PHYTOCHROME CHROMOPHORE BIOSYNTHESIS - TREATMENT OF TETRAPYRROLE-DEFICIENT AVENA EXPLANTS WITH NATURAL AND NON-NATURAL BILATRIENES LEADS TO FORMATION OF SPECTRALLY ACTIVE HOLOPROTEINS

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  • 5 January 1989
    • journal article
    • research article
    • Vol. 264  (1) , 183-189
Abstract
Etiolated Avena seedling grown in the presence of 4-amino-5-hexynoic acid, an inhibitor of 5-aminolevulinic acid synthesis in plants, contain less than 10% of the spectrally detectable levels of phytochrome found in untreated seedlings (Elich, T. D., and Lagarias, J. C. (1988) Plant Physiol. 88, 747-751). In this study, incubation of explants from such seedlings with [14C]biliverdin IX.alpha. led to rapid covalent incorporation of radiolabel into to a single 124-kDa polypeptide in soluble protein extracts. Immunoprecipitation experiments confirmed that the protein was phytochrome. Parallel experiments were performed with four unlabeled linear tetrapyrroles, the naturally occurring biliverdin IX.alpha. isomer, two non-natural isomers, biliverdin XIII.alpha. and biliverdin III.alpha., and phycocyanobilin-the cleaved prosthetic group of the light-harvesting antenna protein C-phycocyanin. In all cases, except for the III.alpha. isomer of biliverdin, a time-dependent recovery of photoreversible phytochrome was observed. The newly formed phytochrome obtained after incubation with biliverdin IX.alpha. exhibited spectral characteristics identical with those of the native protein. In contrast, the spectral properties of phytochromes formed during incubation with biliverdin XIII.alpha. and phycocyanobilin differed significantly from those of the native chromoprotein. These results indicate that biliverdin IX.alpha. is an intermediate in the biosynthesis of the phytochrome chromophore and that phytochromes with prosthetic groups derived from bilatrienes having non-natural D-ring substituents are photochromic.