Effects of divalent cations and EDTA on spectral properties of phytochrome in particulate fractions from etiolated pea epicotyls

The photoreversible absorbance change of phytochrome in suspensionsof a 20,000xg particulate fraction (20kP) prepared from a 1,000xgsupernatant (1kS) of etiolated pea epicotyl extracts decreasedremarkably in the presence of 5 mM Cu²⁺, Zn²⁺ and Co²⁺, butremained unchanged in 5 mM Ca²⁺, Mg²⁺, Fe²⁺ or Mn²⁺. This spectraldistortion of phytochrome was more evident in soluble preparationsand in suspensions of pellets prepared from red light (R)-irradiatedtissues than it was in suspensions of pellets prepared in thedark from etiolated tissues that received no actinic irradiation. When Cu²⁺ was added to the red-light-absorbing form of phytochrome(Pr) in resuspended pellets prepared from R-irradiated tissues,the distortion of its difference spectrum took place after irradiationwith the first actinic R. In contrast, when Cu²⁺ was added tothe far-red-light-absorbing form of phytochrome (Pfr) in thesame resuspended pellet, no distortion was seen, unless thePfr in the pellet was first photoconverted to Pr and then photoconvertedback to Pfr. Spectral distortion of Pr remained small during dark incubationat 25°C when suspensions of 20kPs were prepared and incubatedwith a buffer containing EDTA, whether the 20kP was preparedfrom nonirradiated tissue or from R-irradiated tissues. But,when EDTA was added to a suspension of 20kP prepared from 1kS,after the 1kS was irradiated with R in the presence of 10 mMCaCl₂, the spectral distortion of Pr in 20kP occurred instantaneously. (Received April 14, 1980; )