Characterization of a transcription factor unique to race 1 isolates of Cochliobolus carbonum

HC-toxin is a cyclic tetrapeptide produced by race 1 isolates of the filamentous fungus Cochliobolus carbonum. Genetic and biochemical data indicate that HC-toxin is the key determinant of virulence in interactions between toxin-producing isolates of the fungus and maize lines homozygously recessive at the Hm locus. Production of HC-toxin segregates as a single genetic locus, TOX2, in crosses between race 1 isolates and race 2 isolates that do not produce the toxin. Molecular analyses of the TOX2 locus have revealed that at least six linked genes are necessary for HC-toxin biosynthesis. One of these genes, TOXE, was shown to be necessary for the full expression of most of the genes within the TOX2 locus. In this study the product of the TOXE gene is shown to be a transcription factor that binds to a conserved DNA sequence present in one or more copies within the promoters of the genes that comprise the TOX2 locus. Through mutational analysis of the TOXE protein, regions critical for proper DNA binding and transcriptional activation were identified. The biochemical properties of TOXE as well as its role in HC-toxin production and pathogenicity are discussed.