Reciprocal regulation of distinct asparagine synthetase genes by light andmetabolites in Arabidopsis thaliana

In plants, the amino acid asparagine serves as an important nitrogen transp ort compound whose levers are dramatically regulated by light in many plant species, including Arabidopsis thaliana. To elucidate the mechanisms regul ating the flux of assimilated nitrogen into asparagine, we examined the reg ulation of the gene family for asparagine synthetase in Arabidopsis. In add ition to the previously identified ASN1 gene, we identified a novel class o f asparagine synthetase genes in Arabidopsis (ASN2 and ASN3) by functional complementation of a yeast asparagine auxotroph. The proteins encoded by th e ASN2/3 cDNAs contain a Pur-F type glutamine binding triad suggesting that they, like ASN1, encode glutamine-dependent asparagine synthetase isoenzym es. However, the ASN2/3 iso-enyzmes form a novel dendritic group with monoc ot AS genes which is distinct from all other dicot AS genes including Arabi dopsis ASN1. In addition to these distinctions in sequence, the ASN1 and AS N2 genes are reciprocally regulated by light and metabolites. Time-course e xperiments reveal that light induces revels of ASN2 mRNA while it represses levels of ASN1 mRNA in a kinetically reciprocal fashion. Moreover, the lev els of ASN2 and ASN1 mRNA are also reciprocally regulated by carbon and nit rogen metabolites. The distinct regulation of ASN1 and ASN2 genes combined with their distinct encoded isoenzymes suggest that they may play different roles in nitrogen metabolism, as discussed in this paper.