An asymmetric distribution of positrons in the Galactic disk revealed by gamma-rays.

Gamma-ray line radiation at 511 keV is the signature of electron-positron annihilation. Such radiation has been known for 30 years to come from the general direction of the Galactic Centre, but the origin of the positrons has remained a mystery. Stellar nucleosynthesis, accreting compact objects, and even the annihilation of exotic dark-matter particles have all been suggested. Here we report a distinct asymmetry in the 511-keV line emission coming from the inner Galactic disk ( approximately 10-50 degrees from the Galactic Centre). This asymmetry resembles an asymmetry in the distribution of low mass X-ray binaries with strong emission at photon energies >20 keV ('hard' LMXBs), indicating that they may be the dominant origin of the positrons. Although it had long been suspected that electron-positron pair plasmas may exist in X-ray binaries, it was not evident that many of the positrons could escape to lose energy and ultimately annihilate with electrons in the interstellar medium and thus lead to the emission of a narrow 511-keV line. For these models, our result implies that up to a few times 10(41) positrons escape per second from a typical hard LMXB. Positron production at this level from hard LMXBs in the Galactic bulge would reduce (and possibly eliminate) the need for more exotic explanations, such as those involving dark matter.

Interaction of sulfate assimilation with carbon and nitrogen metabolism in Lemna minor.

Cysteine synthesis from sulfide and O-acetyl-L-serine (OAS) is a reaction interconnecting sulfate, nitrogen, and carbon assimilation. Using Lemna minor, we analyzed the effects of omission of CO(2) from the atmosphere and simultaneous application of alternative carbon sources on adenosine 5'-phosphosulfate reductase (APR) and nitrate reductase (NR), the key enzymes of sulfate and nitrate assimilation, respectively. Incubation in air without CO(2) led to severe decrease in APR and NR activities and mRNA levels, but ribulose-1,5-bisphosphate carboxylase/oxygenase was not considerably affected. Simultaneous addition of sucrose (Suc) prevented the reduction in enzyme activities, but not in mRNA levels. OAS, a known regulator of sulfate assimilation, could also attenuate the effect of missing CO(2) on APR, but did not affect NR. When the plants were subjected to normal air after a 24-h pretreatment in air without CO(2), APR and NR activities and mRNA levels recovered within the next 24 h. The addition of Suc and glucose in air without CO(2) also recovered both enzyme activities, with OAS again influenced only APR. (35)SO(4)(2-) feeding showed that treatment in air without CO(2) severely inhibited sulfate uptake and the flux through sulfate assimilation. After a resupply of normal air or the addition of Suc, incorporation of (35)S into proteins and glutathione greatly increased. OAS treatment resulted in high labeling of cysteine; the incorporation of (35)S in proteins and glutathione was much less increased compared with treatment with normal air or Suc. These results corroborate the tight interconnection of sulfate, nitrate, and carbon assimilation.

Effect of glucose on assimilatory sulphate reduction in Arabidopsis thaliana roots.

With the aim of analysing the relative importance of sugar supply and nitrogen nutrition for the regulation of sulphate assimilation, the regulation of adenosine 5'-phosphosulphate reductase (APR), a key enzyme of sulphate reduction in plants, was studied. Glucose feeding experiments with Arabidopsis thaliana cultivated with and without a nitrogen source were performed. After a 38 h dark period, APR mRNA, protein, and enzymatic activity levels decreased dramatically in roots. The addition of 0.5% (w/v) glucose to the culture medium resulted in an increase of APR levels in roots (mRNA, protein and activity), comparable to those of plants kept under normal light conditions. Treatment of roots with d-sorbitol or d-mannitol did not increase APR activity, indicating that osmotic stress was not involved in APR regulation. The addition of O-acetyl-l-serine (OAS) also quickly and transiently increased APR levels (mRNA, protein, and activity). Feeding plants with a combination of glucose and OAS resulted in a more than additive induction of APR activity. Contrary to nitrate reductase, APR was also increased by glucose in N-deficient plants, indicating that this effect was independent of nitrate assimilation. [35S]-sulphate feeding experiments showed that the addition of glucose to dark-treated roots resulted in an increased incorporation of [35S] into thiols and proteins, which corresponded to the increased levels of APR activity. Under N-deficient conditions, glucose also increased thiol labelling, but did not increase the incorporation of label into proteins. These results demonstrate that (i) exogenously supplied glucose can replace the function of photoassimilates in roots; (ii) APR is subject to co-ordinated metabolic control by carbon metabolism; (iii) positive sugar signalling overrides negative signalling from nitrate assimilation in APR regulation. Furthermore, signals originating from nitrogen and carbon metabolism regulate APR synergistically.

Flux control of sulphate assimilation in Arabidopsis thaliana: adenosine 5'-phosphosulphate reductase is more susceptible than ATP sulphurylase to negative control by thiols.

The effect of externally applied L-cysteine and glutathione (GSH) on ATP sulphurylase and adenosine 5'-phosphosulphate reductase (APR), two key enzymes of assimilatory sulphate reduction, was examined in Arabidopsis thaliana root cultures. Addition of increasing L-cysteine to the nutrient solution increased internal cysteine, gamma-glutamylcysteine and GSH concentrations, and decreased APR mRNA, protein and extractable activity. An effect on APR could already be detected at 0.2 mm L-cysteine, whereas ATP sulphurylase was significantly affected only at 2 mm L-cysteine. APR mRNA, protein and activity were also decreased by GSH at 0.2 mm and higher concentrations. In the presence of L-buthionine-S, R-sulphoximine (BSO), an inhibitor of GSH synthesis, 0.2 mm L-cysteine had no effect on APR activity, indicating that GSH formed from cysteine was the regulating substance. Simultaneous addition of BSO and 0.5 mm GSH to the culture medium decreased APR mRNA, enzyme protein and activity. ATP sulphurylase activity was not affected by this treatment. Tracer experiments using (35)SO(4)(2-) in the presence of 0.5 mm L-cysteine or GSH showed that both thiols decreased sulphate uptake, APR activity and the flux of label into cysteine, GSH and protein, but had no effect on the activity of all other enzymes of assimilatory sulphate reduction and serine acetyltransferase. These results are consistent with the hypothesis that thiols regulate the flux through sulphate assimilation at the uptake and the APR step. Analysis of radioactive labelling indicates that the flux control coefficient of APR is more than 0.5 for the intracellular pathway of sulphate assimilation. This analysis also shows that the uptake of external sulphate is inhibited by GSH to a greater extent than the flux through the pathway, and that the flux control coefficient of APR for the pathway, including the transport step, is proportionately less, with a significant share of the control exerted by the transport step.