The Complexity of the Prto PfrPhototransformation Kinetics Is an Intrinsic Property of Native Phytochrome*

Several possible origins of the complex phytochrome red to far‐red light‐absorbing phytochrome (PrPfr) phototransformation kinetics in the nanosecond‐to‐second time range have been examined. Heterogeneity based on protein sequence is ruled out as an origin of the multi‐component kinetics because recombinant 124 kDa oat phytochrome A apoprotein reconstituted with phytochro‐mobilin and the native protein are very similar in this regard throughout this time range. The Prforms of native 124 kDa oat phytochrome A and of a homogeneous recombinant 65 kDa chromoprotein fragment exhibit thermochromic properties interpreted as arising in each case from the presence of two Prspecies in thermal equilibrium. They exhibit identical photochemical properties. The complex kinetics therefore cannot result from Prheterogeneity either. Thus, the presence of two Prforms in equilibrium (Pr,675and Pr,655) and the complex multiex‐ponential PrPfrphototransformation kinetics observed in all time ranges are intrinsic properties of the homogeneous holoprotein of oat phytochrome A.