recombination with the formation of hybrid (coding-to-signal) joints. These appear to occur in cells already undergoing somatic hypermutation in which, some authors are convinced, RAG genes are silenced. We recently proposed that the latter type of V<sub>H</sub> replacement might occur by homologous recombination initiated by the activity of AID (activation-induced cytidine deaminase), which is essential for somatic hypermutation and gene conversion. The latter has been observed in other species, but not in human Ig genes, so far. In this paper, we present a new analysis of sequences published as examples of the second type of rearrangement. This not only shows that AID recognition motifs occur in recombination regions but also that some sequences show replacement of central sections by a sequence from another gene, similar to gene conversion in the immunoglobulin genes of other species. These observations support the proposal that this type of rearrangement is likely to be AID-mediated rather than RAG-mediated and is consistent with gene conversion

X-shooter Spectroscopy and HST Imaging of 15 Massive Quiescent Galaxies at z ≳ 2

We present a detailed analysis of a large sample of spectroscopically confirmed massive quiescent galaxies (MQGs; log(M*/M ⊙) ~ 11.5) at z ≳ 2. This sample comprises 15 galaxies selected in the COSMOS and UDS fields by their bright K-band magnitudes and followed up with Very Large Telescope (VLT) X-shooter spectroscopy and Hubble Space Telescope (HST)/WFC3 H_(F160W) imaging. These observations allow us to unambiguously confirm their redshifts, ascertain their quiescent nature and stellar ages, and reliably assess their internal kinematics and effective radii. We find that these galaxies are compact, consistent with the high-mass end of the stellar mass–size relation for quiescent galaxies at z = 2. Moreover, the distribution of the measured stellar velocity dispersions of the sample is consistent with the most massive local early-type galaxies from the MASSIVE Survey, showing that evolution in these galaxies is dominated by changes in size. The HST images reveal, as surprisingly high, that 40% of the sample has tidal features suggestive of mergers and companions in close proximity, including three galaxies experiencing ongoing major mergers. The absence of velocity dispersion evolution from z = 2 to 0, coupled with a doubling of the stellar mass, with a factor of 4 size increase and the observed disturbed stellar morphologies, supports dry minor mergers as the primary drivers of the evolution of the MQGs over the last 10 billion yr

New constraints on the physical conditions in H2-bearing GRB-host damped Lyman-alpha absorbers

International audienceWe report the detections of molecular hydrogen (H 2), vibrationally-excited H 2 (H * 2), and neutral atomic carbon (C i), an efficient tracer of molecular gas, in two new afterglow spectra of GRBs 181020A (z = 2.938) and 190114A (z = 3.376), observed with X-shooter at the Very Large Telescope (VLT). Both host-galaxy absorption systems are characterized by strong damped Lyman-α absorbers (DLAs) and substantial amounts of molecular hydrogen with log N(H i, H 2) = 22.20 ± 0.05, 20.40 ± 0.04 (GRB 181020A) and log N(H i, H 2) = 22.15±0.05, 19.44±0.04 (GRB 190114A). The DLA metallicites, depletion levels, and dust extinctions are within the typical regimes probed by GRBs with [Zn/H] = −1.57 ± 0.06, [Zn/Fe] = 0.67 ± 0.03, and A V = 0.27 ± 0.02 mag (GRB 181020A) and [Zn/H] = −1.23 ± 0.07, [Zn/Fe] = 1.06 ± 0.08, and A V = 0.36 ± 0.02 mag (GRB 190114A). In addition, we examine the molecular gas content of all known H 2-bearing GRB-DLAs and explore the physical conditions and characteristics required to simultaneously probe C i and H * 2. We confirm that H 2 is detected in all C i-and H * 2-bearing GRB absorption systems, but that these rarer features are not necessarily detected in all GRB H 2 absorbers. We find that a large molecular fraction of f H 2 10 −3 is required for C i to be detected. The defining characteristic for H * 2 to be present is less clear, though a large H 2 column density is an essential factor. We also find that the observed line profiles of the molecular-gas tracers are kinematically "cold", with small velocity offsets of δv < 20 km s −1 from the bulk of the neutral absorbing gas. We then derive the H 2 excitation temperatures of the molecular gas and find that they are relatively low with T ex ≈ 100−300 K, however, there could be evidence of warmer components populating the high-J H 2 levels in GRBs 181020A and 190114A. Finally, we demonstrate that even though the X-shooter GRB afterglow campaign has been successful in recovering several H 2-bearing GRB-host absorbers, this sample is still hampered by a significant dust bias excluding the most dust-obscured H 2 absorbers from identification. C i and H * 2 could open a potential route to identify molecular gas even in low-metallicity or highly dust-obscured bursts, though they are only efficient tracers for the most H 2-rich GRB-host absorption systems