Phytochrome chromophore biosynthesis

Etiolated Avena sativa L. seedlings grown in the presence of gabaculine of 4-amino-5-hexynoic, inhibitors of 5-aminolevulinic acid synthesis in plants, contain reduced levels of spectrally detectable phytochrome compared with untreated seedlings but continue to accumulate phytochrome apoprotein. Inhibitor-treated seedling explants administered either 5-aminolevulinic acid or biliverdin IX;Parallel experiments were also performed with three other linear tetrapyrroles, two non-natural isomers of biliverdin--biliverdin III;Addition of phycocyanobilin to soluble extracts of inhibitor-treated seedlings resulted in covalent attachment of this bilatriene to apophytochrome with a concomitant increase in phytochrome photoreversible spectral activity. The newly formed photoactive species displayed spectral properties similar to those observed in the previous in vivo studies. The increase in spectral activity was consistent with conversion of all of the pre-existing apoprotein to functionally active holoprotein. A photoreversible, covalent adduct with a similar optical spectrum also formed when immunopurified apophytochrome was incubated with phycocyanobilin. When biliverdin IX...