Recombinant phytochrome A in yeast differs by its spectroscopic and photochemical properties from the major phyA ' and is close to the minor phyA '':Evidence for posttranslational modification of the pigment in plants

Previously, two pools of phytochrome A (phyA' and phyA") have been detected by in situ low-temperature fluorescence spectroscopy and photochemistry; i t was suggested that they might differ in the nature of their posttranslati onal modification. In order to verify this possibility Arabidopsis and rice (Oryza) phyA were expressed in yeast and the pigments were assembled in vi vo with phycocyanobilin (PCB) and phytochromobilin (P PhiB). The resulting recombinant phytochromes in the red-light-absorbing form (Pr) were characte rized in the yeast cell by (1) the fluorescence emission spectra; (2) the t emperature dependence of Pr fluorescence intensity and activation energy of fluorescence decay: and (3) the extent of photoconversion of Pr into photo product lumi-R (gamma (1)) or far-red-light absorbing form (Pfr) (gamma (2) ). Both Arabidopsis phyA/PCB and Oryza phyA/P PhiB had low gamma (1) of ca 0.05, allowing their attribution to the Pr" phenomenological type of phytoc hrome comprising phyA", phyB and cryptogam phytochromes. The spectroscopic properties of Oryza phyA/P PhiB were also very close to phyA", However, bot h investigated holoproteins differed from phyA", both with respect to the c haracter of temperature dependence of the fluorescence yield and activation energy. Thus, recombinant Oryza phyA/P PhiB is similar but not identical t o phyA", The data demonstrate that the low-abundance-fraction plant phyA (p hyA") comes from the same gene as the major (phyA') fraction. Because both endogenous phyA fractions differ from the phytochrome expressed in yeast, t hey appear to be posttranslationally modified and/or bound to partner prote ins or cellular substructures. However, the character of the presumed chemi cal modification is different in phyA' and phyA" and its extent is more pro found in the case of the former.