The importance of the N-terminus for activity of Ptr ToxB, a chlorosis-inducing host-selective toxin produced by Pyrenophora tritici-repentis

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.