Primary photoprocesses of undegraded phytochrome excited with red and blue light at 77 K.

1. Red light irradiation of phytochrome (Pr) at 77 K produces an intermediate absorbing at 696 nm. The photostationary state concentration of this intermediate is rapidly established with that of Pr as the result of spectral overlap between the Qy band of Pr and the Qx band of the intermediate. 2. The 696 nm intermediate reverts back to Pr preferentially without yielding a substantial amount of Pfr upon thawing the 77 K sample to higher temperatures. 3. Blue light irradiation of Pr with or without exogenous FMN at 77 K results in the formation of two intermediates absorbing at 684 nm and 696 nm. The 684 nm intermediate is photochemically converted to the 696 nm intermediate at 77 K. Possibilities for the preferential formation of the 684 nm intermediate with blue light are discussed. 4. At 277 K, blue light irradiation of phytochrome (Pr) containing exogenous FMN increases the rate of phototransformation from Pr to Pfr three times over Pr having no FMN. On the other hand, exogenous FMN has no effect on the rate of transformation of Pr to Pfr by red light. 5. Energy transfer occurs from FMN to Pr at 77 K, initiating the photoprocesses of the Pr. The energy transfer apparently occurs within flavin-phytochrome complexes.