THE METHYLOTROPHIC YEAST PICHIA-PASTORIS SYNTHESIZES A FUNCTIONALLY ACTIVE CHROMOPHORE PRECURSOR OF THE PLANT PHOTORECEPTOR PHYTOCHROME

Induction of the expression of an algal phytochrome cDNA in the methyl otrophic yeast Pichia pastoris led to time-dependent formation of phot oactive holophytochrome without the addition of exogenous bilins, Both in vivo and in vitro difference spectra of this photochromic species are very similar to those of higher plant phytochrome A, supporting th e conclusion that this species possesses a phytochromobilin prosthetic group, Zinc blot analyses confirm that a bilin chromophore is covalen tly bound to the algal phytochrome apoprotein. The hypothesis that P. pastoris contains phytochromobilin synthase, the enzyme that converts biliverdin IX alpha to phytochromobilin, was also addressed in this st udy. Soluble extracts from P. pastoris were able to convert biliverdin to a bilin pigment, which produced a native difference spectrum upon assembly with oat apophytochrome A. HPLC analyses confirm that biliver din is converted to both 3E- and 3Z-isomers of phytochromobilin, These investigations demonstrate that the ability to synthesize phytochromo bilin is not restricted to photosynthetic organisms and support the hy pothesis of a more widespread distribution of the phytochrome photorec eptor.