Abstract
Two molybdenum cofactor (MoCo)-deficient mutants, C290 and C384, were isolated from 204, 500 M2 seedlings of rice (Oryza sativa L., ssp. indica, cv. IR30) by means of chlorate-resistance. The two mutations were monogenic and recessive. Biochemical analysis showed that the activities of nitrate reductase (NR) and xanthine dehydrogenase (XDH) in the mutants were considerably lower than in the wild type IR30, which indicated that the two mutants were actually MoCo-deficient types. On the other hand, nitrite reductase (NiR) activity and nitrate content in the two mutants were higher than IR30. An F1 strain of cross between the mutants was mutant, indicating that the mutations were allelic. However, the mutants were able to complement the defects in two previously isolated mutants involved in the biosynthesis of MoCo, cnx1 and cnx2. Therefore, genes carried by C290 and C384 define a new locus, which was designated c__ofactor for n__itrate reductase and x__anthine dehydrogenase 3 (cnx 3). In contrast to cnx1 mutant, adding 0.5 mM molybdate into growth medium did not recover NADH-NR activity in C290 and C384. Therefore, the cnx 3 gene presumably encodes a different step from cnx1 in the MoCo biosynthetic pathway.
