Molecular Cloning of the Gene Encoding the L-Asparaginase Gene of Arabidopsis thaliana

In plants, ammonium is first assimilated into the amino acids L-Gln and L-glutamate and then channeled into L-aspar- tate and L-Asn. These amino acids serve as nitrogen donors in biosynthetic reactions, as well as compounds for intercellular nitrogen transport. In amide-transporting plants, Asn can act as the sole nitrogen source for the growth of plant tissues. In higher plants the catabolism of Asn is thought to occur mainly through two routes. The first pathway involves the hydrolysis of Asn, releasing ammonia and aspartate by as- paraginase activity. The second pathway proceeds via transamination in a reaction catalyzed by the asparaginase:2- oxoacid transaminase (Lea et al., 1990). L-Asparaginase is a key enzyme for Asn utilization by plants that plays an im- portant role in the nitrogen metabolism of developing plant tissues (Sieciechowicz et al., 1988). The ammonia released from the hydrolysis of the translocated Asn is utilized in the synthesis of all nitrogen-containing compounds of the cell and in particular in the synthesis of the amino acids needed