Transport mechanisms in the ammonium transporter family

Ammonium transport is mediated by membrane proteins of the ubiquitous Amt/Rh family. Despite the availability of different X-ray structures that provide many insights on the ammonium permeation process, the molecular details of its mechanism remain controversial. The X-ray structures have revealed that the pore of the Amt and Rh proteins is characterized by a hydrophobic portion about 12 Å long in which electronic density was observed in crystallographic study of AmtB from Escherichia coli. This electronic density was initially only observed when crystals were grown in presence of ammonium salt and was thus attributed to ammonia (NH₃) molecules, and lead the authors to suggest that the conduction mechanism in the Amt/Rh proteins involves the single-file diffusion of NH₃ molecules. However, other X-ray crystallography results and molecular mechanics simulations suggest that the pore of AmtB could also be filled with water molecules. The possible presence of water molecules in the pore lumen calls for a reassessment of the growing consensus that Amt/Rh proteins work as plain NH₃ channels. Indeed, functional experiments on plant ammonium transporters and rhesus proteins suggest a variety of permeation mechanisms including the passive diffusion of NH₃, the antiport of NH₄⁺/H⁺, the transport of NH₄⁺, or the cotransport of NH₃/H⁺. We discuss these mechanisms in light of some recent functional and simulation studies on the AmtB transporter and illustrate how they can be reconciled with the available high resolution X-ray data.     

Identification and genomovar assignation of clinical strains of Pseudomonas stutzeri

The identification of Pseudomonas stutzeri clinical isolates through conventional phenotypic methods was compared with identification through partial rpoD gene sequencing. We observed that commercial phenotypic systems easily confuse P. stutzeri with other Pseudomonas species. We also demonstrated that most of the clinical strains of P. stutzeri herein studied (79%) belonged to genomovar 1 of the species. We propose the use of partial rpoD gene sequence analysis as a complementary molecular tool for the precise routine identification and genomovar assignation of P. stutzeri clinical isolates, as well as for typing and epidemiological studies.     

Comparative sensitivity and resistance of some strains of Pseudomonas aeruginosa and Pseudomonas stutzeri to antibacterial agents

A comparison has been made of the sensitivities to various antibiotic and non-antibiotic substances of some strains of Pseudomonas aeruginosa and P. stutzeri, the latter including strains isolated from eye and other cosmetic products and from other sources. Whereas P. aeruginosa strains showed a high resistance to cetrimide and to benzalkonium chloride, the P. stutzeri strains were generally more sensitive to these and to chlorhexidine. The P. stutzeri strains were also more sensitive to the various antibiotics tested. The loss of the ability to transfer an R factor by two strains of P. aeruginosa caused no significant change in their drug sensitivity pattern.     

Pseudomonas stutzeri pneumonia and septicemia in a patient with multiple myeloma.

A case of septicemia caused by Pseudomonas stutzeri belonging to the unusual biotype Vb-3 in a patient with multiple myeloma is described. The origin of the septicemia was attributed to a community-acquired pneumonia. The bacteriology and pathogenicity of P. stutzeri are reviewed.     

Physiological role of the putative ammonium transporter RhCG in the mouse.

Ammonium excretion into urine is a major process essential to the regulation of acid-base homeostasis. We have shown that Rh-type proteins, including renal RhCG, belong to the Mep/Amt family of ammonium transporters and promote bi-directional ammonium transport upon heterologous expression in yeast. To study the physiological role of RhCG and to test a potential function in ammonium excretion, we have generated mice bearing an invalidation of the corresponding gene.     

Comparison of aspartate transcarbamoylase regulation in Pseudomonas alcaligenes and Pseudomonas mendocina

The regulation of aspartate transcarbamoylase activity in cell extracts of Pseudomonas alcaligenes ATCC 14909 and Pseudomonas mendocina ATCC 25411 was compared. Under saturating substrate concentrations, pyrophosphate, CTP, UDP and ADP were highly inhibitory of the P. alcaligenes transcarbamoylase activity while pyrophosphate, UDP, ADP, ATP and GTP were the most effective inhibitors of the P. mendocina transcarbamoylase. By examining transcarbamoylase inhibition by ribonucleotide triphosphates, it was possible to differentiate these species assigned to different DNA homology groups and such an analysis might prove useful in the reclassification of Pseudomonas species.     

Genetic regulation of melatonin excretion in urine

The melatonin excretion in urine was determined in 107 individuals from 23 nuclear families. Complex segregation analysis showed that the melatonin production might be regulated by an additive major gene.     

Involvement of the cytoskeleton in regulation of collagen synthesis and secretion

11-day chick embryo skin fibroblasts administered in vitro with colchicine or cytochalasin B, have been investigated for collagen synthesis and secretion. The cytoskeleton, examined by confocal laser scanning immunofluorescence microscope, exibited in colchicine treated cells a clear loss of microtubules, in cytochalasin B treated cells a diffuse staining and patches of intracellular fluorescence caused by actin disassembly. Cytochalasin B treatment decreased endocellular type I collagen synthesis and the total uptake of 3H-proline, while enhanced collagen secretion. Colchicine did not interfere with the collagen secretion and the label uptake, whereas it increased endocellular collagen synthesis. In addition, both drugs induced an increase of the alpha2:alpha1 ratio of collagen chains. The results obtained suggest that the cytoskeleton controls not only the morphological organization of cell components, but also the organization of biochemical processes.     

