Abstract
The molybdenum cofactor (Mo-co) content of cultured cells of Nicotiana tabacum was estimated by the ability of the acid and heat treated extract to restore NADPH-nitrate reductase (NR) of Neurospora crassa nit-1. Addition of nitrate to wild-type cells grown on amino acids caused a rapid increase in Mo-co activity concomitant with induction of NADH-NR. Time course experiments revealed both the constitutive and the induced levels of Mo-co activity to be dependent on culture age, the maximum occurring during log phase. According to their Mo-co activity, the 12 NR-deficient mutant cell lines tested were shown to fall into 3 groups: Group I nia mutants are characterized by a low Mo-co level, which does not increase either during log phase or after addition of nitrate. Group II nia mutants are similar to wild type. The four cnx mutants tested display a constitutive-high Mo-co level, being not further inducible by nitrate. Possible mechanisms for the common regulation of Mo-co and NR synthesis are discussed.
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