Generation of cDNA probes directed by amino acid sequence: cloning of urate oxidase

Urate oxidase (E.C. 1.7.3.3) catalyzes the oxidation of uric acid to allantoin in most mammals except humans and certain primates. The amino-terminal amino acid sequence for porcine urate oxidase was determined and used in a novel procedure for generating complementary DNA (cDNA) probes to this amino acid sequence. The procedure is based on the polymerase chain reaction and utilizes mixed oligonucleotide primers complementary to the reverse translation products of an amino acid sequence. This rapid and simple cDNA cloning procedure is generally applicable and requires only a partial amino acid sequence. A cDNA probe developed by this procedure was used to isolate a full-length porcine urate oxidase cDNA and to demonstrate the presence of homologous genomic sequences in humans.     

Composition and structure of the martian atmosphere: preliminary results from viking 1

Results from the aeroshell-mounted neutral mass spectrometer on Viking I indicate that the upper atmosphere of Mars is composed mainly of CO(2) with trace quantities of N(2), Ar, O, O(2), and CO. The mixing ratios by volume relative to CO(2) for N(2), Ar, and O(2) are about 0.06, 0.015, and 0.003, respectively, at an altitude near 135 kilometers. Molecular oxygen (O(2)(+)) is a major component of the ionosphere according to results from the retarding potential analyzer. The atmosphere between 140 and 200 kilometers has an average temperature of about 180 degrees +/- 20 degrees K. Atmospheric pressure at the landing site for Viking 1 was 7.3 millibars at an air temperature of 241 degrees K. The descent data are consistent with the view that CO(2) should be the major constituent of the lower martian atmosphere.     

Replacements of Pro86 in phage T4 lysozyme extend an alpha-helix but do not alter protein stability

To investigate the relation between protein stability and the predicted stabilities of individual secondary structural elements, residue Pro86 in an alpha-helix in phage T4 lysozyme was replaced by ten different amino acids. The x-ray crystal structures of seven of the mutant lysozymes were determined at high resolution. In each case, replacement of the proline resulted in the formation of an extended alpha-helix. This involves a large conformational change in residues 81 to 83 and smaller shifts that extend 20 angstroms across the protein surface. Unexpectedly, all ten amino acid substitutions marginally reduce protein thermostability. This insensitivity of stability to the amino acid at position 86 is not simply explained by statistical and thermodynamic criteria for helical propensity. The observed conformational changes illustrate a general mechanism by which proteins can tolerate mutations.     

持续、低剂量金霉素对肉仔鸡免疫机能的抑制作用研究

270只Abore Acre商品代肉用公鸡随机分成3组，金霉素在饲料中添加水平分别为0、50只和150mg/kg，研究饲用金霉素对肉仔鸡法氏囊、脾脏和胸腺等免疫器官发育、以及免疫反应的影响。结果表明50mg/kg的金霉素对肉仔鸡脾脏和21日龄的胸腺无明显抑制作用，对T、B淋巴细胞转化率也无明显抑制作用（P＜0.05），而且显著促进法氏囊的萎缩（P＜0.05）。金霉素对T淋巴细胞转化率有明显的直接抑制作用；50mg/kg和150mg/kg的金素对BAS特异性抗体的产生均有显著的抑制作用（P＜0.01）。研究结果表明，持续、低剂量和金霉素对肉 鸡免疫器官发育和免疫应答具有显著的抑制作用，抑制作用随剂量增加而加强，150mg/kg的金霉素对肉仔鸡具有显著促生长作用。     

Triton's Geyser-Like Plumes: Discovery and Basic Characterization

At least four active geyser-like eruptions were discovered in Voyager 2 images of Triton, Neptune's large satellite. The two best documented eruptions occur as columns of dark material rising to an altitude of about 8 kilometers where dark clouds of material are left suspended to drift downwind over 100 kilometers. The radii of the rising columns appear to be in the range of several tens of meters to a kilometer. One model for the mechanism to drive the plumes involves heating of nitrogen ice in a subsurface greenhouse environment; nitrogen gas pressurized by the solar heating explosively vents to the surface carrying clouds of ice and dark partides into the atmosphere. A temperature increase of less than 4 kelvins above the ambient surface value of 38 +/- 3 kelvins is more than adequate to drive the plumes to an 8-kilometer altitude. The mass flux in the trailing clouds is estimated to consist of up to 10 kilograms of fine dark particles per second or twice as much nitrogen ice and perhaps several hundred or more kilograms of nitrogen gas per second. Each eruption may last a year or more, during which on the order of a tenth of a cubic kilometer of ice is sublimed.     

Quantum-confined stark effect in single CdSe nanocrystallite quantum dots

The quantum-confined Stark effect in single cadmium selenide (CdSe) nanocrystallite quantum dots was studied. The electric field dependence of the single-dot spectrum is characterized by a highly polarizable excited state ( approximately 10(5) cubic angstroms, compared to typical molecular values of order 10 to 100 cubic angstroms), in the presence of randomly oriented local electric fields that change over time. These local fields result in spontaneous spectral diffusion and contribute to ensemble inhomogeneous broadening. Stark shifts of the lowest excited state more than two orders of magnitude larger than the linewidth were observed, suggesting the potential use of these dots in electro-optic modulation devices.