Immunity and resistance to the KP6 toxin of Ustilago maydis

The KP6 toxin of Ustilago maydis, encoded by segmented double-stranded (ds) RNA viruses, is lethal to sensitive strains of the same species and related species. The toxin consists of two polypeptides, α and β, synthesized as a single preprotoxin, which are not covalently linked. Neither polypeptide alone is toxic, but killer activity can be restored by in vitro and in vivo complementation. Killer-secreting strains are resistant to the toxin they produce. Resistance is conferred by a single recessive nuclear gene. This study describes a search for cytoplasmic factors that may confer resistance, also referred to as immunity. The approaches used to detect cytoplasmic immunity included transmission of dsRNA and transmission of virus particles to sensitive cells by cytoduction, cytoplasmic mixing in diploids and infection with viruses. An alternative approach was also used to express cloned cDNAs of the KP6 toxin-encoding dsRNA and of the α and β polypeptides. The results indicated that no immunity to KP6 can be detected. While KP6, α and β polypeptides were expressed by resistant cells, neither KP6 nor β were expressed in sensitive strains. The α polypeptide was expressed in sensitive cells, but it did not confer immunity. These results suggest that neither the preprotoxin nor the α or β polypeptides confer immunity and thus β may be the toxic component of the binary toxin.