THE AVR9 RACE-SPECIFIC ELICITOR OF CLADOSPORIUM-FULVUM IS PROCESSED BY ENDOGENOUS AND PLANT PROTEASES

The avirulence gene avr9 of the fungal tomato pathogen Cladosporium fu lvum encodes a race-specific peptide elicitor that induces a hypersens itive response in tomato plants carrying the complementary resistance gene Cf9. The avr9 gene is highly expressed when C fulvum is growing i n the plant and the elicitor accumulates in infected leaves as a 28-am ino acid (aa) peptide. In C fulvum grown in vitro, the peptide elicito r is not produced in detectable amounts. To produce significant amount s of the AVR9 elicitor in vitro, the coding and termination sequences of the avr9 gene were fused to the constitutive gpd-promoter (glyceral dehyde 3-phosphate dehydrogenase) of Aspergillus nidulans. Transforman ts of C fulvum were obtained that highly expressed the avr9 gene in vi tro and produced active AVR9 peptide elicitors. These peptides were pa rtially sequenced from the N terminus and appeared to consist of 32, 3 3, 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 proc essing 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.