Alpine vascular plant species richness: the importance of daily maximum temperature and pH

Species richness in the alpine zone varies dramatically when communities are compared. We explored (i) which stress and disturbance factors were highly correlated with species richness, (ii) whether the intermediate stress hypothesis (ISH) and the intermediate disturbance hypothesis (IDH) can be applied to alpine ecosystems, and (iii) whether standing crop can be used as an easily measurable surrogate for causal factors determining species richness in the alpine zone. Species numbers and standing crop were determined in 14 alpine plant communities in the Swiss Alps. To quantify the stress and disturbance factors in each community, air temperature, relative air humidity, wind speed, global radiation, UV-B radiation, length of the growing season, soil suction, pH, main soil nutrients, waterlogging, soil movement, number of avalanches, level of denudation, winter dieback, herbivory, wind damage, and days with frost were measured or observed. The present study revealed that 82% of the variance in␣vascular species richness among sites could be explained by just two abiotic factors, daily maximum temperature and soil pH. Daily maximum temperature and pH affect species richness both directly and via their effects on other environmental variables. Some stress and disturbance factors were related to species richness in a monotonic way, others in an unimodal way. Monotonic relationships suggest that the harsher the environment is, the fewer species can survive in such habitats. In cases of unimodal relationships (ISH and IDH) species richness decreases at both ends of the gradients due to the harsh environment and/or the interaction of other environmental factors. Competition and disturbance seemed only to play a secondary role in the form of fine-tuning species richness in specific communities. Thus, we concluded that neither the ISH nor the IDH can be considered useful conceptual models for the alpine zone. Furthermore, we found that standing crop can be used as an easily measurable surrogate for causal factors determining species richness in the alpine zone, even though there is no direct causality.     

Conformational analysis of the 3'-5'-cyclic dinucleotide d less than pApA greater than by means of molecular mechanics

The conformational properties of the cyclic dinucleotide d less than pApA greater than were studied by means of molecular mechanics calculations in which a multiconformation analysis was combined with minimum energy calculations. In this approach models of possible conformers are built by varying the torsion angles of the molecule systematically. These models are then subjected to energy minimization; in the present investigation use was made of the AMBER Force field. It followed that the lowest energy conformer has a pseudo-two-fold axis of symmetry. In this conformer the deoxyribose sugars adopt a N-type conformation. The conformation of the sugar-phosphate backbone is determined by the following torsion angles: alpha +, beta t, gamma +, epsilon t and zeta +. The conformation of this ringsystem corresponds to the structure derived earlier by means of NMR spectroscopy and X-ray diffraction. The observation of a preference for N-type sugar conformations in this molecule can be explained by the steric hindrance induced between opposite H3' atoms when one sugar is switched from N- to S-type puckers. The sugars can in principle switch from N- to S-type conformations, but this requires at least the transition of gamma + to gamma -. In this process the molecule obtains an extended shape in which the bases switch from a pseudo-axial to a pseudo-equatorial position. The calculations demonstrate that, apart from the results obtained for the lowest energy conformation, the 180 degrees change in the propagation direction of the phosphate backbone can be achieved by several different combinations of the backbone torsion angles. It appeared that in the low energy conformers five higher order correlations are found. The combination of torsion angles which are involved in changes in the propagation direction of the sugar-phosphate backbone in DNA-hairpin loops and in tRNA, are found in the dataset obtained for cyclic d less than pApA greater than. It turns out, that in the available examples, 180 degrees changes in the backbone direction are localized between two adjacent nucleotides.     

The Hemagglutinin of Bat-Associated Influenza Viruses Is Activated by TMPRSS2 for pH-Dependent Entry into Bat but Not Human Cells

New World bats have recently been discovered to harbor influenza A virus (FLUAV)-related viruses, termed bat-associated influenza A-like viruses (batFLUAV). The internal proteins of batFLUAV are functional in mammalian cells. In contrast, no biological functionality could be demonstrated for the surface proteins, hemagglutinin (HA)-like (HAL) and neuraminidase (NA)-like (NAL), and these proteins need to be replaced by their human counterparts to allow spread of batFLUAV in human cells. Here, we employed rhabdoviral vectors to study the role of HAL and NAL in viral entry. Vectors pseudotyped with batFLUAV-HAL and -NAL were able to enter bat cells but not cells from other mammalian species. Host cell entry was mediated by HAL and was dependent on prior proteolytic activation of HAL and endosomal low pH. In contrast, sialic acids were dispensable for HAL-driven entry. Finally, the type II transmembrane serine protease TMPRSS2 was able to activate HAL for cell entry indicating that batFLUAV can utilize human proteases for HAL activation. Collectively, these results identify viral and cellular factors governing host cell entry driven by batFLUAV surface proteins. They suggest that the absence of a functional receptor precludes entry of batFLUAV into human cells while other prerequisites for entry, HAL activation and protonation, are met in target cells of human origin.