A cyclic peptide synthetase gene required for pathogenicity of the fungus Cochliobolus carbonum on maize.
A single locus for host-selective pathogenicity (Tox2) in C. carbonum governs production of a cyclic tetrapeptide named HC-toxin. A chromosomal region, 22 kilobases (kb) long, containing a 15.7-kb open reading frame (HTS1) encoding a multifunctional cyclic peptide synthetase was isolated. The 22-kb chromosomal region is duplicated in toxin-producing isolates of the fungus but is absent from the genomes of isolates which do not produce the toxin. Mutants of the fungus with disruptions in both copies of HTS1, at either of 2 different sites within HTS1, were engineered by DNA-mediated transformation. Disruption of both copies at either site resulted in loss of ability to produce HC-toxin and loss of host-selective pathogenicity, but the mutants displayed different biochemical phenotypes depending on the site of disruption. It is concluded that TOX2 encodes, at least in part, a large, multifunctional biosynthetic enzyme and that the evolution of host range in C. carbonum involved the insertion or deletion of a large piece of chromosomal DNA.