The cycling of organic nitrogen through the atmosphere

Atmospheric organic nitrogen (ON) appears to be a ubiquitous but poorly understood component of the atmospheric nitrogen deposition flux. Here, we focus on the ON components that dominate deposition and do not consider reactive atmospheric gases containing ON such as peroxyacyl nitrates that are important in atmospheric nitrogen transport, but are probably not particularly important in deposition. We first review the approaches to the analysis and characterization of atmospheric ON. We then briefly summarize the available data on the concentrations of ON in both aerosols and rainwater from around the world, and the limited information available on its chemical characterization. This evidence clearly shows that atmospheric aerosol and rainwater ON is a complex mixture of material from multiple sources. This synthesis of available information is then used to try and identify some of the important sources of this material, in particular, if it is of predominantly natural or anthropogenic origin. Finally, we suggest that the flux of ON is about 25 per cent of the total nitrogen deposition flux.     

Expansion of the nucleoplasmic reticulum requires the coordinated activity of lamins and CTP:phosphocholine cytidylyltransferase alpha

The nucleoplasmic reticulum (NR), a nuclear membrane network implicated in signaling and transport, is formed by the biosynthetic and membrane curvature-inducing properties of the rate-limiting enzyme in phosphatidylcholine synthesis, CTP:phosphocholine cytidylyltransferase (CCT) alpha. The NR is formed by invagination of the nuclear envelope and has an underlying lamina that may contribute to membrane tubule formation or stability. In this study we investigated the role of lamins A and B in NR formation in response to expression and activation of endogenous and fluorescent protein-tagged CCTalpha. Similarly to endogenous CCTalpha, CCT-green fluorescent protein (GFP) reversibly translocated to nuclear tubules projecting from the NE in response to oleate, a lipid promoter of CCT membrane binding. Coexpression and RNA interference experiments revealed that both CCTalpha and lamin A and B were necessary for NR proliferation. Expression of CCT-GFP mutants with compromised membrane-binding affinity produced fewer nuclear tubules, indicating that the membrane-binding function of CCTalpha promotes the expansion of the NR. Proliferation of atypical bundles of nuclear membrane tubules by a CCTalpha mutant that constitutively associated with membranes revealed that expansion of the double-bilayer NR requires the coordinated assembly of an underlying lamin scaffold and induction of membrane curvature by CCTalpha.     

Using lower trophic level factors to predict outcomes in classical biological control of insect pests

Importation of exotic natural enemies for biological control of insect pests entails risks to the environment. Pre-release estimates of the likelihood of achieving successful control would be helpful in avoiding ineffective importations. Based on strong evidence of multi-trophic level interactions in terrestrial ecosystems, we tested whether variation in ecological and biological factors found at the plant and herbivore trophic levels (levels one and two) could be used to create a simple, empirically based formula, capable of estimating the probability of successful biological control against holometabolous insect pests. We constructed a database consisting of 828 records of biological control attempts against 91 pest insect species and used stepwise logistic regression to test whether five basic features of the ecosystem, crop, and pest (habitat type, crop use, pest order, pest feeding niche, and damage severity) were correlated with rates of successful control. Natural enemy characteristics were not included in the model. The final model included 10 significant independent variables, nine of which were two-way interactions; all five basic ecosystem features appeared in significant interactions. The model provided good estimates of historical success rates against pest species in the data set. In a further test, the model was able to correctly rank amenability to biological control for 10 pest species not included in the original data set. These results provide evidence that lower trophic level factors can be useful in the search for a predictive formula for biological control.     

Interactions of helical polypepetide segments which span the hydrocarbon region of lipid bilayers. Studies of the gramicidin A lipid-water system.

The polypeptide gramicidin A in a dimeric form is considered to form a helical structure which spans the hydrocarbon region of lipid bilayers. In the present investigation it is used as a model for the interactions of the polypeptide segments of transmembrane proteins within the hydrocarbon region of the lipid bilayers of biomembrane structures. A variety of physical techniques (X-ray diffraction, differential scanning calorimetry, optical and electron microscopy, Raman and electron spin resonance spectroscopy) are applied to a study of the interactions of this polypeptide within the phospholipid bilayers of dimyristoyl and dipalmitoyl lecithins in water, at temperatures both above and below the main endothermic phase transition of the pure lipids.Above the transition temperature of the lipid, the Raman studies show that the polypeptide perturbs the fluid lipid environment and causes a marked decrease in the number of gauche isomers of the lipid hydrocarbon chains, even at quite low relative molar concentrations of the polypeptide to lipid (1:150). At concentrations of phospholipid to polypeptide of less than 5:1, the electron spin resonance studies show the existence of two lipid regions within the bilayer. One region corresponds to the relatively fluid lipid region normally observed at these temperatures and the other to a relatively rigid lipid region. The latter is considered to arise from clusters of the polypeptide in which some of the lipid is entrapped.Below the lipid phase transition temperature, the pretransition endotherm observed with pure lipid-water systems is removed by small molar concentrations of the polypeptide (1:50) and the rippled appearance observed in freeze-fracture electron micrographs with pure dimyristoyl lecithin-water dispersions is replaced by a smooth appearance.The main lipid phase transition becomes broadened by the presence of increasing amounts of the polypeptide within the lipid bilayer as indicated by calorimetry, and electron spin resonance spectroscopy. The enthalpy of the lipid transition decreases linearly with increasing amounts of the polypeptide until, with dipalmitoyl lecithin, a concentration of approximately 20 lipids per polypeptide is reached. This is considered to correspond to the onset of an aggregation process which produces localised polypeptide-lipid clusters within the plane of the membrane.At concentrations of polypeptide less than five lipids per polypeptide, freezefracture electron microscopy shows the presence of liposomes with smooth fracture faces. At higher polypeptide concentrations, sheet-like structures are observed with smooth fracture faces.When a mixed lipid-water system (dilauroyl and dipalmitoyl lecithin) containing low concentrations of the polypeptide is slowly cooled, the calorimetric evidence shows that the polypeptide moves preferentially into the lower melting region of the bilayer, whereas at higher polypcptide eoncentrations a mixing of the two lipids takes place.The various results are discussed to provide insight pertinent to the organisation, interactions, aggregation properties, boundary layer and packing arrangements of helical polypeptides and proteins in reconstituted systems and natural biomembranes.     

