A new family of integral membrane proteins involved in transport of aromatic amino acids in Escherichia coli.

The nucleotide sequence of tnaB of the tryptophanase operon of Escherichia coli is presented. TnaB is a tryptophan-specific permease that is homologous to Mtr, a second tryptophan-specific permease, and to TyrP, a tyrosine-specific permease. Each member of this family appears to contain 11 membrane-spanning domains.     

Transient expression of the calcitonin receptor by enteric neurons of the embryonic and early post-natal mouse

Calcitonin receptor-immunoreactivity (CTR-ir) was found in enteric neurons of the mouse gastrointestinal tract from embryonic day 13.5 (E13.5) to post-natal day 28 (P28). CTR-ir occurred in cell bodies in ganglia of the myenteric plexus extending from the esophagus to the colon and in nerve cells of the submucosal ganglia of the small and large intestines. CTR-ir was also found in vagal nerve trunks and mesenteric nerves. Counts in the ileal myenteric plexus revealed CTR-ir in 80% of neurons. CTR-ir was clearly evident in the cell bodies of enteric neurons by E15.5. The immunoreactivity reached maximum intensity between P1.5 and P12 but was weaker at P18 and barely detectable at P28. The receptor was detected in nerve processes in the intestine for only a brief period around E17.5, when it was present in one to two axonal processes per villus in the small intestine. In late gestation and soon after birth, CTR-ir was also evident in the mucosal epithelium. The perinatal expression of CTR within the ENS suggests that the calcitonin/CTR system may have a role in the maturation of enteric neurons. Signals may reach enteric neurons in milk, which contains high levels of calcitonin.     

The cloning and over-expression of PABA synthase in E. coli

Both the genes encoding E. coli p-aminobenzoic acid synthase have been cloned and an overproducing strain has been obtained. The partial purification of the large subunit is described. The kinetic properties of the cloned enzyme, while similar to those reported for the B. subtilis enzyme, show some differences to those reported for the S. griseus enzyme.     

Spatio-temporal variability and environmental controls of methane fluxes at the forest–tundra ecotone in the Fennoscandian mountains

We report on temporal and spatial variability in net methane (CH₄) fluxes measured during the thaw period of 1999 and 2000 at three study sites along a c. 8° latitudinal gradient in the Fennoscandian mountain range and across the mountain birch‐tundra ecotone. All of the sites studied here were underlain by well‐drained mesic soils. In addition, we conducted warming experiments in the field to simulate future climate change. Our results show significant CH₄ uptake at mesic sites spanning the forest‐tundra ecotone: on average 0.031 and 0.0065 mg CH₄ m^?2 h^?1 during the 1999 and 2000 thaw periods, respectively, in Abisko (Sweden), and 0.019 and 0.032 mg CH₄ m^?2 h^?1 during 2000 in Dovrefjell and Joatka (Norway), respectively. These values were both temporally and spatially highly variable, and multiple regression analysis of data from Abisko showed no consistent relationship with soil‐moisture status and temperature. Also, there was no consistent difference in CH₄ fluxes between forest and tundra plots; our data, therefore, provide no support for the hypothesis that conversion of tundra to mountain birch forest, or vice versa, would result in a systematic change in the magnitude or direction of net CH₄ fluxes in this region. Experimental warming treatments were associated with a 2.4 °C increase in soil temperatures (5 cm depth) in 1999 in Abisko, but no consistent soil warming was noted at any of the three field locations during 2000. In spite of this, there were significant treatment effects, principally early during the thaw period, with increased CH₄ uptake compared with control (ambient) plots. These results suggest that direct effects of air warming on vegetation processes (e.g. transpiration, root exudation and nutrient assimilation) can influence CH₄ fluxes even in predominantly methanotrophic environments. We conclude that net CH₄ oxidation is significant in these cold, mesic soils and could be strengthened in an environmental change scenario involving a combination of (i) an increase in the length of the thaw period and (ii) increased mean temperatures during this period in combination with decreased soil‐moisture content.     

