Inorganic nitrogen assimilation in the non‐N2‐fixing cyanobacterium Phormidium laminosum. II. Effect of the nitrogen source on the nitrite reductase levels

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Abstract
The effect of different inorganic nitrogen sources on the cellular levels of nitrite reductase (NiR. EC 1.7.7.1) activity has been studied in the filamentous non‐N2‐fixing cyanobacterium Phormidium laminosum (strain OH‐1‐p.Cl,). Nitrate‐grown cells gave the highest NiR in cell‐free extracts [ca 165 nmol of nitrite reduced (mg protein)‐1 min‐], whereas no activity could be detected in extracts from ammonium‐grown cells. The in vivo effect of ammonium on NiR was similar to that exerted by chloramphenicol, suggesting that de novo synthesis of protein was probably repressed by this ion. When ammonium was removed from the culture medium, a rapid increase of de novo synthetized NiR occurred, and the appearance of the enzyme was slightly stimulated by the presence in the medium of either nitrate or nitrite. However, remarkably high levels of NiR [around 1.2 μmol of nitrite reduced (mg protein) −1min−1] could be routinely measured in nitrogen‐deficient cells, indicating that the enzyme was ammonium‐repressible rather than nitrate‐ or nitrite‐inducible.