Fungal S-adenosylmethionine synthetase and the control of development and secondary metabolism in Aspergillus nidulans

The filamentous fungus Aspergillus nidulans carries a single gene for the S-adenosylmethionine (SAM) synthetase SasA, whereas many other organisms possess multiple SAM synthetases. The conserved enzyme catalyzes the reaction of methionine and ATP to the ubiquitous methyl group donor SAM. SAM is the main methyl group donor for methyltransferases to modify DNA, RNA, protein, metabolites, or phospholipid target substrates. We show here that the single A. nidulans SAM synthetase encoding gene sasA is essential. Overexpression of sasA, encoding a predominantly cytoplasmic protein, led to impaired development including only small sterile fruiting bodies which are surrounded by unusually pigmented auxiliary Hülle cells. Hülle cells are the only fungal cell type which does not contain significant amounts of SasA. Sterigmatocystin production is altered when sasA is overexpressed, suggesting defects in coordination of development and secondary metabolism. SasA interacts with various metabolic proteins including methionine or mitochondrial metabolic enzymes as well as proteins involved in fungal morphogenesis. SasA interaction to histone-2B might reflect a putative epigenetic link to gene expression. Our data suggest a distinct role of SasA in coordinating fungal secondary metabolism and development.

► The single SAM synthetase gene sasA is essential in Aspergillus nidulans.
► sasA is highly expressed during vegetative growth.
► Overexpression of sasA leads to sterile microcleistothecia with pigmented Hülle cells.
► Overexpression of sasA reduces sterigmatocystin production.
► The cytoplasmic SasA was co-purified with proteins involved in growth and metabolism.