A NOVEL AUTOPHOSPHORYLATION MEDIATED REGULATION OF NITRITE REDUCTASE IN CANDIDA-UTILIS

The assimilatory nitrite reductase catalyses the conversion of nitrite to ammonia. The enzyme from Candida utilis has been previously purifi ed to homogeneity and shown to be a heterodimer consisting of 58 kDa a nd 66 kDa subunits. The enzyme has also been shown to be induced by ni trate and repressed by ammonium ions. The levels of nitrite reductase mRNA, its protein and the enzyme activity were modulated together indi cating that the primary level of regulation of this enzyme existed at the transcriptional level. Here me report that the 58 kDa and 66 kDa s ubunits of the enzyme were differentially phosphorylated under the ind uced and repressed conditions, indicating a second level of regulation . The highly phosphorylated 66 kDa subunit was shown to be dephosphory lated by calf intestinal alkaline phosphatase. The enzymatic activity associated with the native enzyme also decreased due to the dephosphor ylation. Each of the subunits could undergo autophosphorylation at ser ine/threonine residues as demonstrated by thin layer chromatography an d recognition by antibodies to phosphoamino acids. The presence of sim ilar phosphorylated subunits under in vivo conditions has also been de monstrated. A model has been proposed to explain the posttranslational regulation of the enzyme. (C) 1997 Federation of European Biochemical Societies.