EXPRESSION OF AN ARABIDOPSIS ASPARTATE KINASE HOMOSERINE DEHYDROGENASE GENE IS METABOLICALLY REGULATED BY PHOTOSYNTHESIS-RELATED SIGNALS BUT NOT BY NITROGENOUS COMPOUNDS

Although the control of carbon fixation and nitrogen assimilation has been studied in detail, relatively little is known about the regulatio n of carbon and nitrogen flow into amino acids. In this paper we repor t our study of the metabolic regulation of expression of an Arabidopsi s aspartate kinase/homoserine dehydrogenase (AK/HSD) gene, which encod es two linked key enzymes in the biosynthetic pathway of aspartate fam ily amino acids. Northern blot analyses, as well as expression of chim eric AK/HSD-beta-glucuronidase constructs, have shown that the express ion of this gene is regulated by the photosynthesis-related metabolite s sucrose and phosphate but not by nitrogenous compounds. In addition, analysis of AK/HSD promoter deletions suggested that a CTTGACTCTA seq uence, resembling the binding site for the yeast GCN4 transcription fa ctor, is likely to play a functional role in the expression of this ge ne. Nevertheless, longer promoter fragments, lacking the GCN4-like ele ment, were still able to confer sugar inducibility, implying that the metabolic regulation of this gene is apparently obtained by multiple a nd redundant promoter sequences. The present and previous studies sugg est that the conversion of aspartate into either the storage amino aci d asparagine or aspartate family amino acids is subject to a coordinat ed, reciprocal metabolic control, and this biochemical branch point is a part of a larger, coordinated regulatory mechanism of nitrogen and carbon storage and utilization.