In Arabidopsis thaliana, blocking histidine biosynthesis with a specific inhibitor of imidazoleglycerolphosphate dehydratase caused increased expression of eight genes involved in the biosynthesis of aromatic amino acids, histidine, lysine, and purines. A decrease in expression of glutamine synthetase was also observed. Addition of histidine eliminated the gene-regulating effects of the inhibitor, demonstrating that the changes in gene expression resulted from histidine-pathway blockage. These results show that plants are capable of cross-pathway metabolic regulation.

cDNA clones encoding imidazoleglycerolphosphate dehydratase (ICPD; EC 4.2.1.19) from Arabidopsis thaliana were isolated by complementation of a bacterial auxotroph. The predicted primary translation product shared significant identity with the corresponding sequences from bacteria and fungi. As in yeast, the plant enzyme is monofunctional, lacking the histidinol phosphatase activity present in the Escherichia coli protein. ICPD mRNA was present in major organs at ali developmental stages assayed. The Arabidopsis genome appears to contain two genes encoding this enzyme, based on DNA gel blot and polymerase chain reaction analysis.

lmidazoleglycerolphosphate dehydratase (ICPD; EC 4.2.1.1 9), which is involved in the histidine biosynthetic pathway of Arabidopsis fhaliana and wheat (Triticum aestivum), has been expressed in insect cells using the baculovirus expression vector system. Nterminal amino acid sequencing indicated that recombinant IGPDs (rlCPDs) were produced as mature forms via nonspecific proteolytic cleavages in the putative transit peptide region. The wheat rlCPD contained one M n atom per subunit, and the M n was involved in the assembly of the subunits to form active ICPDs. Protein-blotting analysis, using antibodies raised against the wheat rlCPD, indicated that ICPD was located in the chloroplasts of wheat. The rlCPDs of Arabidopsis and wheat, which were 86% identical in their primary structures deduced from the cDNAs, exhibited similar properties in terms of the molecular mass, pH optimum, and the K,,, for the substrate, imidazoleglycerolphosphate. However, the nonselective herbicides 3-amino-l,2,4-triazole and a newly synthesized triazole [(l R', 3Re)-[3-hydroxy-3-(2H-[1,2,4]triazole-3-yl)-cyclohexyl]-phosphonic acid], inhibited Arabidopsis and wheat ICPDs in a mixed-type and a competitive manner, respectively.

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