The Arabidopsis photomorphogenic mutant hy1 is deficient in phytochrome chromophore biosynthesis as a result of a mutation in a plastid heme oxygenase

The HY1 locus of Arabidopsis is necessary for phytochrome chromophore biosy nthesis and is defined by mutants that show a long hypocotyl phenotype when grown in the light. We describe here the molecular cloning of the HY1 gene by using chromosome walking and mutant complementation. The product of the HY1 gene shows significant similarity to animal heme oxygenases and contai ns a possible transit peptide for transport to plastids. Heme oxygenase act ivity was detected in the HY1 protein expressed in Escherichia coli. Heme o xygenase catalyzes the oxygenation of heme to biliverdin, an activity that is necessary for phytochrome chromophore biosynthesis. The predicted transi t peptide is sufficient to transport the green fluorescent protein into chl oroplasts. The accumulation of the HY1 protein in plastids was detected by using immunoblot analysis with an anti-HY1 antiserum. These results indicat e that the Arabidopsis HY1 gene encodes a plastid heme oxygenase necessary for phytochrome chromophore biosynthesis.