PROCESSING AND SECRETION OF A VIRALLY ENCODED ANTIFUNGAL TOXIN IN TRANSGENIC TOBACCO PLANTS - EVIDENCE FOR A KEX2P PATHWAY IN PLANTS

Ustilago maydis is a fungal pathogen of maize. Some strains of U. mayd is encode secreted polypeptide toxins capable of killing other suscept ible strains of U. maydis. We show here that one of these toxins, the KP6 killer toxin, is synthesized by transgenic tobacco plants containi ng the viral toxin cDNA under the control of a cauliflower mosaic viru s promoter. The two components of the KP6 toxin, designated alpha and beta, with activity and specificity identical to those found in toxin secreted by U. maydis cells, were isolated from the intercellular flui d of the transgenic tobacco plants. The beta polypeptide from tobacco was identical in size and N-terminal sequence to the U.maydis KP6 beta polypeptide. Processing of the KP6 preprotoxin in U.maydis requires a subtilisin-like processing protease, Kex2p, which is present in both animal and fungal cells and is required for processing of (among other things) small secreted polypeptide hormones and secreted toxins. Our findings present evidence for Kex2p-like processing activity in plants . The systemic production of this viral killer toxin in crop plants ma y provide a new method of engineering biological control of fungal pat hogens in crop plants.