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.

Nitric oxide (NO) is a potent intercellular signal in mammals that mediates key aspects of blood pressure, hormone release, nerve transmission and the immune response of higher organisms. Proteins homologous to full-length mammalian nitric oxide synthases (NOSs) are found in lower multicellular organisms. Recently, genome sequencing has shown that some bacteria contain genes coding for truncated NOS proteins; this is consistent with reports of NOS-like activities in bacterial extracts. Biological functions for bacterial NOSs are unknown, but have been presumed to be analogous to their role in mammals. Here we describe a gene in the plant pathogen Streptomyces turgidiscabies that encodes a NOS homologue, and we reveal its role in nitrating a dipeptide phytotoxin required for plant pathogenicity. High similarity between bacterial NOSs indicates a general function in biosynthetic nitration; thus, bacterial NOSs constitute a new class of enzymes. Here we show that the primary function of Streptomyces NOS is radically different from that of mammalian NOS. Surprisingly, mammalian NO signalling and bacterial biosynthetic nitration share an evolutionary origin.

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SummaryPotato scab is a globally important disease caused by polyphyletic plant pathogenic Streptomyces species. Streptomyces acidiscabies , Streptomyces scabies and Streptomyces turgidiscabies possess a conserved biosynthetic pathway for the nitrated dipeptide phytotoxin thaxtomin. These pathogens also possess the nec1 gene which encodes a necrogenic protein that is an independent virulence factor. In this article we describe a large (325-660 kb) pathogenicity island (PAI) conserved among these three plant pathogenic Streptomyces species. A partial DNA sequence of this PAI revealed the thaxtomin biosynthetic pathway, nec1 , a putative tomatinase gene, and many mobile genetic elements. In addition, the PAI from S. turgidiscabies contains a plant fasciation ( fas ) operon homologous to and colinear with the fas operon in the plant pathogen Rhodococcus fascians . The PAI was mobilized during mating from S. turgidiscabies to the nonpathogens Streptomyces coelicolor and Streptomyces diastatochromogenes on a 660 kb DNA element and integrated site-specifically into a putative integral membrane lipid kinase. Acquisition of the PAI conferred a pathogenic phenotype on S. diastatochromogenes but not on S. coelicolor . This PAI is the first to be described in a Gram-positive plant pathogenic bacterium and is responsible for the emergence of new plant pathogenic Streptomyces species in agricultural systems.