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<jats:p> Laboratory simulations of shocks created with a high-energy laser demonstrate that the efficacy of organic production depends on the molecular, not just the elemental composition of the shocked gas. In a methane-rich mixture that simulates a low-temperature equilibrium mixture of cometary material, hydrogen cyanide and acetylene were produced with yields of 5 × 10 <jats:sup>17</jats:sup> molecules per joule. Repeated shocking of the methane-rich mixture produced amine groups, suggesting the possible synthesis of amino acids. No organic molecules were produced in a carbon dioxide–rich mixture, which is at odds with thermodynamic equilibrium approaches to shock chemistry and has implications for the modeling of shock-produced organic molecules on early Earth. </jats:p>

<jats:p> Piezoelectric materials have many applications at cryogenic temperatures. However, the piezoelectric response below 10 kelvin is diminished, making the use of these materials somewhat marginal. Results are presented on strontium titanate (SrTiO <jats:sub>3</jats:sub> ), which exhibits a rapidly increasing piezoelectric response with decreasing temperature below 50 kelvin; the magnitude of its response around 1 kelvin is comparable to that of the best materials at room temperature. This “giant” piezoelectric response may open the way for a broad class of applications including use in ultralow-temperature scanning microscopies and in a magnetic field–insensitive thermometer. These observations, and the possible divergence of the mechanical response to electric fields at even lower temperatures, may arise from an apparent quantum critical point at absolute zero. </jats:p>

<jats:p>A numerical model shows that in Antarctic sea ice, increased flooding in regions with thick snow cover enhances primary production in the infiltration (surface) layer. Productivity in the freeboard (sea level) layer is also determined by sea ice porosity, which varies with temperature. Spatial and temporal variation in snow thickness and the proportion of first-year ice thus determine regional differences in sea ice primary production. Model results show that of the 40 teragrams of carbon produced annually in the Antarctic ice pack, 75 percent was associated with first-year ice and nearly 50 percent was produced in the Weddell Sea.</jats:p>

<jats:p>The kinetics of a first-order, solid-solid phase transition were investigated in the prototypical nanocrystal system CdSe as a function of crystallite size. In contrast to extended solids, nanocrystals convert from one structure to another by single nucleation events, and the transformations obey simple unimolecular kinetics. Barrier heights were observed to increase with increasing nanocrystal size, although they also depend on the nature of the nanocrystal surface. These results are analogous to magnetic phase transitions in nanocrystals and suggest general rules that may be of use in the discovery of new metastable phases.</jats:p>

<jats:p>Fossils from a large-bodied hominoid from early Miocene sediments of Uganda, along with material recovered in the 1960s, show features of the shoulder and vertebral column that are significantly similar to those of living apes and humans. The large-bodied hominoid from Uganda dates to at least 20.6 million years ago and thus represents the oldest known hominoid sharing these derived characters with living apes and humans.</jats:p>

<jats:p> Multiple endocrine neoplasia–type 1 (MEN1) is an autosomal dominant familial cancer syndrome characterized by tumors in parathyroids, enteropancreatic endocrine tissues, and the anterior pituitary. DNA sequencing from a previously identified minimal interval on chromosome 11q13 identified several candidate genes, one of which contained 12 different frameshift, nonsense, missense, and in-frame deletion mutations in 14 probands from 15 families. The <jats:italic>MEN1</jats:italic> gene contains 10 exons and encodes a ubiquitously expressed 2.8-kilobase transcript. The predicted 610–amino acid protein product, termed menin, exhibits no apparent similarities to any previously known proteins. The identification of <jats:italic>MEN1</jats:italic> will enable improved understanding of the mechanism of endocrine tumorigenesis and should facilitate early diagnosis. </jats:p>

<jats:p>Membrane-bound immunoglobulin (mIg) of the IgG, IgA, and IgE classes have conserved cytoplasmic tails. To investigate the function of these tails, a B cell line was transfected with truncated or mutated γ2a heavy chains. Transport to the endosomal compartment of antigen bound by the B cell antigen receptor did not occur in the absence of the cytoplasmic tail; and one or two mutations, respectively, in the Tyr-X-X-Met motif of the tail partially or completely interrupted the process. Experiments with chimeric antigen receptors confirmed these findings. Thus, a role for the cytoplasmic tail of mIg heavy chains in endosomal targeting of antigen is revealed.</jats:p>

<jats:p>B cells use immunoglobulin M (IgM) and IgD as antigen receptors, but after contact with antigen they can switch and use IgG, IgA, or IgE. In mice lacking the transmembrane and cytoplasmic domains of IgE, serum IgE is reduced by more than 95 percent and, after immunization, specific responses are negligible. In mice lacking most of the cytoplasmic tail of IgE, serum IgE levels are reduced by 50 percent and specific responses are reduced by 40 to 80 percent, without a clear secondary response. Thus, membrane expression is indispensable for IgE secretion in vivo, and the cytoplasmic tail influences the degree and quality of the response.</jats:p>

<jats:p>In antibody responses, B cells switch from the expression of immunoglobulin (Ig) μ and δ heavy (H) chains to that of other Ig classes (α, γ, or ɛ), each with a distinct effector function. Membrane-bound forms of α, γ, and ɛ, but not μ and δ, have highly conserved cytoplasmic tails. Mutant mice unable to express membrane γ1 H chains or producing tailless γ1 H chains failed to generate efficient IgG1 responses and IgG1 memory. H chain membrane expression after class switching is thus required for these functions, and class switching equips the B cell antigen receptor with a regulatory cytoplasmic tail that naı̈ve B cells lack.</jats:p>

<jats:p> The three-dimensional structure of the enzyme 3-oxo-Δ <jats:sup>5</jats:sup> -steroid isomerase (E.C. 5.3.3.1), a 28-kilodalton symmetrical dimer, was solved by multidimensional heteronuclear magnetic resonance spectroscopy. The two independently folded monomers pack together by means of extensive hydrophobic and electrostatic interactions. Each monomer comprises three α helices and a six-strand mixed β-pleated sheet arranged to form a deep hydrophobic cavity. Catalytically important residues Tyr <jats:sup>14</jats:sup> (general acid) and Asp <jats:sup>38</jats:sup> (general base) are located near the bottom of the cavity and positioned as expected from mechanistic hypotheses. An unexpected acid group (Asp <jats:sup>99</jats:sup> ) is also located in the active site adjacent to Tyr <jats:sup>14</jats:sup> , and kinetic and binding studies of the Asp <jats:sup>99</jats:sup> to Ala mutant demonstrate that Asp <jats:sup>99</jats:sup> contributes to catalysis by stabilizing the intermediate. </jats:p>