Distinctive properties of mammalian DNA polymerases.

DNA polymerase-alpha and -beta can be distinguished from one another by the differential effects of N-ethylmaleimide, KCl, ara-CTP and temperature, as well as on the basis of sedimentation. The sensitivity of DNA polymerase-beta to elevated temperatures as compared to DNA polymerase-alpha provides a new means of distinguishing between these two enzymes even in crude extracts and a possible probe for determining their function. DNA polymerase-alpha and -beta share several properties in common, including the ability to readily incorporate dUTP in place of dTTP. The Km for dUTP varies from 10 to 30 micron with different preparations of DNA polymerase-alpha and -beta. Thus, in mammalian cells, dUMP could be incorporated into DNA, and if excised by an endonuclease, would lead to discontinuities. Initial analyses of fidelity in direct comparative studies indicate that beta-class DNA polymerases are highly accurate in base selection when copying poly[d(A-T)]. Less than one molecule of dGMP is incorporated for every 12 000-45 000 molecules of dAMP and dTMP polymerized. DNA polymerase-alpha is somewhat less accurate, making one mistake for every 4000-10 000 correct nucleotides incorporated. Since both polymerases lack an exonucleolytic activity, this accuracy must be the result of selectivity for the complementary nucleotide by the polymerase.     

Distinctive features of dietary phosphate supply.

Dietary phosphate has profound effects on growth and renal handling of the compound. On the basis of changes in growth rate and food intake, after alterations in phosphate load, our laboratory previously suggested that these effects are mediated by intestinal signals (Landsman A, Lichtstein D, Bacaner M, and Ilani A. Br J Nutr 86: 217-223, 2001). The aim of this study was to further evaluate the role of dietary phosphate on food intake and appetite and specific organ growth, and to test for the presence of a serum factor that may affect renal phosphate handling in phosphate-resupplied rats. The experimental design was based on a comparison between groups of rats receiving identical low-phosphate diets but drinking water containing either phosphate or chloride. We show that 1) changes in food intake after alterations in phosphate load occurred in parallel with variations in digestive system distention, suggesting that dietary phosphate has also a direct effect on appetite; 2) dietary phosphate-dependent growth has a specific effect on the growth of liver and epididymal fat; and 3) serum of rats supplied with phosphate contains a factor that inhibits sodium-dependent phosphate transport in a model of renal proximal tubule cells. Collectively, these observations are in accord with the hypothesis that factor(s) emanating from the digestive system in response to dietary phosphate load may be involved in growth, appetite and renal handling of phosphate.     

Distinctive properties of the hepatitis B virus envelope proteins.

Using recombinant adenoviral vectors, we expressed and characterized the large, middle, and major envelope proteins of hepatitis B virus (HBV). Cells infected with the recombinant adenovirus which contained the large envelope gene (HS1.HP) expressed predominantly large envelope and small but detectable quantities of middle (4%) and major (6%) envelope proteins in the cell lysate. No HBV envelope proteins were detected in the culture medium from HS1.HP-infected cells. Cells infected with recombinant adenovirus which contained the middle envelope gene (HS2.HP) expressed and secreted the middle and major envelope proteins in a molar ratio of 3:1. Cells infected with the recombinant adenovirus which contained the major envelope gene (HS.HP) expressed and secreted major envelope proteins. The HBV envelope proteins secreted by cells infected with either HS2.HP or HS.HP were assembled in 22-nm particles, as shown by velocity sedimentation rate determination, buoyant densities, and electron microscopy. Cells coinfected with a recombinant adenovirus which contained the large envelope gene and with either HS2.HP or HS.HP expressed similar quantities of the large, middle, and major envelope proteins in the cell lysates. Secretion of the major and middle envelope proteins was inhibited more than 95% by the presence of the large envelope proteins. These results suggest that differential biosynthesis, transport, and processing of the envelope proteins occur during HBV infection, allowing efficient assembly and secretion of virions and hepatitis B surface antigen particles.     

Distinctive features of the two classes of eukaryotic peptide deformylases.

