The AVR9 Race-Specific Elicitor of Cladosporium fulvum Is Processed by Endogenous and Plant Proteases

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Abstract
The avirulence gene avr9 of the fungal tomato pathogen Cladosporium fulvum encodes a race-specific peptide elicitor that induces a hypersensitive response in tomato plants carrying the complementary resistance gene Cf9. The avr9 gene is highly expressed when C. fulvum is growing in the plant and the elicitor accumulates in infected leaves as a 28-amino acid (aa) peptide. In C. fulvum grown in vitro, the peptide elicitor is not produced in detectable amounts. To produce significant amounts of the AVR9 elicitor in vitro, the coding and termination sequences of the avr9 gene were fused to the constitutive gpd-promoter (glyceraldehyde 3-phosphate dehydrogenase) of Aspergillus nidulans. Transformants of C. fulvum were obtained that highly expressed the avr9 gene in vitro and produced active AVR9 peptide elicitors. These peptides were partially sequenced from the N terminus and appeared to consist of 32, 33, and 34 aa's, respectively, and are the precursors of the mature 28-aa AVR9 peptide. We demonstrated that plant factors process the 34-aa peptide into the mature 28-aa peptide. We present a model for the processing of AVR9 involving cleavage of a signal peptide during excretion and further maturation by fungal and plant proteases into the stable 28-aa peptide elicitor.