Je. Thompson et al., The second naphthol reductase of fungal melanin biosynthesis in Magnaporthe grisea - Tetrahydroxynaphthalene reductase, J BIOL CHEM, 275(45), 2000, pp. 34867-34872
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.