A PHYTOCHROME STUDY USING 2-LASER 2-COLOR FLASH-PHOTOLYSIS - I-700 ISA MANDATORY INTERMEDIATE IN THE P(R)-]P(FR) PHOTOTRANSFORMATION

The phototransformation of native (124 kDa) oat phytochrome, P(r) --> P(fr) has been studied at 10-degrees-C by two-laser/two-color flash ph otolysis. The overall P(r) --> P(fr) reaction yield did not vary with temperature within the range 4-21-degrees-C. Following the excitation of P(r) with a single 15 ns laser flash at 650 nm, the formation of P( fr) was quantitatively measured in a time-resolved experiment in the p resence of a second 8 ns laser flash at 710 nm delayed from the initia l flash. The second laser flash causes at 1.0 s after the initial lase r flash a depletion of the intermediate I700 as well as a reduction of the P(fr) absorption at 730 nm. The depletion of I700 correlates quan titatively with the reduction of P(fr) formation. The absorption spect ra of I700 and of the following intermediate, I(bl), were calculated a ssuming that the amount of P(r), which is photoconverted by a single l aser flash, equals the amount of P(fr) formed.