Dothistroma pini , a Forest Pathogen, Contains Homologs of Aflatoxin Biosynthetic Pathway Genes

Abstract

Homologs of aflatoxin biosynthetic genes have been identified in the pine needle pathogen Dothistroma pini. D. pini produces dothistromin, a difuranoanthraquinone toxin with structural similarity to the aflatoxin precursor versicolorin B. Previous studies with purified dothistromin suggest a possible role for this toxin in pathogenicity. By using an aflatoxin gene as a hybridization probe, a genomic D. pini clone was identified that contained four dot genes with similarity to genes in aflatoxin and sterigmatocystin gene clusters with predicted activities of a ketoreductase (dotA), oxidase (dotB), major facilitator superfamily transporter (dotC), and thioesterase (dotD). A D. pini dotA mutant was made by targeted gene replacement and shown to be severely impaired in dothistromin production, confirming that dotA is involved in dothistromin biosynthesis. Accumulation of versicolorin A (a precursor of aflatoxin) by the dotA mutant confirms that the dotA gene product is involved in an aflatoxin-like biosynthetic pathway. Since toxin genes have been found to be clustered in fungi in every case analyzed so far, it is speculated that the four dot genes may comprise part of a dothistromin biosynthetic gene cluster. A fifth gene, ddhA, is not a homolog of aflatoxin genes and could be at one end of the dothistromin cluster. These genes will allow comparative biochemical and genetic studies of the aflatoxin and dothistromin biosynthetic pathways and may also lead to new ways to control Dothistroma needle blight.

Phylogenomic analysis of type I polyketide synthase genes in pathogenic and saprobic ascomycetes

“…In a third example, Dothistroma pini makes dothistromin with products of genes that are orthologous to those of the aflatoxin biosynthetic pathway found in Aspergillus spp. (48).…”

The Population Dynamics of Aflatoxigenic Aspergilli

“…They encode proteins with unknown functions and probably are not necessary for ST biosynthesis. A homolog to stcC is found in the dothistromin biosynthetic cluster (Bradshaw, Bhatnagar et al 2002). Another gene loss appears to have occurred relatively recently, perhaps within the last 1 million years.…”

“…The genes for AF/ST/dothistromin synthesis have been retained for greater than 125 million years and it is probable that the polyketides encoded by these clusters enabled the fungi to survive and prosper as free-living organisms in diverse environments (Bradshaw, Bhatnagar et al 2002;Gomez and Nosanchuk 2003). We assume that the reason for maintaining such gene clusters is that the precursor metabolites are critical for fungal adaptation.…”

Genetic structure of the foliar pathogen Dothistroma septosporum in northwest British Columbia suggests recombination and high gene flow.

“…Ferguson et al (1986) have shown that the toxin causes chromosome abnormalities in human lymphocytes and Skinnider et al (1989) found an increase in frequency of sister chromatid exchange. Also of interest is that the biosynthetic pathway leading to the production of dothistromin is very similar to the pathway that leads to aflatoxin production, a human carcinogen (Bradshaw et al 2002). Studies on the toxin are important both for developing methods of control of this pathogen, but also for the development of safety protocols for forest workers.…”

“…Worker exposure to toxin was greater in wet conditions versus dry and was also affected by season and environment (Elliott et al 1989). The increasing incidence of Dothistroma disease epidemics in the Northern hemisphere is also of concern (Bradshaw et al 2002). An understanding of risks is essential for development of safety protocols for forest workers.…”