Pyrenophora tritici-repentis, the causal agent of tan spot of wheat, produces Ptr ToxB (ToxB), a chlorosis-inducing host-selective toxin. To define the sequence requirements for chlorosis induction, the biological activity of chimeric proteins that contain combinations of three coding regions from ToxB and/or an inactive allele toxb, was tested. Results indicate that, although the entire ToxB sequence is required to trigger full symptom development, the N-terminal region plays a critical role in the activity of the toxin. Results of site-directed mutagenesis of the two amino acids in the N-terminal region that differ between ToxB and toxb indicate that: 1) each of these two amino acid is essential for complete ToxB activity and 2) the sequence surrounding the first cysteine residue is necessary for proper folding of the protein. We conclude that ToxB does not contain a single contiguous motif sufficient to confer chlorosis-inducing activity.
Bibliographical noteFunding Information:
We are grateful to A. Karplus for helpful discussions. We wish to thank the Mass Spectrometry Facility at Oregon State University for their assistance with the MALDI-TOF analysis. This research was supported by the National Research Initiative (NRI) Microbial Biology: Microbial Associations with Plants Program of the USDA Cooperative State Research, Education and Extension Service (CSREES, grant number 2006-55600-16619 ). Funding for K. Cardwell was provided by a HHMI (Howard Hughes Medical Institute) summer undergraduate research fellowship and two URISC (Undergraduate Research, Innovation, Scholarship and Creativity) awards from Oregon State University. M. Figueroa Betts was supported by a National Science Foundation Minority Postdoctoral Fellowship (award number 0706881).
- Host-selective toxin
- Pyrenophora tritici-repentis