Identification of peptide synthetase-encoding genes from filamentous fungi producing host-selective phytotoxins or analogs.

Race 1 of Cochliobolus carbonum (Cc) makes a cyclic tetrapeptide, HC-toxin, that is necessary for its virulence on certain genotypes of maize. The synthesis of HC-toxin is catalyzed by a 570-kDa multifunctional enzyme, HC-toxin synthetase (HTS). The gene encoding HTS (HTS1) is absent from other races of Cc and from other species of Cochliobolus. Four other unrelated filamentous fungi make cyclic peptides closely related to HC-toxin, raising the possibility that the corresponding cyclic peptide synthetase (CPS)-encoding genes have moved between these fungi by horizontal gene transfer. Degenerate PCR primers were designed based on several highly conserved amino acid (aa) motifs common to known CPS domains and used to amplify genomic sequences from different fungi. PCR products representing CPS genes from Diheterospora chlamydosporia, which makes the HC-toxin analog chlamydocin, Cylindrocladium macrosporum, which makes the analog Cyl-2, and C. victoriae, which makes the unrelated cyclic pentapeptide victorin, were cloned and analysed. Their sequences are more related to HTS1 than to other cloned CPS, but the percent aa identity is not consistent with very recent horizontal movement of these genes.