What is a quasispecies?

A quasispecies is a well-defined distribution of mutants that is generated by a mutation-selection process. Selection does not act on a single mutant but on the quasispecies as a whole. Experimental systems have been designed to study quasispecies evolution under laboratory conditions. More recently, virus populations have been called quasispecies to indicate their extensive genetic heterogeneity. The most prominent examples are probably the human immunodeficiency viruses HIV-1 and HIV-2. The quasispecies nature of HIV has formed the basis of a model that provides a mechanism for the pathogenesis of acquired immunodeficiency syndrome (AIDS) in humans. This article focuses on the nature of the quasispecies concept and its implications for evolutionary biology and virology.     

Models of interactions between HIV and other pathogens.

We investigate possible interactions between HIV and other pathogens that would arise if HIV replication were enhanced by the activation of T helper cells specific to other pathogens. Using mathematical models of the population dynamics of T helper cells, HIV and other pathogens we address three facets of the interactions between HIV and other pathogens: enhanced HIV replication due to immune stimulation by other pathogens; modified immune control of other pathogens due to immunosuppression by HIV; and the vicious circle formed by positive feedback between these two effects. The models predict that there is a correlation between higher levels of activated TH cells and disease progression and that there is a threshold number of activated TH cells above which the HIV infected immune system is unable to control pre-established pathogens. This threshold marks the boundary between a suppressed but still functioning immune system and the vicious circle of CD4 cell depletion that marks the final stages of AIDS.     

Serotonin N-acetyltransferase (NAT) induction in mammalian retina: role of cyclic AMP and calcium ions.

In retinas and pineal glands of rat, rabbit and hen, activities of the penultimate (and key regulatory) enzyme in melatonin biosynthesis, serotonin N-acetyltransferase (NAT), display distinct diurnal variations, with high and low values during dark and light phase of a 12-h dark: 12-h light illumination cycle. Two-hour incubation (during daytime hours in light) of isolated pineal glands of the studied vertebrates, or the retinas, with 50 microM forskolin (plus 100 microM 3-isobutyl-1-methylxanthine, IBMX-a phosphodiesterase inhibitor), and 1 mM dibutyryl-cAMP, markedly increased the tissue NAT activity. The same procedures significantly enhanced the enzyme activity of rat retina in light, however, only during nighttime hours. The forskolin (+ IBMX)-induced increase of NAT activity in rat retina was significantly lower in a calcium-free medium, and substantially enhanced when calcium concentration was raised from 1.3 mM to 3.9 mM. Treatment of rats with IBMX or aminophylline, and rabbits with aminophylline, increased NAT activity in their pineal glands irrespective of the time of the day, whereas both phosphodiesterase inhibitors significantly increased the enzyme activity of rat retina only when injected during the subjective dark hours. It is concluded that, by analogy to vertebrate pineal gland, in vertebrate retina an increase of NAT activity (and consequently melatonin formation), stimulated both physiologically (i. e. at night), or pharmacologically, involves a cAMP- and calcium dependent process of the enzyme induction.     

25Mg NMR studies of yeast enolase and rabbit muscle pyruvate kinase.

25Mg NMR spectroscopy was used to study the interactions of the activating cations with their respective binding sites in the enzymes yeast enolase and rabbit muscle pyruvate kinase (PK). Titration of Mg2+ with enolase allows for the calculation of 1/T2 for Mg2+ bound at site I of 1510 s-1 and a quadrupolar coupling constant chi = 0.30 MHz. Titration of Mg2+ with enolase in the presence of 2-phosphoglycerate (PGA) and Zn2+, where Zn2+ binds specifically at site I, gives a 1/T2 for Mg2+ bound at site II of 4000 s-1 (chi = 0.49 MHz). The Mg2+ at site II appears to be more anisotropic than Mg2+ at site I. The titration of site I of the enolase-Mg-PGA-Mg complex with Zn2+ or Mn2+ shows a simple displacement of the Mg2+. No paramagnetic effects by Mn2+ on 25Mg relaxation were observed. Temperature studies of the 25Mg resonance show that fast exchange of the Mg2+ occurs under these conditions. From the lack of a paramagnetic effect, the distance between the cations at sites I and II must be more than 6-9 A. This distance limits the location, hence the function, of the cation at site II for catalytic activity. Titration of Mg2+ with PK gives a 1/T2 for bound Mg2+ of 2200 s-1 (chi = 0.24 MHz). A titration of Mg2+ with PK in the presence of the inhibitor oxalate gives a 1/T2 of 400 s-1. The temperature dependence of 25Mg relaxation in the PK-Mg-oxalate complex is consistent with slow exchange (Ea = 6.1 +/- 1.6 kcal/mol). The enzyme-bound cation is more tightly sequestered by the addition of a ligand that binds directly to the cation. An investigation of the 25Mg relaxation in the PK-Mn-oxalate-Mg-ATP complex, where the Mg2+ is bound to the nucleotide and the Mn2+ was enzyme bound, was not successful due to precipitation of PK under experimental conditions and the short T2 relaxation for 25Mg in this complex. The applications of 25Mg NMR have been useful in partially describing the properties of the bound Mg2+ in these two metal-requiring enzymes.     

