Nitrate is one of the major sources of nitrogen for higher plants and is taken up from the soil by active transporters coupled with H ϩ across the plasma membrane (PM) of root cells. Nitrate uptake systems have been classified into two groups: low-affinity transport systems (LATS) and high-affinity transport systems (HATS). The LATS contribute to nitrate uptake at high nitrate concentrations above 1 mM whereas the HATS operate at micromolar concentrations of external nitrate and display MichaelisMenten kinetics saturating at 0.2-0.5 mM nitrate. The HATS are further divided into two categories: constitutive HATS (cHATS) and inducible HATS (iHATS), which are significantly affected by the supply of external nitrate. Many studies of the molecular basis of nitrate uptake reveal the existence of two gene families, namely the NRT1 and NRT2 families, which potentially encode for LATS and HATS respectively. NRT2 genes are identified in a variety of organisms including fungi, certain yeasts, green algae, and higher plants ( Unkles et al. 1991;Quesada et al. 1994;Trueman et al. 1996;Pérez et al. 1997;Quesada et al. 1997;Amarasinghe et al. 1998;Zhuo et al., 1999;Araki and Hasegawa 2006;Tsujimoto et al. 2007). In most species, NRT2 genes are members of a multigene family: for example, seven Arabidopsis genes (AtNRT2.1-AtNRT2.7) and four rice genes (OsNRT2.1-OsNRT2.4) have been found in their genomes (Orsel et al. 2002; Araki and Hasegawa 2007), and at least four NRT2 genes (HvNRT2.1-HvNRT2.4) exist in barley (Vidmar 2000a). Amino acid sequences deduced from these genes indicate that the NRT2 proteins are typically 480-510 amino acids in length and predicted to be integral to membranes with 12 transmembrane helices (Forde 2000).It has been well documented that iHATS activity is strongly induced by nitrate supply, and is down-regulated by the accumulation of nitrate assimilation products, especially ammonium and glutamine (Crawford and Glass 1998). In several plant species, it has been shown that a particular member of the NRT2 gene family (e.g., NpNRT2.1 for Nicotiana plumbaginifolia, AtNRT2.1 for Arabidopsis, HvNRT2.1 for barley) contribute to iHATS, because those transcript levels are highly correlated with changes in iHATS activity in such species (Krapp et al. 1998;Lejay et al. 1999;Zhuo et al. 1999;Vidmar et al. 2000a Abstract A high affinity transport system (HATS) for nitrate in plants is operated by a two-component NRT2/NAR2 transport system. However, the regulation and localization of NRT2 and NAR2 at protein level are largely unknown and especially so in crop plant species. In this study with barley (Hordeum vulgare), membrane localization, protein expression in the roots, and a direct protein-protein interaction of HvNRT2 and HvNAR2 proteins were investigated. Immunochemical analysis showed that both HvNRT2 and HvNAR2 proteins were co-localized in the plasma membrane of the roots. Expression of HvNRT2 and HvNAR2 proteins was more strongly induced by treatment with higher concentrations of external nitrate, while H...

The purpose of this study was to determine whether the gamma-aminobutyric acid (GABA) affects the rate of brain protein synthesis in male rats. Two experiments were done on five or three groups of young rats (5 wk) given the diets containing 20% casein administrated 0 mg, 25 mg, 50 mg, 100 mg or 200 mg/100 g body weight GABA dissolved in saline by oral gavage for 1 day (d) (Experiment 1), and given the diets contained 0%, 0.25% or 0.5% GABA added to the 20% casein diet (Experiment 2) for 10 d. The plasma concentration of growth hormone (GH) was the highest in rats administrated 50 mg and 100 mg/100 g body weight GABA. The concentration of serum GABA increased significantly with the supplementation groups. The fractional (Ks) rates of protein synthesis in brain regions, liver and gastrocnemius muscle increased significantly with the 20% casein + 0.25% GABA diet and still more 20% casein + 0.5% GABA compared with the 20% casein diet. In brain regions, liver and gastrocnemius muscle, the RNA activity [g protein synthesized/(g RNA . d)] significantly correlated with the fractional rate of protein synthesis. The RNA concentration (mg RNA/g protein) was not related to the fractional rate of protein synthesis in any organ. Our results suggest that the treatment of GABA to young male rats are likely to increase the concentrations of plasma GH and the rate of protein synthesis in the brain, and that RNA activity is at least partly related to the fractional rate of brain protein synthesis.

SummaryThe purpose of this study was to find whether the synthesis and degradation of

We have shown that urinary urea excretion increased in rats fed a low quality protein. The purpose of present study was to determine whether an addition of dietary limiting amino acids affected urea synthesis in rats fed a low gluten diet. Experiments were done on three groups of rats given diets containing 10% gluten, 10% gluten +0.5% L-lysine or 10% gluten+0.5% L-lysine, 0.2% L-threonine and 0.2% L-methionine for 10 d. The urinary excretion of urea, and the liver concentrations of serine and ornithine decreased with the addition of dietary L-lysine, L-threonine and L-methionine. The fractional and absolute rates of protein synthesis in tissues increased with the treatment of limiting amino acids. The activities of hepatic urea-cycle enzymes was not related to the urea excretion. These results suggest that the addition of limiting amino acids for the low gluten diet controls the protein synthesis in tissues and hepatic ornithine and decline urea synthesis.