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Photoactivation of NAD Kinase through Phytochrome

Phosphate Donors and Cofactors

^a Biological Institute, Faculty of Science, Nagoya University, Nagoya 464, Japan, Department of Agricultural Chemistry, Faculty of Agriculture, Nagoya University, Nagoya 464, Japan

The specificities of phosphate donors and the effects of metal chelating agents and divalent metal ions on NAD kinase activation by phytochrome-far red-absorbing form (Pfr) were examined. ATP was the most efficient phosphorylating agent. Uridine 5'-triphosphate, cytidine 5'-triphosphate (CTP), inosine 5'-triphosphate, and guanosine 5'-triphosphate in this order caused significant phosphorylation in the dark. Under red light, striking photoactivation of NAD kinase was obtained with ATP and subsequently CTP.

In the presence of exogenous Mg²⁺, which is required for NAD kinase activity, -nitroso--naphthol, cyanide, and dimethylglyoxime, strongly inhibited the activation by red light without affecting the level of NAD kinase in the dark.

Of the divalent cations tested with the KCN-treated phytochrome preparation, only Co²⁺ was effective for photoactivation of NAD kinase. Even when Mg²⁺, an essential component of NAD kinase, was added to the assay system, the further addition of Co²⁺ was required for the activation of NAD kinase by Pfr.