A gene encoding a Delta 9 fatty acid desaturase (Delta 9-desaturase) was isolated from L. edodes. The open reading frame of this gene (named Le-FAD1) consisted of 1416 bp. The predicted protein of 471 amino acids shares 47-57% sequence identity with Delta 9-desaturases from other fungi. Expression of the Le-FAD1 ORF complemented genetic disruption of the Saccharomyces cerevisiae OLE1 gene (Sc-OLE1). Fatty acid analysis of the Le-FAD1-expressing S. cerevisiae ole1-disrupted strain showed that Le-FAD1 protein had slight activity with palmitic acid (16:0) as the substrate and high activity with stearic acid (18:0). Analysis of transcriptional expression showed that the levels of Le-FAD1 mRNA in the primordium and fruiting body of L. edodes were higher than in mycelia cultivated at 18 degrees C as well as 25 degrees C. These results correlated with the increase of unsaturated fatty acids (UFAs) in total lipids during fruiting-body formation.