Toward a systems approach to understanding plant cell walls

One of the defining features of plants is a body plan based on the physical properties of cell walls. Structural analyses of the polysaccharide components, combined with high-resolution imaging, have provided the basis for much of the current understanding of cell walls. The application of genetic methods has begun to provide new insights into how walls are made, how they are controlled, and how they function. However, progress in integrating biophysical, developmental, and genetic information into a useful model will require a system-based approach.     

Biosynthesis of streptococcal cell walls: A rhamnose polysaccharide

Protoplast membranes from Streptococcus pyogenes incorporated rhamnose into a preexisting polysaccharide when incubated with thymidine diphosphate rhamnose-C(14). This polysaccharide, when extracted from the membranes, did not give a precipitin reaction with group A antisera, but could be coprecipitated with added group A polysaccharide by acetone. It is presumed to be a precursor to group specific polysaccharide of the streptococcal cell wall.     

Nitrogen fixation by floating diatom mats: a source of new nitrogen to oligotrophic ocean waters

Nitrogen fixation, apparently by bacterial endosymbionts, is associated with intertwining chains of two species of the diatom Rhizosolenia. In situ fixation rates were enhanced by incubation in the dark, whereas concurrent shipboard experiments either underestimated or did not detect nitrogen fixation. This is the first example of nitrogen fixation associated with a bacteria-diatom symbiosis in the pelagic zone, and it indicates that these systems may contribute a significant amount of "new" nitrogen to oligotrophic waters.     

一种新的氨基酸序列进化距离及其应用

在选择的思想下,建立了蛋白质氨基酸序列的进化动力学方程,提出了一个新的蛋白质氨基酸序列的进化距离———选择进化距离,该距离统一了前人提出的各种氨基酸序列的进化距离;将该动力学方程转换为线性模型后,利用最小二乘法可实现模型中参数b的估计;最后,通过17个物种的细胞色素b的氨基酸序列说明了该选择进化距离的计算方法,并根据自展法比较了不同进化距离得到的物种进化树。结果表明,利用选择进化距离构建的进化树与其他几种进化距离得到的进化树的拓扑结构一致。选择进化距离为估计蛋白质氨基酸序列的进化距离提供了新方法。     

Immunoglobulin structure: partial amino acid sequence of a Bence Jones protein

Sequence analysis and ordering of the soluble tryptic peptides of one Bence Jones protein and comparison with partial sequence data for another have revealed many structural differences in the half of the molecule with the terminal amino group, but only one structural difference in the half of the molecule having the terminal carboxyl group. Somatic chromosomal rearrangements may effect such changes and account for variability in antibody structure.     

Hexagonal pattern in cell walls of Escherichia coli B

Cell walls, isolated from Escherichia coli B, as examined by electron microscopy and optical diffraction contain a hexagonal lattice structure, the (1,0) planes of which are separated by 140 +/- 8 angstroms. Unless the walls are briefly heated (10 minutes, 90 degrees C) early in the isolation, the hexagonal array cannot always be observed. Enzymatic digestion with pancreatin and amylase improves visualization of the lattice; subsequent treatment with pepsin and sodium dodecylsulfate removes the hexagonal pattern. Protein or lipoprotein globular units within the wall may thus be arranged in a hexagonal array uponthe mucopeptide layer.     

Selection of amino acid sequences in the beta chain of the T cell antigen receptor

The induction of an immune response in mammals is initiated by specifically reactive T lymphocytes. The specificity of the reaction is mediated by a complex receptor, part of which is highly variable in sequence and analogous to immunoglobulin heavy- and light-chain variable domains. The functional specificity of the T cell antigen receptor is, however, markedly different from immunoglobulins in that it mediates cell-cell interactions via the simultaneous recognition of foreign antigens and major histocompatibility complex-encoded molecules expressed on the surface of various lymphoid and nonlymphoid cells. The relation between the structure of the receptor and its functional specificity was investigated by analyzing the primary sequences of the receptors expressed by a series of T lymphocyte clones specific for a model antigen, pigeon cytochrome c. Within this set of T lymphocyte clones there was a striking selection for amino acid sequences in the receptor beta-chain in the region analogous to the third complementarity-determining region of immunoglobulins. Thus, despite the functional differences between T cell antigen receptors and immunoglobulin molecules, analogous regions appear to be important in determining ligand specificity.     

