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Articles

Irradiation-enhanced Phytochrome Pelletability

Requirement for Phosphorylative Energy in Vivo ¹

Carnegie Institution of Washington, 290 Panama Street, Stanford, California 94305

Short, high intensity pulses of red and far red light are used to study, at room temperature, the kinetics of the in vivo dark reaction responsible for irradiation-enhanced phytochrome pelletability. The t for this reaction is 2 seconds at 25 C in both Avena shoots and Zea mays coleoptiles. This is the most rapid phytochrome—far red-absorbing form (Pfr)-mediated cellular response thus far reported. Anoxia, KCN, NaN₃ and carbonyl cyanide p-trifluoromethoxyphenylhydrazone reduce the rate (but not the final extent) of the reaction by more than an order of magnitude. The rate of the reaction under these conditions is strongly correlated with the inhibitor-induced reductions in cellular ATP levels. Likewise, recovery in ATP levels upon withdrawal of the inhibitors is accompanied by a parallel recovery in the rate of the reaction. Cytochalasin B blocks cytoplasmic streaming without diminishing the pelletability response. Colchicine is likewise without effect. These data suggest a requirement for phosphorylative energy in one or more of the Pfr-dependent intracellular events leading to enhanced phytochrome pelletability. The possibility that this event might represent an ATP-dependent modification of the pigment protein itself in the Pfr form is discussed.

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