The Arabidopsis HY2 Gene Encodes Phytochromobilin Synthase, a Ferredoxin-Dependent Biliverdin Reductase

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Abstract
Light perception by the plant photoreceptor phytochrome requires the tetrapyrrole chromophore phytochromobilin (PϕB), which is covalently attached to a large apoprotein. Arabidopsis mutants hy1 and hy2, which are defective in PϕB biosynthesis, display altered responses to light due to a deficiency in photoactive phytochrome. Here, we describe the isolation of the HY2 gene by map-based cloning. hy2 mutant alleles possess alterations within this locus, some of which affect the expression of the HY2 transcript. HY2 encodes a soluble protein precursor of 38 kD with a putative N-terminal plastid transit peptide. The HY2 transit peptide is sufficient to localize the reporter green fluorescent protein to plastids. Purified mature recombinant HY2 protein exhibits PϕB synthase activity (i.e., ferredoxin-dependent reduction of biliverdin IXα to PϕB), as confirmed by HPLC and by the ability of the bilin reaction products to combine with apophytochrome to yield photoactive holophytochrome. Database searches and hybridization studies suggest that HY2 is a unique gene in the Arabidopsis genome that is related to a family of proteins found in oxygenic photosynthetic bacteria.