Dissemination of spores in a glasshouse: Pattern or chaos?

The dissemination of particles in a glasshouse with three united compartments was observed from experiments withLycopodium sp. The experiments were performed in a glasshouse with open and closed ventilation openings, spore sources on two heights above soil surface, and in two consecutive time intervals. The spores were caught with self constructed spore traps placed at regular distances from each other. Air circulation was observed using smoke-puffs. High ventilation caused a rapid cleaning of the air while in a closed glasshouse spores remained suspended for quite a long time. Though the three compartments of the glasshouse were not separated by walls, a so-called cell structure of the individual compartments could be indicated.     

Temporary disturbance of translocation of assimilates in douglas firs caused by low levels of ozone and sulfur dioxide

Douglas firs (Pseudotsuga menziesii [Mirb.] Franco) are suffering strongly from air pollution in western Europe. We studied the effect of low concentrations of ozone (200 micrograms per cubic meter during 3 days) and sulfur dioxide (53 micrograms per cubic meter during 28 days) on translocation of assimilates in 2 year old Douglas firs. The trees were exposed to the pollutants and afterward transferred to a growth chamber adapted to the use of ¹⁴CO₂. Root/soil respiration was measured daily. The results showed a significant decrease of the ¹⁴CO₂ root/soil respiration during the first 1 to 2 weeks after exposure to either ozone or sulfur dioxide. The ultimate level of ¹⁴CO₂ root/soil respiration did not differ significantly, which suggests a recovery of the exposed trees during the first weeks after exposure.     

Enzyme patterns in two species of Xenopus and their hybrids

Differences in the isozyme patterns of Xenopus laevis and Xenopus mulleri have been utilized to examine the expression of alleles of both species in hybrid animals. Mitochondrial MDH and tetrazolium oxidase phenotypes were examined during the development of non-hybrid embryos of each species and of reciprocal hybrids. Early stages of the hybrids resemble the enzyme phenotype of the maternal parent. Appearance of paternal enzyme takes place just prior to the active feeding tadpole stage for both mitochondrial MDH and oxidase. The maternal effect disappears shortly thereafter in early feeding tadpoles, at which point reciprocal hybrids have identical isozyme patterns. There is no evidence for a predominance of one species over the other. Examination of feeding tadpoles and adult toads indicates that both laevis and mulleri expression is stable. The appearance of paternal mitochondrial MDH does not correspond to the time when other mitochondrial components begin to increase in Xenopus. Multiple bands of MDH in both species and of oxidase in laevis are probably not due to the aggregation of subunits produced by different alleles at the same locus. There is no evidence for the formation of “hybrid” molecules consisting of subunits of both species.     

The Na+/H+ Exchanger Present in Trout Erythrocyte Membranes is Not Responsible for the Amiloride-insensitive Na+/Li+ Exchange Activity

The protein responsible for the Na⁺/Li⁺ exchange activity across the erythrocyte membrane has not been cloned or isolated. It has been suggested that a Na⁺/H⁺ exchanger could be responsible for the Na⁺/Li⁺ exchange activity across the erythrocyte membrane. Previously, we reported that in the trout erythrocyte, the Li⁺/H⁺ exchange activity (mediated by the Na⁺/H⁺ exchanger βNHE) and the Na⁺/Li⁺ exchange activity respond differently to cAMP, DMA (dimethyl-amiloride) and O₂. We concluded that the DMA insensitive Na⁺/Li⁺ exchange activity originates from a different protein. To further examine these findings, we measured Li⁺ efflux in fibroblasts expressing the βNHE as the only Na⁺/H⁺ exchanger. Moreover, the internal pH of these cells was monitored with a fluorescent probe. Our findings indicate that acidification of fibroblasts expressing the Na⁺/H⁺ exchanger βNHE, induces a Na⁺ stimulated Li⁺ efflux activity in trout erythrocytes. This exchange activity, however, is DMA sensitive and therefore differs from the DMA insensitive Na⁺/Li⁺ exchange activity. In these fibroblasts no significant DMA insensitive Na⁺/Li⁺ exchange activity was found. These results support the hypothesis that the trout erythrocyte Na⁺/Li⁺ exchange activity is not mediated by the Na⁺/H⁺ exchanger (βNHE) present in these membranes. Received: 6 December 1996/Revised: 11 August 1997     

Heterotrophic Uptake Experiments with 14C-Labeled Histidine in a Histidine-Limited Chemostat

