IN-VIVO CHARACTERIZATION OF PHYTOCHROME-PHYCOCYANOBILIN ADDUCTS IN YEAST

The in vivo reconstitution of phycocyanobilin with apophytochrome lead s to photoreversible adducts in living yeast cells. Investigations wit h the rice phytochrome A phycocyanobilin adduct (PHYA) and the tobacc o phytochrome B phycocyanobilin adduct (PHYB) show that the protein s tability in yeast is independent of the form of the photoreceptor. Aft er in vivo assembly and irradiation with red light, 25.6% of the far-r ed light-absorbing form of PHYB exhibited dark reversion with a half- life time of approximately 20 min. Control experiments with PHYA reve aled no dark reversion, The data indicate that the molecular basis for this reaction is the formation of heterodimers between the red and th e far-red light absorbing form of phytochrome. Electron microscopic in situ localizations and in, vitro sequestering experiments showed that phytochrome A was able to sequester in yeast, On the electron microsc opic level, the sequestered areas of phytochrome from etiolated plants and yeast are indistinguishable, The sequestering reaction in yeast i s independent of the formation of the far-red light absorbing form of phytochrome, Therefore, we discuss a new model for this reaction in pl ant. Since it is unlikely that yeast cells contain elements that disti nguish between phytochrome A and B, we conclude that sequestering and dark reversion reflect intrinsic properties of phytochrome.