thaxtomin phytotoxins produced by plant - pathogenic streptomyces species contain a nitro group that is essential for phytotoxicity . 
 the n , n-dimethyldiketopiperazine core of thaxtomins is assembled from l - phenylalanine and l-4-nitrotryptophan by a nonribosomal peptide synthetase and nitric oxide synthase - generated no is incorporated into the nitro group , but the biosynthesis of the non - proteinogenic amino acid l-4-nitrotryptophan is unclear . here 
 we report that txte , a unique cytochrome p450 , catalyzes l - tryptophan nitration using no and o2 .