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.     

Comment on "Transitions to asexuality result in excess amino acid substitutions"

Paland and Lynch (Reports, 17 February 2006, p. 990) showed that in Daphnia pulex, the ratio of amino acid replacement to silent substitution in mitochondrial genes is higher in asexual lineages than in sexual lineages. If this base-composition bias is maintained by selection, it too should alter following transitions in reproductive mode. Analysis reveals no such change in the genomes of D. pulex.     

Thyroxine: effects on amino acid incorporation into protein in vivo

Treatment of rats with L-thyroxine increases the incorporation in vivo of radioactive amino acids into protein of liver, kidney, and heart, but not of spleen, testis, or brain. The distribution of the effect among the organs is the same as that observed in thyroxine stimulation of oxidative metabolism. Thus, stimulation of protein synthesis seems to be a physiological action of the thyroid hormone.     

Hemoglobins in sheep: multiple differences in amino acid sequences of three beta-chains and possible origins

Among the three adult sheep hemoglobins (A, B, and C), two (A and B) are reportedly products of alleles. The beta-chains of A and B differ by at least seven scattered amino acid residues whereas the beta-sequence of C differs from A by at least 16 residues and from B by at least 21 residues. These changes suggest that the origin of C-beta antedated the divergence of A and B. Five shared differences between A-beta and C-beta with respect to B-beta can be interpreted as the result of selective advantage in favor of B. A complex of additional mechanisms has possibly been involved in maintaining the A-B- C porymorphism.     

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.     

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。     

Conversion of a PI-anchored protein to an integral membrane protein by a single amino acid mutation

Qa-2, a cell-surface glycoprotein anchored by phosphatidylinositol (PI), is structurally related to the class I transplantation antigens H-2 K, D, and L, which are integral membrane glycoproteins. The predicted transmembrane segment of Qa-2 differs from those of H-2 K, D, and L by the presence of an aspartate in place of a valine at position 295. A single base change that replaced this aspartate with valine resulted in cell-surface Qa-2 molecules that were insensitive to hydrolysis by a PI-specific phospholipase C and more resistant to papain cleavage, properties shared by H-2D. Cells expressing Asp----Val mutant Qa-2 proteins were still able to attach a PI anchor to endogenous proteins such as Thy-1 and J11D. It therefore appears that this single amino acid change converts Qa-2 from a PI-linked form into an integral membrane protein.     

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.     

Probing protein electrostatics with a synthetic fluorescent amino acid

Electrostatics affect virtually all aspects of protein structure and activity and are particularly important in proteins whose primary function is to stabilize charge. Here we introduce a fluorescent amino acid, Aladan, which can probe the electrostatic character of a protein at multiple sites. Aladan is exceptionally sensitive to the polarity of its surroundings and can be incorporated site-selectively at buried and exposed sites, in both soluble and membrane proteins. Steady-state and time-resolved fluorescence measurements of Aladan residues at different buried and exposed sites in the B1 domain of protein G suggest that its interior is polar and heterogeneous.     

Effects of thyroxin on amino acid incorporation into protein

The effect of thyroxin on the in vitro incorporation of dl-leucine-l-C(14) into the protein of rat liver homogenates has been investigated. Both thyroxin pretreatment in vivo and thyroxin in vitro at a concentration of 1 x 10(-5) M were found to increase the rate of amino acid incorporation. The increased activity following the thyroxin pretreatment in vivo was found to be localized in the mitochondrial fraction. It is suggested that the acceleration of metabolic rate characteristic of thyroxin action may be secondary to the stimulation of energy-requiring reactions such as protein synthesis.     

Amino acid sequences common to rapidly degraded proteins: the PEST hypothesis

The amino acid sequences of ten proteins with intracellular half-lives less than 2 hours contain one or more regions rich in proline (P), glutamic acid (E), serine (S), and threonine (T). These PEST regions are generally, but not always, flanked by clusters containing several positively charged amino acids. Similar inspection of 35 proteins with intracellular half-lives between 20 and 220 hours revealed that only three contain a PEST region. On the basis of this information, it was anticipated that caseins, which contain several PEST sequences, would be rapidly degraded within eukaryotic cells. This expectation was confirmed by red blood cell-mediated microinjection of 125I-labeled caseins into HeLa cells where they exhibited half-lives of less than 2 hours. The rapid degradation of injected alpha- and beta-casein as well as the inverse correlation of PEST regions with intracellular stability indicate that the presence of these regions can result in the rapid intracellular degradation of the proteins containing them.     

Estimation of the half-life of a secretory protein message

Synthesis of a specific secretory protein was followed in the presence of actinomycin D and cordycepin (3'-deoxyadenosine). The apparent half-life of the messenger RNA for this protein is significantly longer in the presence of actinomycin D than in the presence of cordycepin. These results suggest that actinomycin D studies of half-life in specialized cells may be inaccurate.     

玉米胚芽分离蛋白提取率及氨基酸组分分析

利用水、氯化钠、乙醇、氢氧化钠,分别提取玉米胚芽中的水溶蛋白、盐溶蛋白、醇溶蛋白、碱溶蛋白,确定各种分离蛋白的最佳溶剂条件.样品过80目筛后,脱脂处理,取5 g脱脂样品进行试验.结果表明,水溶蛋白的最佳溶剂量为水80 mL,提取率为37.13％;盐溶蛋白的最佳溶剂浓度为质量分数为6％的氯化钠溶液,提取率为25.34％;...     

Effects of 3-methylcholanthrene and phenobarbital on amino acid incorporation into protein

Treatment of rats with methylcholanthrene and phenobarbital, two compounds known to enhance microsomal enzyme activity, stimulates the incorporation of free or soluble ribonucleic acid bound amino acid into proteins of cell-free liver preparations. Mitochondria, microsomes, and cell sap all contribute to the methylcholanthrene effect on the incorporation of free amino acid.     

不同蛋白质水平的氨基酸平衡日粮对泌乳母猪生产性能、血清指标和乳汁氨基酸含量的影响

本试验旨在探讨理想氨基酸模式下,不同粗蛋白质水平的日粮对泌乳母猪生产性能、血清指标和乳汁氨基酸含量的影响.选用3-6胎次、体况相似、预产期相近的60头长大二元杂交母猪,随机分成4个组,每组15头母猪,分别饲喂粗蛋白质18%(对照组)和粗蛋白质17%(试验1组)、16%(试验2组)、15%(试验3组)低蛋白氨基酸平衡的日粮,试验期从妊娠110天到哺乳期21天结束.结果表明:日粮蛋白质水平对母猪泌乳期日均采食量、体重损失、断奶后发情间隔、仔猪哺乳期窝增重和平均日增重的影响不显著(P>0.05);母猪血清和乳汁游离氨基酸含量处理组间差异均不显著(P>0.05);15%蛋白组和16%蛋白组血清尿素氮含量显著低于18%蛋白组(P<0.05).