Department of Plant Pathology, University of Kentucky, Lexington 40546-0312.
We have previously reported the isolation and characterization of the broad-spectrum antifungal protein, victoriocin, from culture filtrates of a virus-infected isolate of the plant-pathogenic fungus Helminthosporium (teleomorph: Cochliobolus) victoriae. We predicted that the 10-kDa mature victoriocin is derived in vivo from a preprotoxin precursor that is processed by a signal peptidase and kexin-like endopeptidase. We also presented evidence that the victoriocin precursor is encoded by a host gene, designated the victoriocin (vin) gene. In the present study, an H. victoriae genomic DNA library was constructed in the cosmid vector pMLF-2, and a cosmid clone carrying the vin gene and flanking sequences was isolated and used to generate constructs for transformation of virus-free and virus-infected H. victoriae isolates with the vin gene. Culture filtrates of the virus-free vin transformants exhibited high levels of antifungal activity compared with that revealed by the nontransformed virus-free wild-type strain, which exhibited little or no antifungal activity. Moreover, transformation of the wild-type virus-infected H. victoriae strain with the vin gene resulted in still higher production of victoriocin and higher antifungal activity in the culture filtrates of the vin transformants compared with the virus-infected wild-type strain. As previously predicted, the presence in the vin transformants of the preprovictoriocin and its post-translationally generated products, the provictoriocin and the mature victoriocin, was clearly demonstrated. Processing of the victoriocin preprotoxin requires eukaryotic host factors because no processing occurred in an in vitro translation system or in bacteria. It is of interest that some of the virus-free isolates transformed with the vin gene exhibited some features of the virus-induced disease phenotype, including moderate stunting and sectoring. Present data suggests that victoriocin may play an indirect role in disease development. Taken together, these results indicate that victoriocin is the primary protein responsible for the antifungal activity in culture filtrates of virus-infected H. victoriae isolates and that virus infection upregulates the expression of victoriocin. Overproduction of victoriocin may give the slower-growing virus-infected fungal strains some competitive advantage by inhibiting the growth of other fungi.