The fragmented Dothistromin gene cluster and its regulation by AflR

Red band disease, caused by the fungus Dothistroma septosporum, is a devastating disease of pine plantations globally. The red banding is associated with the polyketide toxin dothistromin which is structurally similar to versicolorin B, a precusor of aflatoxin. Unlike most other secondary metabolites, dothistromin is produced mainly during the early exponential growth phase in culture. The D. septosporum genome was fully sequenced by the DOE Joint Genome Institute; this enabled the identification of all putative dothistromin genes, including an ortholog of the aflatoxin regulatory gene AflR, and also revealed that most of the genes are spread over six separate loci on chromosome 12 (1.3 Mb) leading to the question of how such a fragmented cluster is regulated. To address this question, AflR function was analyzed in D. septosporum. Inactivation of the DsAflR gene (¿DsAflR) resulted in a drastic reduction in dothistromin. This contrasts with orthologous ¿AflR mutants in Aspergillus species that produce no aflatoxin. Expression patterns in ¿DsAflR mutants helped to predict the complete set of genes involved in dothistromin production and also showed no correlation between dothistromin gene expression and gene distance from the telomere. An orthologous set of dothistromin genes with similar arrangement was found in the closely related biotrophic tomato pathogen Cladosporium fulvum, even though this species does not produce dothistromin. In C. fulvum, pseudogenization of key biosynthetic genes explains the lack of dothistromin production. The fragmented arrangement of dothistromin genes provides an example of coordinated control of a dispersed set of secondary metabolite genes.