The molybdenum cofactor is shared by nitrate reductase (NR), xanthine
dehydrogenase (XDH), and abscisic acid (ABA) aldehyde oxidase in highe
r plants (M. Walker-Simmons, D.A. Kudrna, R.L. Warner [1989] Plant Phy
siol 90: 728-733). In agreement with this, cnx mutants are simultaneou
sly deficient for these three enzyme activities and have physiological
characteristics of ABA-deficient plants. In this report we show that
aba1 mutants, initially characterized as ABA-deficient mutants, are im
paired in both ABA aldehyde oxidase and XDH activity but overexpress N
R. These characteristics suggest that aba1 is in fact involved in the
last step of molybdenum cofactor biosynthesis specific to XDH and ABA
aldehyde oxidase; aba1 probably has the same function as hxB in Asperg
illus. The significance of NR overexpression in aba1 mutants is discus
sed.