7巧空间 Opel Zafira

1999年,欧宝凭借赛飞利成为首个生产7座小型 MPV 的德国汽车制造商。赛飞利一经推出,当年就占据了同级别车型德国市场的销量冠军,并保持了四年之久。据资料显示,首代赛飞利共生产了140万辆     

Role for the nitrogenase MoFe protein alpha-subunit in FeMo-cofactor binding and catalysis

Two components constitute Mo-dependent nitrogenase (EC 1.18.6.1)--the Fe protein (a homodimer encoded by nifH) and the MoFe protein (an alpha 2 beta 2 tetramer encoded by nifDK). The MoFe protein provides the substrate-binding site and probably contains six prosthetic groups of two types--four Fe-S centres and two Fe- and Mo-containing cofactors. To determine the distribution and catalytic function of these metalloclusters, we and others are attempting to change the catalytic and spectroscopic features of nitrogenase by substituting specific amino acids targeted as potential metallocluster ligands, particularly those to the FeMo-cofactor, which is responsible for the biologically unique electron paramagnetic resonance signal (S = 3/2) of nitrogenase, and is believed to be the N2-reducing site. Here we describe mutant strains of Azotobacter vinelandii that have single amino-acid substitutions within the MoFe protein alpha-subunit. These substitutions alter both substrate-reduction properties and the unique electron paramagnetic resonance signal, indicating that the FeMo-cofactor is associated with both the alpha-subunit and the substrate-reducing site.     

Population biology of infectious diseases: Part II.

In the first part of this two-part article (Nature 280, 361--367), mathematical models of directly transmitted microparasitic infections were developed, taking explicit account of the dynamics of the host population. The discussion is now extended to both microparasites (viruses, bacteria and protozoa) and macroparasites (helminths and arthropods), transmitted either directly or indirectly via one or more intermediate hosts. Consideration is given to the relation between the ecology and evolution of the transmission processes and the overall dynamics, and to the mechanisms that can produce cyclic patterns, or multiple stable states, in the levels of infection in the host population.     

Jasmonate and salicylate induce expression of herbivore cytochrome P450 genes

Jasmonate and salicylate are plant-produced signals that activate plant defence genes after herbivory or pathogen attack. Amplification of these signals, evoked by either enemy attack or experimental manipulation, leads to an increase in the synthesis of toxic compounds (allelochemicals) and defence proteins in the plants. Although the jasmonate and salicylate signal cascades activate different sets of plant defence genes, or even act antagonistically, there is substantial communication between the pathways. Jasmonate and salicylate also contribute to protecting plants against herbivores by causing plants that experience insect damage to increase their production of volatile molecules that attract natural enemies of herbivorous insects. In response to plant defences, herbivores increase their production of enzymes that detoxify allelochemicals, including cytochrome P450s (refs 15, 16). But herbivores are potentially vulnerable to toxic allelochemicals in the duration between ingesting toxins and induction of detoxification systems. Here we show that the corn earworm Helicoverpa zea uses jasmonate and salicylate to activate four of its cytochrome P450 genes that are associated with detoxification either before or concomitantly with the biosynthesis of allelochemicals. This ability to 'eavesdrop' on plant defence signals protects H. zea against toxins produced by host plants.     

A Soft System Framework for the Conservation Management of Material Cultural Heritage

Experience gained from cultural heritage management in Norway and the UK has led to the development of a framework describing the key conservation activities required to sustain a diverse range between material cultural heritage assets. A soft system approach allows the complex interaction between various activities and external factors impacting on cultural heritage management to be examined as a single entity, for example, the contributions made by human interaction, environmental impact and the properties of the material heritage itself, on the preservation or loss of a cultural object. By constructing such a framework it is possible to identify potential indicators that can be used to measure the quality of conservation activities. Arguments are presented for grouping these indicators of impact into social, economic and environmental categories and examples are given of how this approach can be universally applied to real situations.