In vivo characterization of chimeric phytochromes in yeast

Phytochromes are plant photoreceptors that play a major role in photomorpho genesis. Two members of the phytochrome family have been characterized in s ome detail. Phytochrome A, which controls very low fluence and high irradia nce responses, is rapidly degraded in the light, forms sequestered areas of phytochrome (SAPs), and does not exhibit dark reversion in monocotyledonou s seedlings. Phytochrome B mediates red! far-red reversible responses, is s table in the light, and does not form SAPs, We report on the behavior in ye ast of the phytochrome apoproteins of rice PHYA, tobacco PHYB, and chimeric PHYAB and PHYBA and on the behavior of the respective holoprotein adducts after assembly with phycocyanobilin chromophore (PHY*). SAP-like formation in yeast was not observed for PHYB but was detectable for PHYA, PHYAB, and PHYBA. Rice PHYA* did not undergo dark reversion in yeast. Surprisingly, al l other tested phytochrome constructs did exhibit dark reversion, including chimeric phytochromes with a short N-terminal part of tobacco PHYB or pars ley PHYA fused to rice PHYA. Furthermore, the proportion of phytochrome und ergoing dark reversion and the rate of reversion were increased for both th e N terminus-swapped constructs and PHYBA*, These results are discussed wit h respect to structure/function analysis of phytochromes A and B.