Peptide deformylases (PDFs) are essential enzymes of the N-terminal protein processing pathway of eubacteria. The recent discovery of two types of PDFs in higher plants, PDF1A and PDF1B, and the detection of PDF1A in humans, have raised questions concerning the importance of deformylation in eukaryotes. Here, we have characterized fully in vitro and compared the properties of the two classes of eukaryotic PDFs, PDF1A and PDF1B, using the PDFs from Arabidopsis thaliana and Lycopersicon esculentum. We have shown that the PDFs of a given class (1A or 1B) all display similar features, independently of their origin. We also observed similar specificity of all plant PDFs for natural substrate peptides, but identified a number of biochemical differences between the two classes (1A or 1B). The main difference lies at the level of the bound cofactor, iron for PDF1B-like bacterial PDFs, and zinc for PDF1A. The nature of the metal cation has important consequences concerning the relative sensitivity to oxygen of the two plant PDFs. Investigation of the specificity of these enzymes with unusual substrates revealed additional differences between the two types of PDFs, enabling us to identify specific inhibitors with a lower affinity against PDF1As. However, the two plant PDFs were inhibited equally strongly in vitro by actinonin, an antibiotic that specifically acts on bacterial PDFs. Uptake of actinonin by A. thaliana seedlings was used to investigate the function of PDFs in the plant. Because it induces an albino phenotype, we conclude that deformylation is likely to play an essential role in the chloroplast.     

Distinctive properties of glutamine synthetase from the cyanobacterium Anacystis nidulans.

The intracellular levels of glutamine synthetase (GS) in Anacystis nidulans grown under different conditions were determined using a whole-cell assay. Nitrate-grown cells have 64% more GS than cells grown in ammonium sulfate. Nitrogen starvation does not affect GS levels appreciably. Incubation of nitrate-grown cells with ammonium sulfate does not change the ratio of gamma-glutamyl transferase activities stimulated by Mg2+ and Mn2+ ions. An in vitro test of adenylylation indicates that algae do not have an endogenous adenylyl transferase (ATase) and that algal GS is not adenylylatable by the Klebsiella aerogenes ATase. Some characteristics of the GS-membrane complex were determined by centrifugation of the complex under varying conditions of pH and ionic strength. In this way, it was shown that acid pH (4.5) stabilizes the complex and high ionic strength tends to solubilize the enzyme. A simple partial purification of GS (89-fold) was developed based on the sedimentation properties of GS.     

Distinctive signatures of recursion

Although recursion has been hypothesized to be a necessary capacity for the evolution of language, the multiplicity of definitions being used has undermined the broader interpretation of empirical results. I propose that only a definition focused on representational abilities allows the prediction of specific behavioural traits that enable us to distinguish recursion from non-recursive iteration and from hierarchical embedding: only subjects able to represent recursion, i.e. to represent different hierarchical dependencies (related by parenthood) with the same set of rules, are able to generalize and produce new levels of embedding beyond those specified a priori (in the algorithm or in the input). The ability to use such representations may be advantageous in several domains: action sequencing, problem-solving, spatial navigation, social navigation and for the emergence of conventionalized communication systems. The ability to represent contiguous hierarchical levels with the same rules may lead subjects to expect unknown levels and constituents to behave similarly, and this prior knowledge may bias learning positively. Finally, a new paradigm to test for recursion is presented. Preliminary results suggest that the ability to represent recursion in the spatial domain recruits both visual and verbal resources. Implications regarding language evolution are discussed.     

Distinctive adjuvanticity of synthetic analogs of mycobacterial water-soluble components.

Synthetic N-acetyl-muramyl-l-alanyl-d-isoglutamine represents the minimal structure capable of duplicating the activity of mycobacteria in Freund's complete adjuvant. In contrast to mycobacterial adjuvant preparations, that function only in the form of water-in-oil emulsions, this compound and a second synthetic analog (N-acetyl-muramyl-l-alanyl-d-isoglutamic acid) augment the humoral immune responses of mice equally as well as aqueous solutions. Whereas N-acetyl-muramyl-l-alanyl-d-isoglutamic acid administered to guinea pigs in water-in-oil emulsion has no effect, N-acetyl-muramyl-l-alanyl-d-isoglutamine induces delayed hypersensitivity to ovalbumin and azobenezene-arsonate N-acetyl-l-tyrosine and increases the level of antibody against ovalbumin. Under these conditions, challenge with the synthetic adjuvants themselves evokes no skin responses. Moreover, Freund's complete adjuvant sensitizes guinea pigs to tuberculin and to native mycobacterial water-soluble adjuvant but not to the synthetic analogs.