Submitted to: Nitric Oxide
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/3/2004
Publication Date: 1/15/2005
Citation: Wach, M., Kers, J., Krasnoff, S.B., Loria, R., Gibson, D.M. 2005. Nitric oxide synthase inhibitors and nitric oxide donors modulate the biosynthesis of thaxtomin a, a nitrated phytotoxin produced by streptomyces spp. Nitric Oxide. 12:46-53. Interpretive Summary: Several Streptomyces species cause "scab" diseases on potato tubers, sweet potato storage roots and expanded tap roots of radish, beet and similar crops. All of these species produce one or more members of an unusual nitrated dipeptide family known as thaxtomins. Toxin production occurs in diseased tissue and many of the disease symptoms are reproduced by the application of toxin preparations to susceptible plant tissue. In this study, we provide biochemical evidence that the enzyme responsible is a nitric oxide synthase. The Streptomyces enzyme is very similar to the mammalian enzymes that produce nitric oxide, an important signal in cell communication, as well as to other bacterial nitric oxide synthases for which no biological function was known. This is the first instance of a biological role for nitric oxide synthase in production of a secondary metabolite.
Technical Abstract: Evidence for the involvement of a bacterial nitric oxide synthase (NOS) in the biosynthesis of a phytotoxin is presented. Several species of Streptomyces bacteria produce secondary metabolites with unusual nitrogen groups, such as thaxtomin A (ThxA), which contains a nitroindole moiety. ThxA is a phytotoxin made by three pathogenic Streptomyces species that cause common scab of potato. All three species possess a gene homologous to the oxygenase domain of murine inducible NOS, and this gene, stnos, is essential for normal levels of ThxA production. We grew S. turgidiscabies in the presence of several known NOS inhibitors and a nitric oxide (NO) scavenger to determine their effect on ThxA production. The NO scavenger (CPTIO) and four NOS inhibitors (NAME, NMMA, AG, and 7-NI) reduced ThxA production without affecting bacterial growth. A strain of S. turgidiscabies from which the stnos gene had been deleted was grown in the presence of three NO donors (DEANO, SIN, and SNAP), and all three partially restored ThxA production. Our data suggest that bacterial nitric oxide synthases may, at least in part, produce NO for biosynthetic purposes, rather than for cellular signaling, as they do in mammals.