Synthesis of a sulfur-containing amino acid under simulated prebiotic conditions

Ultraviolet irradiation of an aqueous solution of ammonium thiocyanate produces the sulfur-containing amino acid methionine. Synthesis of this class of biocompound fills another important gap in development of an overall picture of how prebiological chemistry may have evolved on primitive Earth.     

Hydrophobicity of amino acid residues in globular proteins

During biosynthesis, a globular protein folds into a tight particle with an interior core that is shielded from the surrounding solvent. The hydrophobic effect is thought to play a key role in mediating this process: nonpolar residues expelled from water engender a molecular interior where they can be buried. Paradoxically, results of earlier quantitative analyses have suggested that the tendency for nonpolar residues to be buried within proteins is weak. However, such analyses merely classify residues as either "exposed" or "buried." In the experiment reported in this article proteins of known structure were used to measure the average area that each residue buries upon folding. This characteristic quantity, the average area buried, is correlated with residue hydrophobicity.     

Circadian rhythmicity in amino acid incorporation in Euglena gracilis

A daily rhythm of amino acid incorporation exists in nondividing cultures of Euglena kept in a light-dark cycle. This rhythmicity persists for the two cycles measured in cultures transferred to constant conditions and is therefore probably circadian in nature.     

HTLV-V: a new human retrovirus isolated in a Tac-negative T cell lymphoma/leukemia

A new human retrovirus was isolated from a continuous cell line derived from a patient with CD4+ Tac- cutaneous T cell lymphoma/leukemia. This virus is related to but distinct from human T cell leukemia/lymphoma virus types I and II (HTLV-I and HTLV-II) and human immunodeficiency virus (HIV-1). With the use of a fragment of provirus cloned from one patient with T cell leukemia, closely related sequences were found in DNA of the cell line and of tumor cells from seven other patients with the same disease; these sequences were only distantly related to HTLV-I. The phenotype of the cells and the clinical course of the disease were clearly distinguishable from leukemia associated with HTLV-I. All patients and the wife of one patient showed a weak serological cross-reactivity with both HTLV-I and HIV-1 antigens. None of the patients proved to be at any apparent risk for HIV-1 infection. The name proposed for this virus is HTLV-V, and the date indicate that it may be a primary etiological factor in the major group of cutaneous T cell lymphomas/leukemias, including the sporadic lymphomas known as mycoses fungoides.     

Adenylyl cyclase amino acid sequence: possible channel- or transporter-like structure

Complementary DNA's that encode an adenylyl cyclase were isolated from a bovine brain library. Most of the deduced amino acid sequence of 1134 residues is divisible into two alternating sets of hydrophobic and hydrophilic domains. Each of the two large hydrophobic domains appears to contain six transmembrane spans. Each of the two large hydrophilic domains contains a sequence that is homologous to a single cytoplasmic domain of several guanylyl cyclases; these sequences may represent nucleotide binding sites. An unexpected topographical resemblance between adenylyl cyclase and various plasma membrane channels and transporters was observed. This structural complexity suggests possible, unappreciated functions for this important enzyme.     

The interleukin-2 T-cell system: a new cell growth model

Synchronized interleukin-2 receptor-positive T cells, homogeneous immunoaffinity-purified interleukin-2, and a monoclonal antibody to interleukin-2 receptors were used to show that only three factors are critical for T-cell cycle progression: interleukin-2 concentration, interleukin-2 receptor density, and the duration of the interleukin-2 receptor interaction. Since the proliferative characteristics of T cells are identical to those of both prokaryotic and all other eukaryotic cells, these findings provide a new model that accounts fully for the variables that determine cell cycle progression.     

