MOLECULAR-CLONING OF AN ALLENE OXIDE SYNTHASE - A CYTOCHROME-P450 SPECIALIZED FOR THE METABOLISM OF FATTY-ACID HYDROPEROXIDES

Allene oxide synthases convert lipoxygenase-derived fatty acid hydrope roxides to unstable allene epoxides. In plants, an allene oxide is a p recursor of the growth regulator jasmonic acid. Previously, we showed that an allene oxide synthase from flaxseed has the spectral propertie s of a cytochrome P450. The relationship to the P450 gene family is no w established from the primary structure deduced from the cDNA. The en coded protein of 536 amino acids has segments at the C terminus that m atch certain well conserved regions in cytochrome P450s. The heme-bind ing cysteine is recognizable at position 489. However, there are unpre cedented modifications in this region, with substitution of two of the three most highly conserved amino adds. Also very unusual is the abse nce of a conserved threonine that normally helps form the O2-binding p ocket in cytochrome P450s. Notably, O2 is not involved in the allene o xide synthase reaction and, furthermore, the enzyme is known to have a weak interaction with CO. While allene oxide synthases are usually de scribed as microsomal, the flax cDNA encodes a 58-amino add signal seq uence characteristic of a mitochondrial or chloroplast transit peptide . Therefore, the enzyme is a type I P450 and most likely is located in chloroplasts. Overall, the flax allene oxide synthase has less-than-o r-equal-to 25% identity to other P450s; it belongs to a newly discover ed gene family, to be designated CYP74. The flaxseed enzyme is prototy pical of this family of enzymes that remain to be characterized in pla nts and animals.