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Title: Functional characterization of a veA-dependent polyketide synthase gene in Aspergillus flavus necessary for the synthesis of asparasone, a sclerotium-specific pigment

item Cary, Jeffrey
item Harris Coward, Pamela
item Ehrlich, Kenneth
item DIANA DI MAVUNGU, JOSE - Ghent University
item MALYSHEVA, SVETLANA - Ghent University
item DE SAEGER, SARAH - Ghent University
item Dowd, Patrick
item SHANTAPPA, SOURABHA - Northern Illinois University
item MARTENS, STACEY - Northern Illinois University
item CALVO, ANA - Northern Illinois University

Submitted to: Fungal Genetics and Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/2/2014
Publication Date: 1/9/2014
Citation: Cary, J.W., Harris-Coward, P.Y., Ehrlich, K., Diana Di Mavungu, J., Malysheva, S.V., De Saeger, S., Dowd, P.F., Shantappa, S., Martens, S.L., Calvo, A.M. 2014. Functional characterization of a veA-dependent polyketide synthase gene in Aspergillus flavus necessary for the synthesis of asparasone, a sclerotium-specific pigment. Fungal Genetics and Biology. 64:25-35.

Interpretive Summary: Filamentous fungi produce a number of secondary metabolic compounds that have been shown to be both of great value (i.e. antibiotics and anti-hypercholesterolemics) and great harm (i.e. aflatoxins and trichothecenes). The genes responsible for producing a particular secondary metabolite are often clustered together on the chromosome. We have identified a secondary metabolic cluster in the fungus Aspergillus flavus and shown it is responsible for the production of a pigment that is specific to sclerotia. Sclerotia are hardened survival structures produced by the fungus that are capable of surviving harsh environmental conditions and then producing more of the fungus when conditions improve. We showed that the pigment provides the sclerotia with increased resistance to insect predation as well as to the deleterious effects of ultraviolet light and extreme heat. Therefore, the pigment plays an important role in the ability of the fungus to survive in the field and propagate. The information obtained in this study indicates that the genes required for production of aflatoxins may have evolved from an ancestral gene cluster that had its origin in production of the described protective pigment.

Technical Abstract: The filamentous fungus, Aspergillus flavus, produces the toxic and carcinogenic, polyketide synthase (PKS)-derived family of secondary metabolites termed aflatoxins. While analysis of the A. flavus genome has identified many other PKSs capable of producing secondary metabolites, to date, only a few other metabolites have been identified. In the process of studying how the developmental regulator, VeA, affects A. flavus secondary metabolism we discovered that mutation of veA caused a dramatic down-regulation of transcription of a polyketide synthase gene belonging to cluster 27 and the loss of the ability of the fungi to produce sclerotia. Inactivation of the cluster 27 pks (pks27) resulted in formation of greyish-yellow sclerotia rather than the dark brown sclerotia normally produced by A. flavus while conidial pigmentation was unaffected. One metabolite produced by Pks27 was identified by thin layer chromatography and mass spectral analysis as the known anthraquinone, asparasone A. Sclerotia produced by pks27 mutants were significantly less resistant to insect predation than were the sclerotia produced by the wild-type and more susceptible to the deleterious effects of ultraviolet light and heat. Normal sclerotia were previously thought to be resistant to damage because of a process of melanization similar to that known for pigmentation of conidia. Our results show that the dark brown pigments in sclerotia derive from anthraquinones produced by Pks27 rather than from the typical tetrahydronapthalene melanin production pathway. To our knowledge this is the first report on the genes involved in the biosynthesis of pigments important for sclerotial survival.