The histidine uptake by bacterial strain HIS 42 was determined with [U-¹⁴C]histidine and through oxygen uptake experiments on samples taken from a histidine-limited chemostat. The uptake of [U-¹⁴C]histidine was characterized by a saturation constant of 12.8 to 78.6 nM histidine. At higher growth rates, the measured maximum uptake rate of histidine was lower than the actual uptake rate in the culture. The percentage of respired substrate (76 to 93%) was about 30 to 40% higher than the comparable value for the culture. The uptake of histidine as analyzed through the measurement of oxygen uptake rates was characterized by a saturation constant of 1.7 to 10.5 μM histidine; the maximum uptake rate was always greater than the actual histidine uptake rate in the culture. By the application of the two cited methods, set up to determine the histidine uptake kinetics, two different uptake processes were analyzed. It appeared that the determination of the histidine uptake through measurement of the oxygen uptake rate showed a better reflection of the actual uptake process of histidine in the culture. With the available data it was impossible to assess a correlation between the uptake of histidine, as determined with [U-¹⁴C]histidine, and the actual metabolism of the bacterial population.     

A small protein in model membranes: a time-resolved fluorescence and ESR study on the interaction of M13 coat protein with lipid bilayers

Model membranes with unsaturated lipid chains containing various amounts of M13 coat protein in the -helical form were studied using time-resolved fluorescence and ESR spectroscopy. The lipid-to-protein (L/P) ratios used were > 12 to avoid protein-protein contacts and irreversible aggregation leading to -polymeric coat protein. In the ESR spectra of the 12-SASL probe in dioleoyl phosphatidylcholine (DOPC) bilayers no second protein induced component is observed upon incorporation of M13 coat protein. However, strong effects are detected on the ESR lineshapes upon changing the protein concentration. The ESR lineshapes are simulated by assuming a fixed ratio between the parallel (D) and perpendicular (D) diffusion coefficients of 4, and an order parameter equal to zero. It is found that increasing the protein concentration from L/P to L/P 15 results in a decrease of the rotational diffusion coefficient D from 3.4 × 10⁷ to 1.9 × 10⁷ s^–1. In the time-resolved fluorescence experiments with DPH-propionic acid as a probe, it is observed that increasing the M13 coat protein concentration causes an increase of the two fluorescent lifetimes, indicating an increase in bilayer order. Analysis of the time-resolved fluorescence anisotropy decay allows one to quantitatively determine the order parameters P₂ and P₄, and the rotational diffusion coefficient D of the fluorescent probe. The order parameters P₂ and P₄ increase from 0.34 to 0.55 and from 0.59 to 0.77, respectively, upon adding M13 coat protein to DOPC bilayers with an L/P ratio of 35. The rotational diffusion coefficient D of the DPH-propionic acid probe decreases on incorporating M13 coat protein, in accordance with the ESR results. It is concluded that M13 coat protein in the -monomeric state is not able to produce a long living lipid boundary shell and consequently an immobilization of the lipids. An overall effect on the lipids is induced, resulting in a reduction in the dynamics and an increase in average lipid order. The hydrophobic region of M13 coat protein is proposed to perfectly match the lipid bilayer, resulting in a relatively small distortion of the bilayer structure of the lipid system.     

Inhibition of the respiration of dormant and swollenStreptomyces spores by chloramphenicol

Chloramphenicol produces a decrease in the respiratory quotient of dormant and swollenStreptomyces antibioticus spores of about 20–25% and 18–24%, respectively, in the absence of protein synthesis as measured by [³H]leucine incorporation and by chemical methods. Rifampin and streptomycin had no inhibitory effect on respiration, thus excluding the possibility of an effect of all protein synthesis inhibitors on respiration. The inhibition of respiration by chloramphenicol was not a consequence of an increase in mortality due to toxicity, nor was it influenced by the developmental stage of the spores. It seems that chloramphenicol affects the activity of some component(s) related to the electron transport chain ofS. antibioticus spores, situated before the cytochrome oxidase level.     

Migrant economies: opportunity structures and potential in different city types

In this paper we wish to appraise how opportunities for migrant economies and their role in urban development may differ among various city types. The article contributes to the debate about the relationship of migrant economies and urban development and takes up two perspectives: it examines local opportunity structures for migrant entrepreneurs and sheds light on migrant economies’ potential for urban development. To address the many interrelated historical and contemporary processes in cities that influence migrant economies, we adopt the rescaling and the mixed embeddedness approaches. Studies on the role of migrant economies in urban development have predominantly focused on metropolises. Based on mixed-methods case studies in two medium-sized German cities, we ask how different city types influence the opportunities and potential of migrant economies for urban development.