Allene oxide synthases convert lipoxygenase-derived fatty acid hydroperoxides to unstable allene epoxides. In plants, an allene oxide is a precursor of the growth regulator jasmonic acid. Previously, we showed that an allene oxide synthase from flaxseed has the spectral properties of a cytochrome P450. The relationship to the P450 gene family is now established from the primary structure deduced from the cDNA. The encoded protein of 536 amino acids has segments at the C terminus that match certain well conserved regions in cytochrome P450s. The heme-binding cysteine is recognizable at position 489. However, there are unprecedented modifications in this region, with substitution of two of the three most highly conserved amino acids. Also very unusual is the absence of a conserved threonine that normally helps form the O2-binding pocket in cytochrome P450s. Notably, O2 is not involved in the allene oxide synthase reaction and, furthermore, the enzyme is known to have a weak interaction with CO. While allene oxide synthases are usually described as microsomal, the flax cDNA encodes a 58-amino acid signal sequence 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 < or = 25% identity to other P450s; it belongs to a newly discovered gene family, to be designated CYP74. The flaxseed enzyme is prototypical of this family of enzymes that remain to be characterized in plants and animals.