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Articles

In Vivo Properties of Membrane-bound Phytochrome ^1,2

^a The Biological Laboratories, Harvard University, Cambridge, Massachusetts 02138

After a 3-minute irradiation with red light, which saturates the phototransformation from the red light-absorbing form of phytochrome to the far red light absorbing form of phytochrome, about 40% of the phytochrome extractable from hooks of etiolated squash seedlings (Cucurbita pepo L. cv. Black Beauty) can be pelleted as Pfr at 17,000g after 30 minutes. Dark controls yield only 2 to 4% pelletable phytochrome in the form Pr. If a dark period intervenes between red irradiation and extraction, the bound Pfr gradually loses its photoreversibility. The time course for this destruction parallels the time course for phytochrome destruction in vivo following saturating red irradiation. The soluble fraction of phytochrome remains constant. These results suggest that in squash seedlings phytochrome destruction is related exclusively to the fraction which becomes membrane-bound. The induction of phytochrome binding by red light is not completely reversible by far red. In plants given saturating red followed immediately by saturating far red light, 12% of the phytochrome is found in the bound fraction as Pr if the phytochrome extraction is immediate. If a dark period intervenes between red-far red treatment and extraction, the bound phytochrome is released within 2 hours. A model of the binding properties of phytochrome, based on molecular interaction at the membrane is proposed, and possible consequences for the mechanism of action of phytochrome are discussed.

⁴ Present address: Institut für Biologie III, D7800 Freiburg, Germany, Schänzlestrasse 9-11. To whom correspondence should be addressed.

⁵ Present address: Department of Plant Biology, Carnegie Institution of Washington, Stanford, Calif. 94305.

¹ This work was supported by National Science Foundation Grant GB-30964X to W.R.B., by The Department of Plant Biology, Carnegie Institute of Washington, Stanford, Calif., by the Deutsche Forschungsgemeinschaft (SFB46) and by the Délégation Générale à la Recherche Scientifique et Technique. During the work, D.M. held a Max Kade Foundation Fellowship, and J.B. a North Atlantic Treaty Organization Fellowship. The authors are very grateful for this aid.

² Dedicated to Solon A. Gordon.