Some strains of the plant-pathogenic fungus Ustilago maydis secrete toxins 
(killer toxins) that are lethal to susceptible strains of the same fungus. There 
are three well-characterized killer toxins in U. maydis-KP1, KP4, and KP6-which 
are secreted by the P1, P4, and P6 subtypes, respectively. These killer toxins 
are small polypeptides encoded by segments of an endogenous, persistent 
double-stranded RNA (dsRNA) virus in each U. maydis subtype. In P4 and P6, the 
M2 dsRNA segment encodes the toxin. In this work, the KP1 killer toxin was 
purified for internal amino acid sequence analysis, and P1M2 was identified as 
the KP1 toxin-encoding segment by sequence analysis of cDNA clones. The KP1 
toxin is a monomer with a predicted molecular weight of 13.4kDa and does not 
have extensive sequence similarity with other viral anti-fungal toxins. The P1M2 
segment is different from the P4 and P6 toxin-encoding dsRNA segments in that 
the 3' non-coding region of its plus strand has no sequence homology to the 3' 
ends of the plus strands of P1M1, P4M2, or P6M2.