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.