Viscosity dependence of primary photoprocesses of 124 kilodalton phytochrome.

To characterize the nature of primary photoprocesses of phytochrome which serves as the red-far red reversible photoreceptor for photomorphogenesis in plants, viscosity dependence of the fluorescence lifetimes of phytochrome isolated from etiolated oat seedling (Avena sativa L.) has been investigated. The fluorescence decay of phytochrome exhibited approximately two components, one with lifetime in the range of 50-70 ps and another with 1.1-1.2 ns in phosphate buffer with or without 40-67% glycerol. However, relative amplitudes of these decay components were found to be strongly viscosity dependent. Thus, the longer decay component increased from 2-5% in phosphate buffer to approximately 20% in 67% glycerol-phosphate buffer. These results have been interpreted in terms of primary reaction from the excited singlet state of phytochrome, yielding a photoreversible intermediate whose rate of formation and decay were apparently viscosity-dependent. Further, the viscosity dependence is consistent with the primary reaction involving conformational changes of the chromophore/its apoprotein environment.