A mutation in Aspergillus nidulans which affects the regulation of nitrite reductase and is tightly linked to its structural gene

Summary

The selection of nis-5, a mutation which is tightly linked to the structural genes for nitrate reductase (niaD) and nitrite reductase (niiA) but which only affects nitrite reductase activities, is described. nis-5 single mutants have only 40% of the wild type activity of nitrite reductase after induction by nitrate and, for this reason, grow poorly on nitrate and nitrite. Nitrate reductase activity is not affected, and nis-5 is shown to complement with a niaD^- mutation but not with a niiA^- mutation.

When grown without inducer, nis-5 strains have higher than the non-induced wild type activity of nitrite reductase. This low, constitutive activity is insensitive to repression by ammonium. These facts explain why the nis-5 mutation weakly suppresses many nirA^- and areA^r mutations for utilization of nitrite.

Three of the possible explanations of this unusual phenotype are considered. Studies of nitrite reductase in cell-free extracts provided no evidence for the already unlikely possibility that nis-5 is a structural gene mutation resulting in the observed phenotype because of alteration in the catalytic activity and/or stability of the nitrite reductase.

A more plausible explanation is that it defines a receptor site for either the nirA gene product and/or the areA gene product. However, no evidence for this has yet been obtained from a study of double mutants carrying nis-5 and areA or nirA mutations.

A third possibility is that nis-5 creates a new, but inefficient promoter or ‘initiator’, which is not subject to the normal control systems (and therefore causes constitutive, deprepressed synthesis) but whose physical presence reduces maximal enzyme synthesis. The presence of a translocation in nis-5 strains suggests a means by which niiA could come to be under the control of another promoter/initiator.