Apparent dependence of the light activation of nitrate reductase and sucrose-phosphate synthase activities in spinach leaves on protein synthesis

The activity of spinach leaf nitrate reductase (NR) responds rapidly and reversibly to light/dark transitions by a mechanism which is correlated with changes in protein phosphorylation (J.L. Huber, S.C. Huber, W.H. Campbell and M.G. Redinbaugh (1992) Arch. Biochem. Biophys. in press). Phosphorylation of the NR protein appears to increase sensitivity of the enzyme to Mg²⁺ inhibition, without affecting activity in the absence of Mg²⁺. In the present study, we have compared the light/dark modulation of sucrose-phosphate synthase (SPS; also known to be regulated by protein phosphorylation) and NR (assayed±Mg²⁺) in spinach leaves exposed to extended darkness and treated with inhibitors of gene expression. When intact plants were subjected to extended darkness, the capacity for light-dependent increase in NR protein and activation, inferred (indirectly) from the enzyme activities with and without Mg²⁺ declined even though the capacity for photosynthesis and activation of SPS remained constant. The component of the in situ light activation of both enzymes that appears to involve dephosphorylation could be prevented and reversed by inhibitors of transcription (cordycepin) and cytoplasmic protein synthesis (cycloheximide), in the absence of a large effect on photosynthetic activity. As expected, okadaic acid, a potent and specific inhibitor of type 1 and 2A protein phosphatases, also prevented and reversed the activation of both enzymes in vivo. The results are taken to suggest that the protein phosphatase(s) that dephosphorylates and activates both enzymes is itself activated by light in a process that involves protein synthesis. Overall, NR and SPS share many common features of control, but are not identical in terms of regulation.