|Bignell, Dawn R|
Submitted to: Molecular Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/10/2007
Publication Date: 11/30/2007
Citation: Joshi, M.V., Bignell, D.D., Johnson, E.G., Sparks, J.P., Gibson, D.M., Loria, R. 2007. The AraC/XylS regulator TxtR modulates Thaxtomin Biosynthesis and Virulence in Streptomyces scabies. Molecular Microbiology. 66:633-642. Interpretive Summary: Several Streptomyces species cause “scab” diseases on potato tubers, sweet potatoes, and tap roots of radish, beet and similar crops, due to the presence of a family of phytotoxins, thaxtomins, that are critical for pathogenicity. Thaxtomin inhibits cellulose biosynthesis by some unknown mechanism. This work describes the regulation of the biosynthetic pathway by a gene that is very similar to a previously described gene regulatory family. The protein produced by this regulator gene was shown to bind cellobiose, and subsequently turn on the genes involved in thaxtomin biosynthesis. These studies provide a model for plant pathogenic Streptomyces-host interactions where the cell wall components are key to the regulation of this pathway.
Technical Abstract: Streptomyces scabies is the best studied of those streptomycetes that cause an economically important disease known as potato scab. The phytotoxin thaxtomin is made exclusively by these pathogens and is required for virulence. Here we describe regulation of thaxtomin biosynthesis by TxtR, a member of the AraC/XylS family of transcriptional regulators. The txtR gene is imbedded in the thaxtomin biosynthetic pathway and is located on a conserved pathogenicity island in S. scabies, S. turgidiscabies and S. acidiscabies. Thaxtomin biosynthesis was abolished and virulence was almost eliminated in the txtR deletion mutant of S. scabies 87.22. Accumulation of thaxtomin biosynthetic gene (txtA, txtB,txtC, nos) transcripts was reduced compared with the wild-type S. scabies 87.22. NOS-dependent nitric oxide production by S. scabies was also reduced in the mutant. The TxtR protein bound cellobiose, an inducer of thaxtomin production, and transcription of txtR and thaxtomin biosynthetic genes was upregulated in response to cellobiose. TxtR is the first example of an AraC/XylS family protein regulated by cellobiose. Together, these data suggest that cellobiose, the smallest oligomer of cellulose, may signal the availability of expanding plant tissue, which is the site of action of thaxtomin.