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The Dark Reactions of Rye Phytochrome in Vivo and in Vitro¹

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

The dark reactions of Secale cereale L. cv. Balbo phytochrome have been investigated in coleoptile tips and in extensively purified extracts of large molecular weight phytochrome. Destruction, but not reversion, was detected in vivo. The effects of various inhibitors of an in vitro phytochrome-degrading protease did not support a view of proteolytic attack as the basis of in vivo destruction. In vitro, rye phytochrome (about 240,000 molecular weight) reverted extremely rapidly, even at 5 C. The reversion curves were resolved into two first order components. The previously studied 60,000 molecular weight species, obtained by controlled proteolysis of large rye phytochrome, showed a similar two-component pattern, but a much slower over-all reversion rate. This reduction in rate was caused mainly by the reversion of a greater percentage of the small phytochrome as the slow component. Sodium dithionite markedly accelerated the reversion rate of both large and small forms, but oxidants, at concentrations low enough to avoid chromophore destruction, had no effect. Both large and small crude Avena sativa L. phytochrome showed two-component reversion kinetics.

¹ Research was supported by grants from the National Science Foundation (GB-15572) and E. I. duPont de Nemours and Co. to W. R. B. and a National Science Foundation Predoctoral Fellowship to C. S. P.

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