Tungsten can be substituted for molybdenum in the molybdenum cofactor of nitrate reductase, resulting in inactive enzyme (Deng et al, 1989).

In Nicotiana tabacum plants supplied with tungstate, NR activity declines, whereas NR protein accumulates. NR mRNA level normally exhibits a day-night fluctuation; in tungstate fed plants, this day-night fluctuation was abolished --- NR mRNA level remained high (Deng et al, 1989).

Substitution of molybdenum with tungsten may lead to an over-expression of the NR structural gene, possibly by eliminating a negative feedback control effect of one or several of the metabolites derived from the catalytic action of nitrate reduction, such as nitrite, ammonium or glutamine (Deng et al, 1989).

Deng M, Moureaux T, Caboche M 1989 Tungstate, a molybdate analog inactivating nitrate reductase, deregulates the expression of the nitrate reductase structural gene. Plant Physiol. 91: 304-309.