Reverse mutants of the nitrate reductase-deficient mutant B 25 of Arabidopsis thaliana

Revertants of B25, a nitrate reductase-deficient mutant of Arabidopsis thaliana (L.) Heinh, were isolated with a high frequency. All 7 independently arisen revertants were mutations in the same suppressor gene su, which is unlinked to the originally mutated gene rgn. The mutant character shows up both in growth on nitrate as the sole nitrogen source and in susceptibility to chlorate. When judged for these properties the mutant alleles are either dominant for both, recessive for both or dominant for growth on nitrate and recessive for the effect of chlorate, when compared to the wildtype allele. Whereas the original mutant B25 exhibits no or very little nitrate reductase activity, the activities of the revenants were in the range of 0.4 to 1.5 of the wildtype activity. Physiological characteristics of nitrate reductase from the revertants are the same as those from the wildtype. Probably rgn is not the structural gene for nitrate reductase. The ability to assemble the nitrate reductase complex from its subunits, which was absent in mutant B25, appears to have been restored in the revertants.