Functional analysis of an S ‐adenosylhomocysteine hydrolase homolog of chestnut blight fungus

S ‐adenosylhomocysteine ( SAH ), formed after donation of the methyl group of S ‐adenosylmethionine ( SAM ) to a methyl acceptor, is reversibly hydrolyzed to adenosine ( ADO ) and homocysteine ( HCY ) by S ‐adenosylhomocysteine hydrolase ( SAHH ). In chestnut blight fungus ( C ryphonectria parasitica ), sahh , a hypovirus‐regulated gene that encodes a deduced SAHH protein was shown to have an SAHH enzymatic activity in vitro . Deletion of sahh resulted in the increased accumulation of intracellular SAH and SAM but decreased ADO , and a remarkably increased accumulation of transcripts that encode adenosine kinase, methionine adenosyltransferase, and an O ‐methyltransferase, key components of the methylation pathway. The Δ sahh knockout mutants showed a phenotype of slower growth rate, fewer aerial hyphae, loss of orange pigment, absence of asexual fruiting bodies and conidia, and a significant reduction in virulence. Deletion of sahh significantly reduced the accumulation level of transcripts of the cyp1 that encodes cyclophilin A as well as genes of the heterotrimeric G ‐protein signaling pathways including cpga1 , cpgb1 , and cpgc1 and ste12 , a target activated by the MAP kinase cascade. Taken together, we demonstrated that SAHH is required for virulence and multiple traits of phenotype in C . parasitica , by regulation of the expression of genes involved in key process of the cell.