Regulative fine-tuning of the two novel DAHP isoenzymes aroFp and aroGp ofthe filamentous fungus Aspergillus nidulans

Two novel genes, aroF and aroG, from the filamentous fungus Aspergillus nid ulans were isolated and the regulative fine-tuning between the encoded, dif ferentially regulated 3-deoxy-D-arabino-heptulosonate-7-phosphate (DAHP) sy nthases was analyzed. A wide range of DAHP synthase isoenzymes of various o rganisms are known, but only a few have been characterized further. DAHP sy nthases (EC 4.1.2.15) catalyze the first committed step of the shikimate pa thway, which is a putative target for anti-weed drugs. The reaction is the condensation of erythrose-4-phosphate (E4P) and phosphoenolpyruvate (PEP) t o yield DAHP. The two purified DAHP synthases showed different affinities f or the substrates: 175 muM for PEP and 341 muM for E4P for the aroFp isoenz yme and weaker affinities of 239 muM (PEP) and 475 muM (E4P) for the aroGp isoenzyme. The enzymes are differentially regulated by tyrosine (aroFp) and phenylalanine (aroGp). The calculated k(cat) values are 7.0 s(-1) for the tyrosine-inhibitable (aroFp) and 5.5 s(-1) for the phenylalanine inhibitabl e (aroGp) enzyme. Tyrosine is a competitive inhibitor of the aroFp DAHP syn thase in its reaction with PEP. Phenylalanine is a competitive inhibitor of the isoenzyme aroGp in its reaction with E4P. Both enzymes are inhibited b y the chelating agent EDTA, which indicates a metal ion as cofactor.