葡萄糖转运蛋白1在癌症中的表达及其调控

作为葡萄糖溶质载体2A家族的成员,葡萄糖转运蛋白1(glucose transporter 1,Glut1)主要负责调控细胞糖转运及糖代谢。研究发现在癌症中其表达量都有明显提高,为癌细胞增殖、侵袭迁移等多种生命活动提供足够的葡萄糖及ATP,与癌症的发生发展存在密切关系。因此,以Glut1为治疗靶标,干扰癌细胞葡萄糖代谢,将为癌症诊断及其治疗提供新的思路。     

Involvement of different receptors in the regulation of human posture

Compensatory electromyographic (EMG) responses and several biomechanical parameters were studied following impulsive disturbance of the lower limbs during stance on a treadmill. Treadmill acceleration impulses were backwards or forwards directed, or their direction was inverted after 30 ms. Backwards directed impulses were followed by gastrocnemius and forwards directed ones by tibialis anterior EMG responses (latency 65-75 ms) whose duration depended on impulse duration. When the direction of the impulse was inverted, the respective antagonistic leg muscles were activated, with a delay of 68 to 75 ms after onset of stretch of these muscles. The behaviour of the EMG responses could best be correlated to the displacement at the ankle joint and may be described in terms of a stretch reflex response. The function of this stretch reflex mechanism is suggested to be connected with the control of the body's centre of gravity in order to prevent falling. Head movements induced by the impulses showed little correlation with the appearance of the EMG responses, suggesting that the vestibular system is unlikely to be significantly involved in the generation of these responses.     

Identification and characterization of novel competence genes comA and exbB involved in natural genetic transformation of Pseudomonas stutzeri.

After transposon mutagenesis we identified a novel gene (comA) of Pseudomonas stutzeri which is essential for natural genetic transformation. The putative amino acid sequence is similar to ComA orthologs of other transformable bacteria including Neisseria gonorrhoeae (ComA), Haemophilus influenzae (Rec-2), Bacillus subtilis (ComEC) and Streptococcus pneumoniae (CelB). Downstream of comA two partially overlapping open reading frames termed exbB and exbD were found coding for putative proteins similar to proteins required for macromolecule uptake in Escherichia coli and present in other Gram-negative bacteria. Insertional inactivation of exbB decreased the transformability to 20% of that of the wild type. The binding of 3H-labeled DNA and its uptake into a DNase-resistant state in the comA and exbB strains were similar to the wild type, suggesting that these proteins are involved in a late step of transformation, presumably in the translocation of DNA from the periplasm into the cytosol. The question of whether the translocation process occurs separately from the step of single-strand formation is discussed.     

Role of glutamine synthetase activity in the urea regulation of heterocyst and nitrogenase formation in the cyanobacterium Anabaena cycadeae

Growth and urea regulation of heterocyst and nitrogenase formation have been studied in the diazotrophic cyanobacterium Anabaena cycadeae and its glutamine auxotrophic mutant strain lacking glutamine synthetase (GS) activity. The parent strain having normal GS activity utilized urea as sole nitrogen source without producing N₂-fixing heterocysts. In contrast, the mutant strain lacking GS activity could not utilize urea as sole nitrogen source although similar to the parent strain it lacked N₂-fixing heterocysts in urea-medium. Both parent and mutant strain showed high levels of urea uptake and urease activity in the presence and absence of a GS inhibitor, l-methionine-dl-sulphoximine (MSX). Urea-dependent NH production by MSX-untreated cells was only confined to the mutant strain lacking GS activity whereas the parent strain having normal GS activity produced NH only in the presence of MSX. These results suggest that (i) GS is the sole NH assimilating as well as glutamine-forming route in heterocystous N₂-fixing cyanobacteria; (ii) while GS activity is necessarily required for the assimilation of urea as sole nitrogen source, it is however, not required for the urea inhibition of heterocysts and nitrogenase formation; (iii) that NH resulting from the hydrolysis of urea and not GS-mediated pathway of NH assimilation appears to be the initial repressor signal for heterocysts and nitrogenase formation; and (iv) GS does not control the formation of N₂-fixing heterocysts, urea uptake and urease activity systems.     

Role of hepatic opioid receptors in the regulation of bile excretion

Experiments on isolated rat liver perfused with Ringer-Krebs bicarbonate buffer showed that stimulation of δ-opiate receptors in this organ increases bile flow rate and taurocholate secretion. Stimulation of μ-opiate receptors decelerated bile production and inhibited taurocholate secretion. Acceleration of bile production and stimulation of taurocholate secretion under the influence of dalargin is probably related to its interaction with δ-opiate receptors in the liver. __________ Translated from Byulleten’ Eksperimental’noi Biologii i Meditsiny, Vol. 142, No. 11, pp. 494–496, November, 2006