title: Negative regulation of nitrate reductase gene expression by glutamine or asparagine accumulating in leaves of sulfur-deprived tobacco.
abstract: Tobacco (Nicotiana tabacum L.) plants were subjected to a prolonged period of sulfur-deprivation to characterize molecular and metabolic mechanisms that permit control of primary N-metabolism under these conditions. Prior to the appearance of chlorotic lesions, sulfur-deprived tobacco leaves showed a strong decrease in the sulfate content and changes in foliar enzyme activities, mRNA accumulation and amino-acid pools. The basic amino acids glutamine, asparagine and arginine accumulated in the leaves of sulfur-deprived plants, while the foliar concentrations of aspartate, glutamate, serine or alanine remained fairly unchanged. Maximal extractable nitrate reductase (NR; EC 1.6.6.1) activity decreased strongly in response to sulfur-deprivation. The decrease in maximal extractable NR activity was accompanied by a decline in NR transcripts while the mRNAs of the plastidic glutamine synthetase (EC 6.1.3.2) or the beta-subunit of the mitochondrial ATP synthase were much less affected. Nitrate first accumulated in leaves of tobacco during sulfur-deprivation but then declined. An appreciable amount of nitrate was, however, present in severely sulfur-depleted leaves. The repression of NR gene expression is, therefore, not related to the decrease in the leaf nitrate level. However, glutamine- and/or asparagine-mediated repression of NR gene transcription is a possible mechanism of control in situations when glutamine and asparagine accumulate in leaves and provides a feasible explanation for the reduction in NR activity during sulfur-deprivation. The removal of reduced nitrogen from primary metabolism by redirection and storage as arginine, asparagine or glutamine combined with the down-regulation of nitrate reduction via glutamine- and/or asparagine-mediated repression of NR gene transcription may contribute to maintaining a normal N/S balance during sulfur-deprivation and indicate that the co-ordination of N- and S-metabolism is retained under these conditions.
authors.affiliation: Lehrstuhl für Genetik, Fakultät für Biologie, Universität Bielefeld, Germany.
authors.forename: A
authors.initials: A
authors.lastname: Migge
authors.forename: C
authors.initials: C
authors.lastname: Bork
authors.forename: R
authors.initials: R
authors.lastname: Hell
authors.forename: T W
authors.initials: TW
authors.lastname: Becker
mesh_terms: D001216:Asparagine / Q000502:physiology*; D015971:Gene Expression Regulation, Enzymologic / Q000502:physiology*; D018506:Gene Expression Regulation, Plant / Q000502:physiology*; D005973:Glutamine / Q000502:physiology*; D050901:Nitrate Reductase; D009565:Nitrate Reductases / Q000235:genetics*; D018515:Plant Leaves / Q000201:enzymology; D010947:Plants, Toxic; D013455:Sulfur / Q000378:metabolism*; D014026:Nicotiana / Q000201:enzymology* / Q000378:metabolism
chemical_list: D005973:Glutamine; D001216:Asparagine; D013455:Sulfur; D009565:Nitrate Reductases; D050901:Nitrate Reductase