The arabidopsis HY2 gene encodes phytochromobilin synthase, a ferredoxin-dependent biliverdin reductase

Light perception by the plant photoreceptor phytochrome requires the tetrapyrrole chromophore phytochromobilin (P PhiB), which is covalently attached to a large apoprotein. Arabidopsis mutants hy1 and hy2, which are defective in P PhiB 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 P PhiB synthase activity (i.e., ferredoxin-dependent reduction of biliverdin IX alpha to P PhiB), as confirmed by,MPLC and by the ability of the bilin reaction products to combine with apophytochrome to yield photoactive holaphytochrome; 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.