Transposon-like sequences at the TOX2 locus of the plant-pathogenic fungus Cochliobolus carbonum

The ascomycete fungus Cochliobolus carbonum race 1 is pathogenic on certain genotypes of maize due to the production of HC-toxin, a host-specific cyclic peptide. HC-toxin production is controlled, at least in part, by a duplicated 22-kb region of DNA that is found only in toxin-producing isolates of the fungus. This 22-kb region of DNA is flanked by a repetitive element. We have sequenced the element and found an interrupted reading frame that would encode a product similar to transposases from the fungal transposons Fot1 of Fusarium oxysporum and Pot2 of Magnaporthe grisea. The individual element cloned from C. carbonum is likely to function neither in cis nor trans, as it had a nonsense mutation in frame and several substitutions in its terminal inverted repeats. However, similar elements in the C. carbonum genome may be active, as the putative transposase-encoding region hybridized to mRNA of the size predicted by the reading frame. The element was found in varying copy number in the genomes of all Cochliobolus spp. examined, giving a distinct fingerprint in each species and race tested. The sequence similarity of the C. carbonum repetitive element to other fungal transposons, along with its presence in multiple copies per genome, strongly suggest that the C. carbonum repetitive element is a member of the Fot1 family of fungal transposons.