Non-polluting wood and pulp delignification: Biomimetic ligninase system

Summary We report the delignification ofPinus radiata D Don,Eucalyptus globulus andEucalyptus grandis woods (formic acid treated and untreated) by 2 h treatment with a hemin/hydrogen peroxide system. The untreated chips and sawdust ofE. globulus were 30% and 50% delignified respectively. No significant effects were found forP. radiata sawdust;P. radiata treated chips (organosolv pulp) did not show any further delignification upon hemin/peroxide action, 25% delignification was achieved in untreated chips. In the case ofE. grandis untreated wood the delignification was better in sawdust than in chips, but in smaller percentage than in the otherEucalyptus species. This relation is maintained in substrates, treated with formic acid or untreated. The delignification of chips in both species ofEucalyptus was improved when they were pre-treated with formic acid. The loss of lignin in theE. grandis andE. globulus sawdust (pre-treated with formic acid) was 79% and 75% respectively.     

Glass beads as a solid matrix in in vitro study of the role of polyamines in cold hardiness of white clover

Glass beads were used as an alternative to agar in the study of the role of polyamines in cold hardiness of white clover. Plantlet growth performance, in vitro hardening and cold stress tolerance were similar on both the agar solidified medium and the liquid medium in glass beads matrix. Glass beads allowed media exchange without plant transfer and an easy monitoring of the uptake of putrescine synthesis inhibitor, ¹⁴C difluoromethylornithine from the medium. The matrix is recommended in in vitro studies of whole plant physiology, screening procedures and bioassays where media exchange and/or uniform application of a selection pressure is required. There is also 60% saving on media components and the beads can be re-used after acid wash.Abbreviations DFMA difluoremethylarginine - DFMO difluoromethylornithine - MS Murashige and Skoog salts - Kcpm thousand counts per minute     

Agglutinin from Limulus polyphemus. Purification with formalinized horse erythrocytes as the affinity adsorbent.

We have purified an agglutinin from the hemolymph of Limulus polyphemus about 1500-3000-fold by adsorption to formalinized horse erythrocytes, elution with N-acetylneuraminic acid and subsequent fractionation on Sephadex G-200. Recovery was in the range of 50 percent. On ultracentrifugation the agglutinin behaves as an homogenous protein with a molecular weight of about 460 000. On polyacrylamide gel electrophoresis of the dissociated protein in sodium dodecylsulfate we found a single prominent diffuse band with an apparent molecular weight of 22 000 plus or minus 2000. This band contained carbohydrate as determined by periodic acid-Schiff staining. The intensity of staining compared with standards suggested a carbohydrate content of less than 4 percent. The protein contains a preponderance of acidic amino acids and has an isoelectric point of 4.83.5 residues per 1000 of glucosamine were detected on amino acid analysis. Agglutination of formalinized horse erythrocytes by the purified protein is inhibited not only by N-acetylneuraminic acid but also by D-glucuronic acid; but not by a number of other monosaccharides. D-Glucuronic acid may be used in place of N-acetylneuraminic acid as the eluting sugar in the purification procedure.     

Radiometric Detection of Bacteremia in Neonates

The predicted prevalence of false positive blood cultures due to hyperactive neonatal blood cells in a radiometric detection system was confirmed. Suppression of this blood background radioactivity in the system was achieved by using a hypertonic medium containing 10% sucrose. The radiometric system produced accurate results as fast as the conventional blood culturing method, saved labor and minimized the recovery of extraneous contaminants.