NUCLEAR-LOCALIZATION ACTIVITY OF PHYTOCHROME-B

Phytochromes are soluble red/far-red-light photoreceptor proteins whic h mediate various photomorphogenic responses of plants. Despite much e ffort, the signal transduction mechanism of phytochrome has remained o bscure. Phytochromes are encoded by a small multigene family in Arabid opsis. Among the members of the family, phytochrome A (phyA) and B (ph yB) are the best characterized. PhyB contains putative nuclear localiz ation signals within its C-terminal region. Transgenic Arabidopsis pla nts were produced which expressed a fusion protein consisting of GUS a nd C-terminal fragments of phyB. GUS staining from the fusion protein in these transgenic plants was observed in the nucleus, which suggests that the nuclear localization signal of the fragment is functional. N ext, it was examined whether the endogenous phyB was detected in the n ucleus. Nuclei were isolated from the light-grown wild-type Arabidopsi s leaves and subjected to the immunoblot analysis. The result indicate d that a substantial fraction of total phyB was recovered in the isola ted nuclei. This result was further confirmed by the immunocytochemica l analysis of the protoplasts. Finally the effects of light treatments on the levels of phyB in the isolated nuclei were examined. Dark adap tation of the plants before the nuclear isolation reduced the levels o f phyB. The reduction was accelerated by irradiation of plants with fa r-red light before the transfer to darkness. Thus, nuclear localizatio n of phyB was suggested to be light-dependent.