Identification of a regulatory phosphorylation site in the hinge 1 region of nitrate reductase from spinach (Spinacea oleracea) leaves

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Abstract
Purified nitrate reductase (NR) from spinach leaves was phosphorylated in vitro by NR‐inactivating kinase on Ser‐543 which is located in the hinge 1 region between the molybdenum‐cofactor and haem‐binding domains. Phosphorylation of Ser‐543 allowed NR to be inhibited by the inhibitor, NIP. Degraded NR preparations in which a proportion of the subunits had lost 45 amino acids from the N‐terminus during purification could be phosphorylated by NR kinase on Ser‐543, but could not subsequently be fully inhibited by NIP, suggesting a role for the N‐terminal tail of NR in NIP binding.