The phenotypes of certain mutant strains of were reported to be pleiotropic for nitrate reduction; these strains were selected for their inability to dissimilate nitrate and were found also to have lost the ability to assimilate nitrate. We now report that the isolation procedure selected two mutations, one in genes encoding the synthesis of dissimilatory nitrate reductase ( or ) and another in one of the genes () encoding the synthesis of assimilatory nitrate reductase. Thus in dissimilatory and assimilatory nitrate reductases are genetically distinct. However, a loss of both enzymes is necessary to prevent slow dissimilatory growth on nitrate. Assimilatory nitrate reductase requires molybdenum to function, as does dissimilatory nitrate reductase. Lesions in affect incorporation of molybdenum into both enzymes, and hence exert a pleiotropic effect.


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