METABOLIC-REGULATION OF THE GENE ENCODING GLUTAMINE-DEPENDENT ASPARAGINE SYNTHETASE IN ARABIDOPSIS-THALIANA

Here, we characterize a cDNA encoding a glutamine-dependent asparagine synthetase (ASN1) from Arabidopsis thaliana and assess the effects of metabolic regulation on ASN1 mRNA levels. Sequence analysis shows tha t the predicted ASN1 peptide contains a purF-type glutamine-binding do main. Southern blot experiments and cDNA clone analysis suggest that A SN1 is the only gene encoding glutamine-dependent asparagine synthetas e in A. thaliana. The ASN1 gene is expressed predominantly in shoot ti ssues, where light has a negative effect on its mRNA accumulation. Thi s negative effect of light on ASN1 mRNA levels was shown to be mediate d, at least in part, via the photoreceptor phytochrome. We also invest igated whether light-induced changes in nitrogen to carbon ratios migh t exert a metabolic regulation of the ASN1 mRNA accumulation. These ex periments demonstrated that the accumulation of ASN1 mRNA in dark-grow n plants is strongly repressed by the presence of exogenous sucrose. M oreover, this sucrose repression of ASN1 expression can be partially r escued by supplementation with exogenous amino acids such as asparagin e, glutamine, and glutamate. These findings suggest that the expressio n of the ASN1 gene is under the metabolic control of the nitrogen to c arbon ratio in cells. This is consistent with the fact that asparagine , synthesized by the ASN1 gene product, is a favored compound for nitr ogen storage and nitrogen transport in dark-grown plants. We have put forth a working model suggesting that when nitrogen to carbon ratios a re high, the gene product of ASN1 functions to re-direct the flow of n itrogen into asparagine, which acts as a shunt for storage and/or long -distance transport of nitrogen.