THE ribonuclease, barnase, produced by Bacillus amyloliquefaciens has a molecular weight of 12,382, consisting of 110 amino-acid residues. It is one of the smallest proteins containing neither disulphide bonds nor non-peptide cross-Bnks which nevertheless maintain a well defined tertiary structure1,2. The next smallest reported enzyme of similar nature is the lysozyme of phage T4, with 160 residues. The barnase structure is reversibly destroyed by denaturing solvents or heat2, in what approximates a one step, highly cooperative, transition. Studies of this reaction should be very useful in illustration approaching the general problem of sequence-determined folding in proteins. In particular, thermodynamically meaningful quantitative differences in the stability of various genetic variants and chemically modified, or synthetic, barnases could be measured. Some work has been reported on the effect of various environmental parameters on the transition3 as well as the effects of modification by carboxypeptidases4. Full utilization of such data requires knowledge of both amino-acid sequence and three dimensional structurs. The complete amino-acid sequence is reported here (Fig. 1). The sequence was obtained by conventional procedures involving analysis of peptides isolated after hydrolysis of the native or modified protein by various proteases.
Global environmental change, related to climate change and the deposition of airborne N‐containing contaminants, has already resulted in shifts in plant community composition among plant functional types in Arctic and temperate alpine regions. In this paper, we review how key ecosystem processes will be altered by these transformations, the complex biological cascades and feedbacks that might result, and some of the potential broader consequences for the earth system. Firstly, we consider how patterns of growth and allocation, and nutrient uptake, will be altered by the shifts in plant dominance. The ways in which these changes may disproportionately affect the consumer communities, and rates of decomposition, are then discussed. We show that the occurrence of a broad spectrum of plant growth forms in these regions (from cryptogams to deciduous and evergreen dwarf shrubs, graminoids and forbs), together with hypothesized low functional redundancy, will mean that shifts in plant dominance result in a complex series of biotic cascades, couplings and feedbacks which are supplemental to the direct responses of ecosystem components to the primary global change drivers. The nature of these complex interactions is highlighted using the example of the climate‐driven increase in shrub cover in low‐Arctic tundra, and the contrasting transformations in plant functional composition in mid‐latitude alpine systems. Finally, the potential effects of the transformations on ecosystem properties and processes that link with the earth system are reviewed. We conclude that the effects of global change on these ecosystems, and potential climate‐change feedbacks, cannot be predicted from simple empirical relationships between processes and driving variables. Rather, the effects of changes in species distributions and dominances on key ecosystem processes and properties must also be considered, based upon best estimates of the trajectories of key transformations, their magnitude and rates of change. 相似文献
Agrobacterium tumefaciens is a plant pathogenic bacterium that causes neoplastic growths, called ‘crown gall’, via the transfer and integration of transferred DNA (T‐DNA) from the bacterium into the plant genome. We characterized an acetosyringone (AS)‐induced tumour‐inducing (Ti) plasmid gene, tzs (trans‐zeatin synthesizing), that is responsible for the synthesis of the plant hormone cytokinin in nopaline‐type A. tumefaciens strains. The loss of Tzs protein expression and trans‐zeatin secretions by the tzs frameshift (tzs‐fs) mutant is associated with reduced tumorigenesis efficiency on white radish stems and reduced transformation efficiencies on Arabidopsis roots. Complementation of the tzs‐fs mutant with a wild‐type tzs gene restored wild‐type levels of trans‐zeatin secretions and transformation efficiencies. Exogenous application of cytokinin during infection increased the transient transformation efficiency of Arabidopsis roots infected by strains lacking Tzs, which suggests that the lower transformation efficiency resulted from the lack of Agrobacterium‐produced cytokinin. Interestingly, although the tzs‐fs mutant displayed reduced tumorigenesis efficiency on several tested plants, the loss of Tzs enhanced tumorigenesis efficiencies on green pepper and cowpea. These data strongly suggest that Tzs, by synthesizing trans‐zeatin at early stage(s) of the infection process, modulates plant transformation efficiency by A. tumefaciens. 相似文献
1. In ant–hemipteran mutualisms, ants receive carbohydrates in the form of honeydew, while hemipterans receive protection from natural enemies. In the absence of natural enemies, however, the direct effects of tending are generally less well known. We hypothesised that with increasing tending intensity (ant to aphid ratio), aphid performance would increase initially, then decrease at high tending levels due to the metabolic cost of producing high quality honeydew. 2. We tested our hypothesis in a greenhouse experiment by manipulating Argentine ant (Linepithema humile Mayr) colony size while holding constant the initial size of aphid (Chaitophorus populicola Thomas) aggregations. The two parameters associated with survival, aphid survivorship to maturity and longevity, declined with increasing tending intensity, whereas per capita birth rate and time to first reproduction showed no relationship to attendance. The intrinsic rate of increase declined only at relatively high tending levels, suggesting a nonlinearity in the effect of tending intensity. 3. Tending intensity measured in the experiment was similar to that observed in free‐living aggregations of C. populicola. Furthermore, the per capita recruitment rate of ants to free‐living aphid aggregations was negatively density‐dependent, indicating that small aggregations tend to experience the highest levels of tending intensity. This finding suggests that the aphid's intrinsic rate of increase may be positively density‐dependent, mediated by the aphid's mutualistic interaction with the ant. 4. In the Argentine ant–C. populicola interaction, experimental manipulation of colony size revealed a direct cost of ant attendance that was conditional upon tending intensity. Experiments that manipulate only ant presence or absence may yield an incomplete understanding of the mutualistic interaction if underlying nonlinearities exist. 相似文献
The new names Gentiana membranulifera T. N. Ho and G. nudicaulisKurz var. assamensis T. N. Ho are proposed. The new combinations Gentiana lateriflora Hemsl. var. uncifolia
(H. J. Lam) T. N. Ho, G. sumatrana Ridl.var. humifusa (S. Moore) T. N. Ho, G. quadrifaria Bl. var. wightii (Kusnez.)T. N. Ho, G. loerzingii Ridl. var. timida (Kerr) T. N. Ho, and G. membranulifera T. N. Ho var. recurvata (Kusnez.) T. N. Ho are made. 相似文献
Mycobacterium tuberculosis(M.tb) and human immunodeficiency virus(HIV) co-infection has become a public health issue worldwide. Up to now, there have been many unresolved issues either in the clinical diagnosis and treatment of M.tb/HIV coinfection or in the basic understanding of the mechanisms for the impairments to the immune system by interactions of these two pathogens. One important reason for these unsolved issues is the lack of appropriate animal models for the study of M.tb/HIV coinfection. This paper reviews the recent development of research on the animal models of M.tb/HIV co-infection, with a focus on the non-human primate models. 相似文献