Induction of arginase and ornithine transaminase in the fission yeast Schizosaccharomyces pombe

Regulation of the nitrogen transfer pathway in the arbuscular mycorrhizal symbiosis: gene characterization and the coordination of expression with nitrogen flux

The arbuscular mycorrhiza (AM) brings together the roots of over 80% of land plant species and fungi of the phylum Glomeromycota and greatly benefits plants through improved uptake of mineral nutrients. AM fungi can take up both nitrate and ammonium from the soil and transfer nitrogen (N) to host roots in nutritionally substantial quantities. The current model of N handling in the AM symbiosis includes the synthesis of arginine in the extraradical mycelium and the transfer of arginine to the intraradical mycelium, where it is broken down to release N for transfer to the host plant. To understand the mechanisms and regulation of N transfer from the fungus to the plant, 11 fungal genes putatively involved in the pathway were identified from Glomus intraradices, and for six of them the full-length coding sequence was functionally characterized by yeast complementation. Two glutamine synthetase isoforms were found to have different substrate affinities and expression patterns, suggesting different roles in N assimilation. The spatial and temporal expression of plant and fungal N metabolism genes were followed after nitrate was added to the extraradical mycelium under N-limited growth conditions using hairy root cultures. In parallel experiments with (15)N, the levels and labeling of free amino acids were measured to follow transport and metabolism. The gene expression pattern and profiling of metabolites involved in the N pathway support the idea that the rapid uptake, translocation, and transfer of N by the fungus successively trigger metabolic gene expression responses in the extraradical mycelium, intraradical mycelium, and host plant.

Arginine catabolism in the phototrophic bacterium Rhodobacter capsulatus E1F1. Purification and properties of arginase

The phototrophic bacterium Rhodobacter capsulatus E1F1 grew with L-arginine or L-homoarginine as nitrogen source under light/anaerobiosis. However, when L-arginine was used as the only source of both carbon and nitrogen, the bacterium exhibited weak growth levels and the excess of nitrogen was excreted to the medium as ammonia. By contrast, L-ornithine was used under phototrophic conditions as either nitrogen or carbon source. Other compounds of the arginine catabolic pathways, such as putrescine or proline, also supported phototrophic growth of this bacterium. Under heterotrophic/dark conditions, R. capsulatus always showed a low growth rate with those nitrogen compounds. Cells growing on media containing L-arginine, L-homoarginine or L-ornithine induced an Mn(2+)-dependent arginase activity regardless of the presence of ammonium ions and other readily utilizable nitrogen sources. Arginase activity was strongly inhibited by Zn2+, Cu2+, borate, L-cysteine, L-ornithine and gamma-guanidinobutyrate. Mercurials also inactivated arginase, the activity being partially restored by the presence of thiols. Arginase was purified to electrophoretic homogeneity and found to consist of four identical subunits of 31 kDa. The molecular parameters and kinetic constants of arginase from R. capsulatus E1F1 resembled those previously described for the Saccharomyces cerevisiae enzyme rather than those of bacterial arginases.