The filamentous fungus, Aspergillus flavus, produces the toxic and carcinogenic, polyketide-derived family of secondary metabolites termed aflatoxins (AFs). In addition to the AF biosynthetic gene cluster, analysis of the A. flavus genome has identified 55 gene clusters predicted to be associated with secondary metabolism. To date, very few of the metabolites produced by these clusters have been identified. Secondary metabolism is controlled by global regulators such as LaeA and VeA. In a veA knockout mutant we identified a significantly down-regulated polyketide synthase (PKS) gene belonging to cluster 27. The metabolite produced by this cluster was unknown butin silico cluster analysis predicted that cluster 27 would consist of the PKS and four other genes. qRT-PCR analysis confirmed that expression of the cluster 27 PKS (pks27) gene was down-regulated in the veA mutant. The function of pks27 was determined by gene knockout and complementation approaches. Inactivation of the pks27gene resulted in loss of the dark melanin-like pigment associated with A. flavus sclerotia. Sclerotia are asexual, survival structures produced by condensation of mycelia and function as propagules in the field. Conidial pigmentation did not appear to be affected by the pks27 knockout. Complementation of the pks27 knockout strain with a copy of the pks27 gene under control of the inducible A. oryzae amyB promoter restored wild-type sclerotial pigmentation. The pks27 mutant is being examined for sclerotial resistance to temperature and UV light and the metabolite is being characterized by HPLC-MS. To our knowledge this is the first report on the identification of a gene that encodes a sclerotia-specific pigment. The pigment likely plays a role in sclerotial resistance to environmental stresses and fungal survival.
Full conference title:
- ASM 112th (2012)