The second naphthol reductase of fungal melanin biosynthesis in Magnaporthe grisea - Tetrahydroxynaphthalene reductase

Mutants of Magnaporthe grisea harboring a defective gene for 1,3,8-trihydro xynaphthalene reductase retain the capability to produce scytalone, thus su ggesting the existence of a second naphthol reductase that can catalyze the reduction of 1,3,6,8-tetrahydroxynaphthalene to scytalone within the funga l melanin biosynthetic pathway. The second naphthol reductase gene was clon ed from M. grisea by identification of cDNA fragments with weak homology to the cDNA of trihydroxynaphthalene reductase. The amino acid sequence for t he second naphthol reductase is 46% identical to that of trihydroxynaphthal ene reductase. The second naphthol reductase was produced in Esherichia col i and purified to homogeneity. Substrate competition experiments indicate t hat the second reductase prefers tetrahydroxynaphthalene over trihydroxynap hthalene by a factor of 310; trihydroxynaphthalene reductase prefers trihyd roxynaphthalene over tetrahydroxynaphthalene by a factor of 4.2. On the bas is of the 1300-fold difference in substrate specificities between the two r eductases, the second reductase is designated tetrahydroxynaphthalene reduc tase, Tetrahydroxynaphthalene reductase has a 200-fold larger K-i for the f ungicide tricyclazole than that of trihydroxynaphthalene reductase, and thi s accounts for the latter enzyme being the primary physiological target of the fungicide. M. grisea mutants lacking activities for both trihydroxynaph thalene and tetrahydroxynaphthalene reductases do not produce scytalone, in dicating that there are no other metabolic routes to scytalone.