Studies on temperature-sensitive mutants affecting the assimilatory nitrate reductase of Aspergillus nidulans.

Abstract

In the ascomycete fungus, Aspergillus nidulans, mutations in the niaD gene and in five unlinked cnx genes lead to the loss of assimilatory nitrate reductase, and the inability to grow on nitrate as sole nitrogen source. Mutants have been isolated in the niaD gene and in three of the five cnx genes which are temperature‐sensitive for growth on nitrate and have nitrate reductase activity when grown at the permissive temperature. The half life of nitrate reductase in the wild‐type and in these temperature‐sensitive strains has been measured. Temperature‐sensitive mutants in the cnxE and cnxF genee havs a nitrate reductase with the same half life as in wild‐type strains. Temperature‐sensitive mutants in the niaD and cnxH genes have a nitrate reductase with a shorter half life than the wild‐type enzyme. These results can be interpreted as indicating that the products of the niaD and cnxH genes are structurally involved in the nitrate reductase molecule, whereas the products of the cnxE and cnxF genes are involved in the formation of a cofactor required for nitrate reductase activity.