Metabolic regulation of asparagine synthetase gene expression in maize (Zea mays L.) root tips

Differential hybridization of a cDNA library constructed with poly(A)+ mRNA from 24 h starved maize (Zea mays L.) root tips, resulted in the isolation of a cDNA (called pZSS1) that was highly induced during glucose deprivation. The nucleotide sequence analysis of the full-length cDNA allowed its identification by comparison with sequence data bases. The 586 amino acid sequence encoded by pZSS1 was shown to be about 60% identical to sequences of asparagine synthetases (EC 6.3.5.4) from Asparagus officinalis, Pisum sativum, Arabidopsis thaliana and Brassica oleracea. Southern blot analysis of maize genomic DNA showed that asparagine synthetase may be encoded by at least two genes. The use of a specific probe for the 3' untranslated region of pZSS1 in Northern blot experiments, revealed that the isolated AS gene was essentially expressed in roots of maize seedlings. Time course analysis revealed that maximal expression of the gene corresponding to pZSS1 occurs between 18 and 24 h after the onset of the starvation treatment. The steady-state levels of transcripts in maize root tips were found to change under various incubation conditions. Exogenous supply of metabolizable sugars downregulated the gene expression, while carbohydrate deprivation or feeding with non-metabolizable sugars resulted in the induction of gene expression. In addition to carbohydrate deprivation, the effects of nitrogen metabolite supply and stress conditions indicate that gene expression might be under metabolic control in maize root tips. The intracellular nitrogen to carbon ratio might be an important factor for the regulation of asparagine synthetase gene expression.