Plant carbon allocation drives turnover of old soil organic matter in permafrost tundra soils

Carbon cycle feedbacks from permafrost ecosystems are expected to accelerate global climate change. Shifts in vegetation productivity and composition in permafrost regions could influence soil organic carbon (SOC) turnover rates via rhizosphere (root zone) priming effects (RPEs), but these processes are not currently accounted for in model predictions. We use a radiocarbon (bomb‐¹⁴C) approach to test for RPEs in two Arctic tall shrubs, alder (Alnus viridis (Chaix) DC.) and birch (Betula glandulosa Michx.), and in ericaceous heath tundra vegetation. We compare surface CO₂ efflux rates and ¹⁴C content between intact vegetation and plots in which below‐ground allocation of recent photosynthate was prevented by trenching and removal of above‐ground biomass. We show, for the first time, that recent photosynthate drives mineralization of older (>50 years old) SOC under birch shrubs and ericaceous heath tundra. By contrast, we find no evidence of RPEs in soils under alder. This is the first direct evidence from permafrost systems that vegetation influences SOC turnover through below‐ground C allocation. The vulnerability of SOC to decomposition in permafrost systems may therefore be directly linked to vegetation change, such that expansion of birch shrubs across the Arctic could increase decomposition of older SOC. Our results suggest that carbon cycle models that do not include RPEs risk underestimating the carbon cycle feedbacks associated with changing conditions in tundra regions.     

Uptake and efflux of copper-64 in Menkes''-disease and normal continuous lymphoid cell lines.

The accumulation of copper over 2 h by normal lymphoid cells and those from Menkes'-disease patients (Menkes' cells) was found to be biphasic, with an initial phase of rapid uptake, an approach to steady state at around 40-60 min, followed by a further accumulation phase. The accumulation of copper was not diminished by the addition of a variety of metabolic inhibitors, suggesting that copper uptake is not an active process. The presence of carbonyl cyanide m-chlorophenylhydrazone in the culture medium stimulated the uptake and accumulation of copper in both normal and Menkes' cells to the same absolute level. This effect appeared to be specific for copper, since the accumulation of Zn and Cd was unaffected. Menkes' cells did not differ from normal in their initial rate of copper uptake. Analysis of the uptake curve suggested that the membrane transport of copper involves both passive and facilitated diffusion. Initial rate of efflux from the cells was approximated by two methods. Menkes' cells did not appear to be affected in this function. It seems likely that the basic defect in Menkes' disease involves a step in intracellular copper transport rather than the membrane transport of copper.     

Quantifying landscape‐level methane fluxes in subarctic Finland using a multiscale approach

Quantifying landscape‐scale methane (CH₄) fluxes from boreal and arctic regions, and determining how they are controlled, is critical for predicting the magnitude of any CH₄ emission feedback to climate change. Furthermore, there remains uncertainty regarding the relative importance of small areas of strong methanogenic activity, vs. larger areas with net CH₄ uptake, in controlling landscape‐level fluxes. We measured CH₄ fluxes from multiple microtopographical subunits (sedge‐dominated lawns, interhummocks and hummocks) within an aapa mire in subarctic Finland, as well as in drier ecosystems present in the wider landscape, lichen heath and mountain birch forest. An intercomparison was carried out between fluxes measured using static chambers, up‐scaled using a high‐resolution landcover map derived from aerial photography and eddy covariance. Strong agreement was observed between the two methodologies, with emission rates greatest in lawns. CH₄ fluxes from lawns were strongly related to seasonal fluctuations in temperature, but their floating nature meant that water‐table depth was not a key factor in controlling CH₄ release. In contrast, chamber measurements identified net CH₄ uptake in birch forest soils. An intercomparison between the aerial photography and satellite remote sensing demonstrated that quantifying the distribution of the key CH₄ emitting and consuming plant communities was possible from satellite, allowing fluxes to be scaled up to a 100 km² area. For the full growing season (May to October), ~ 1.1–1.4 g CH₄ m^?2 was released across the 100 km² area. This was based on up‐scaled lawn emissions of 1.2–1.5 g CH₄ m^?2, vs. an up‐scaled uptake of 0.07–0.15 g CH₄ m^?2 by the wider landscape. Given the strong temperature sensitivity of the dominant lawn fluxes, and the fact that lawns are unlikely to dry out, climate warming may substantially increase CH₄ emissions in northern Finland, and in aapa mire regions in general.