Spin-label studies of the sulfhydryl environment in bovine plasma albumin. 1. The N--F transition and acid expansion.

The environment of the sulfhydryl group in plasma albumin was previously characterized by employing spin-labels of varying chain lengths (Hull, H. H., Chang, R., & Kaplan, L. J. (1975) Biochim. Biophys. Acta 400, 132). It was established that the sulfhydryl is in a crevice approximately 10 A deep but this crevice was not identified further. We now report the results of titrating albumin through the acidic conformational transitions while monitoring the electron-spin resonance of the bound nitroxide. With short spin-labels a general change is observed as the pH is lowered but the N--F transition is not discernible. However, with a spin label previously shown to project to the lip on the crevice a clear N--F transition as well as the subsequent acid expansion are observed. These results indicate that the sulfhydryl is in the crevice, formed by the domains of albumin, which opens during the N--F transition. Further results indicate that bound fatty acids do not influence the integrity of the sulfhydryl environment at neutral pH.     

A new theory for the evolution of polyandry as a means of inbreeding avoidance

We propose a novel theory for the evolution of polyandry driven by genetic benefits to females whose offspring interbreed. In species with an ecology characterized by frequent colonization of new habitat patches, consanguineous matings may be common during the early stages of colonization, but genetic diversity may grow as new colonizers arrive. We show that with levels of inbreeding depression similar to those found in predominantly inbreeding populations, a polyandrous female can benefit her descendants since matings among her brood are mainly between half siblings rather than full siblings. We examine the invasion by a polyandrous phenotype using explicit genetic models in which costs of inbreeding are themselves subject to selection. In common with other models of inbreeding, we find that underlying high levels of inbreeding tend to purge deleterious recessive alleles, and hence these are unlikely to maintain sufficient inbreeding depression to favour polyandry. However, if costs of inbreeding are due to overdominance, biologically realistic levels of inbreeding depression result in genetic benefits large enough to favour polyandry provided it is not too costly. The potential significance of polyandry as a mechanism to reduce inbreeding in grandchildren will depend upon the genetic basis of inbreeding depression in natural, inbreeding populations.     

31P nuclear magnetic resonance studies of the association of basic proteins with multilayers of diacyl phosphatidylserine

Lysozyme, cytochrome c, poly(l-lysine), myelin basic protein and ribonuclease were used to form multilayer dispersions containing about 50% protein (by weight) with bovine brain diacyl phosphatidylserine (PS). ³¹P nuclear magnetic resonance shift anisotropies, spin-spin (T₂) and spin-lattice (T₁) relaxation times for the lipid headgroup phosphorus were measured at 36.44 MHz. At pH 7.5, lysozyme, cytochrome c, poly(l-lysine) and ribonuclease were shown to increase the chemical shift anisotropy of PS by between 12–20%. Myelin basic protein altered the shape of the phosphate resonance, suggesting the presence of two lipid components, one of which had a modified headgroup conformation. The presence of cytochrome c led to the formation of a narrow spike at the isotropic shift position of the spectrum. Of the various proteins or peptides we have studied, only poly(l-lysine) and cytochrome c had any effect on the T₁ of PS (1050 ms). Both caused a 20–30% decrease in T₁ of the lamellar-phase phosphate peak. The narrow peak in the presence of cytochrome c had a very short T₁ of 156 ms. The possibility is considered that the cytochrome Fe³⁺ contributes to the phosphate relaxation in this case. The effect of all proteins on the T₂ of the phosphorus resonance was to cause an increase from the value for pure PS (1.6 ms) to between 2 and 5 ms. The results obtained with proteins are compared with the effects of small ions and intrinsic membrane proteins on the order and motion of the headgroups of lipids in bilayers.     

Are Anomalous Invasion Speeds Robust to Demographic Stochasticity?

Two important issues for conservation are the range expansion of species as a result of climate change and the invasion of exotic species. Being able to predict the rate at which species spread is key for successful management. In deterministic models, the invasion speed of a polymorphic population can be faster than that of any of the component phenotypes, and these “anomalous” invasion speeds persist even when the mutation rate between phenotypes is vanishingly small. Here we investigate whether the same phenomenon is observed in a model with demographic stochasticity. The model that we use is discrete in time and space and we carry out numerical simulations to determine the invasion speed of a population that has two morphs which differ in their dispersal abilities. We find that anomalous speeds are observed in the stochastic model, but only when the carrying capacity of the population is large or the mutation rate between morphs is high enough. These results suggest that only species with large population sizes, such as many insect species, may be able to invade faster if they are polymorphic than if there is only a single morph present in the population.