Biological functions of asparagine synthetase in plants

Ammonium is a form of inorganic nitrogen derived from several metabolic pathways, and is assimilated into glutamine, glutamate, asparagine and carbamoylphosphate. These molecules play important roles in nitrogen assimilation, recycling, transport and storage in plants. Ammonium assimilation into asparagine is catalyzed by ammonia-dependent asparagine synthetase encoded by asnA (EC 6.3.1.1) or glutamine-dependent asparagine synthetase encoded by asnB (EC 6.3.5.4) in prokaryotes and eukaryotes. These organisms display a distinct distribution of these two forms of asparagine synthetase. Gene and primary protein structure for asparagine synthetase-A and -B from prokaryotes and eukaryotes is examined. Using nucleotide sequences, we constructed a phylogenetic tree that distinguished two major classes (classes I and II) for ASN genes from a range of organisms. Only the glutamine-dependent asparagine synthetases-B have been identified, and are encoded by a small multigene family in plants. The isoenzyme encoded by each member of the gene family provides asparagine at specific phases of development. These include the nitrogen mobilization in germinating seeds, nitrogen recycling in vegetative organs in response to stress, and nitrogen remobilization during seed embryogenesis. The expression of genes for asparagine synthetase is regulated by light and metabolites. Genetic and molecular data using mutants and transgenic plants have provided insights into the light perception by the photoreceptors, carbon and nitrogen sensing and signal transduction mechanism in the asn regulation. Global analysis of carbon and nitrogen metabolites supports the impact of asn regulation in the synthesis and transport of asparagine in plants.