Light chains of mouse myeloma proteins: partial amino acid sequence

Five kappa chains in the urinary proteins of the BALB/c mouse have the same carboxyl terminal amino acid sequence; this sequence resembles that of kappa light chains in human immunoglobulins. The five chains have amino acid sequence variations at the amino- terminal. The genetic basis for the amino- terminal variations is not understood but could be due either to a mecha nism for differently translating a single genetic message or to the presence of more than one kappa- type structural cistron in the BALB/c genome.     

甘氨酸和鱼蛋白在网纹甜瓜上应用效果比较

[目的]为了提高网纹甜瓜的果实品质,筛选适宜的氨基酸类生物刺激素.[方法]测定网纹甜瓜果实膨大速率、果实产量、口感品质以及果肉养分含量等指标,运用方差分析,比较甘氨酸和鱼蛋白两种氨基酸类生物刺激素处理对品质的影响,进行综合评价.[结果]结果表明,鱼蛋白处理心糖较对照提高13.9％,边糖较对照提高19.9％.不同类型氨基酸含量也高于其他处理,品质方面占明显优势.[结论]授粉后每10d喷施浓度为1 g/L的鱼蛋白,对网纹甜瓜提质效果较好,可在网纹甜瓜生产过程中推广应用.     

猪乳中氨基酸含量的动态分析

采集杜洛克母猪分娩后1、3、5、11、15、20 d的乳汁,通过高效液相法分析氨基酸的含量变化.结果表明,随着泌乳时间的延长,猪乳中17种氨基酸的含量均呈下降趋势,各种氨基酸之间谷氨酸含量最高,而胱氨酸和蛋氨酸的含量最低.     

Transitions to asexuality result in excess amino acid substitutions

Theory predicts that linkage between genetic loci reduces the efficiency of purifying selection. Because of the permanent linkage of all heritable genetic material, asexual lineages may be exceptionally prone to deleterious-mutation accumulation in both nuclear and organelle genes. Here, we show that the ratio of the rate of amino acid to silent substitution (Ka/Ks) in mitochondrial protein-coding genes is higher in obligately asexual lineages than in sexual lineages of the microcrustacean Daphnia pulex. Using a phylogeny-based approach to quantify the frequency of mutational-effect classes, we estimate that mitochondrial protein-coding genes in asexual lineages accumulate deleterious amino acid substitutions at four times the rate in sexual lineages. These results support the hypothesis that sexual reproduction plays a prominent role in reducing the mutational burden in populations.     

Ground water as a silica source for diatom production in a precipitation-dominated lake

The short-term, seasonal input of ground water to a small, precipitation-dominated oligotrophic lake in northern Wisconsin amounts to less than 10 percent of the annual water budget of the lake but accounts for nearly all the external silica loading. Silica is a necessary nutrient for diatoms. A large spring diatom bloom occurs coincident with high silica inputs from ground water when other possible silica sources are low. The mass budgets of ground water and silica in the lake system demonstrate the importance of ground-water solute inputs to the lake.     

Human platelet-derived growth factor (PDGF): amino-terminal amino acid sequence

Human platelet-derived growth factor (PDGF) obtained from outdated human platelets was subjected to amino-terminal amino acid sequence analysis by automated Edman degradation. Despite the apparent presence of limited proteolytic degradation of the protein derived from this method, the sequence analysis reveals two primary peptide sequences and suggests that active PDGF is composed of two, possibly homologous, peptides linked by a disulfide bond or bonds.     

Artificial membranes: a new type of cell for measuring diffusional resistance

A novel technique allows determination of membrane diffusivities and eliminates the complications of the fluid resistance to mass transfer on either side of the membrane. Results on the permeability of a dialysis membrane of the type used in artificial kidneys agree with previous data and are obtained in much shorter time. The measured activation energy for diffusion demonstrates that the transport of salt through the membrane occurs by the same mechanism as the diffusion of salt in water, but that only 10 percent of the membrane surface is effective.