The impact of dark NH4+ and N03assimilation on photosynthetic light harvesting capability of the green alga Selenastrum minutum was monitored by chlorophyll a fluorescence analysis. When cells assimilated NH4+, they exhibited a large decline in the variable fluorescence/maximum fluorescence ratio, the fluorescence yield of photosystem 11 relative to that of photosystem I at 77 kelvin, and 02 ev...

Humid tropical forests are generally characterized by the lack of nitrogen (N) limitation to net primary productivity, yet paradoxically have high potential for N loss. We conducted an intensive field experiment with NH4 and NO3 additions to highly weathered tropical forest soils in Puerto Rico to determine the relative importance of N retention and loss mechanisms. Over one-half of all the NH4...

The mechanisms involved in regulating high-affinity ammonium (NH4+) uptake and the expression of the AtAMT1 gene encoding a putative high-affinity NH4+ transporter were investigated in the roots of Arabidopsis thaliana. Under conditions of steady-state nitrogen (N) supply, transcript levels of the AtAMT1 gene and Vmax values for high-affinity 13NH4+ influx were inversely correlated with levels ...

Increased brain ammonium (NH4(+)/NH3) plays a central role in the manifestation of hepatic encephalopathy (HE), a complex syndrome associated with neurological and psychiatric alterations, which is primarily a disorder of astrocytes. Here, we analysed the influence of NH4(+)/NH3 on the calcium concentration of astrocytes in situ and studied the underlying mechanisms of NH4(+)/NH3-evoked calcium...

Ammonium is a nitrogen source supporting growth of yeast cells at an optimal rate. We recently reported the first characterization of an NH4+ transport protein (Mep1p) in Saccharomyces cerevisiae. Here we describe the characterization of two additional NH4+ transporters, Mep2p and Mep3p, both of which are highly similar to Mep1p. The Mep2 protein displays the highest affinity for NH4+ (Km, 1 to...

Nakhoul, Nazih L., Kathleen S. Hering-Smith, Solange M. Abdulnour-Nakhoul, and L. Lee Hamm. Transport of NH3/NH4 1 in oocytes expressing aquaporin-1. Am J Physiol Renal Physiol 281: F255–F263, 2001.—The aim of this study was to determine whether expressing aquaporin (AQP)-1 could affect transport of NH3. Using ion-selective microelectrodes, the experiments were conducted on frog oocytes (cells ...

Rhbg is a nonerythroid membrane glycoprotein belonging to the Rh antigen family. In the kidney, Rhbg is expressed at the basolateral membrane of intercalated cells of the distal nephron and is involved in NH4+ transport. We investigated the substrate specificity of Rhbg by comparing transport of NH3/NH4+ with that of methyl amine (hydrochloride) (MA/MA+), often used to replace NH3/NH4+, in oocy...

Ammonia is an abundant fermentation product in the forestomachs of ruminants and the intestine of other species. Uptake as NH3 or NH4+ should modulate cytosolic pH and sodium-proton exchange via Na+/H+ exchanger (NHE). Transport rates of Na+, NH4+, and NH3 across the isolated rumen epithelium were studied at various luminal ammonia concentrations and pH values using the Ussing chamber method. T...

A positive selection method for isolation of nitrogenase-derepressed mutant strains of a filamentous cyanobacterium, Anabaena variabilis, is described. Mutant strains that are resistant to a glutamate analog, L-methionine-D,L-sulfoximine, were screened for their ability to produce and excrete NH4' into medium. Mutant strains capable of producing nitrogenase in the presence of NH4' were selected...