A thermophilic and cyanide ion-tolerant bacterium, Bacillus stearothermophilus CN3 isolated from a hot spring in Japan, was found to produce thermostable gamma-cyano-alpha-aminobutyric acid synthase. The enzyme was purified and characterized. The purified enzyme has a molecular mass of approximately 180 kDa and consists of four identical subunits. It was stable in the pH range of 6.0 to 10.5 and up to 60degreesC. The enzyme catalyzed the gamma-replacement reaction of O-acetyl-L-homoserine with cyanide ions. The gene encoding the gamma-cyano-alpha-aminobutyric acid synthase was isolated from B. stearothermophilus CN3. Sequence homology analysis revealed that the gamma-cyano-alpha-aminobutyric acid synthase from the bacterium is O-acetyl-L-homoserine sulfhydrylase. A recombinant plasmid, constructed by ligation of the cloned gene and an expression vector, was introduced into Escherichia coli JM109. The transformed E. coli cells overexpressed gamma-cyano-alpha-aminobutyric acid synthase. The heat stable gamma-cyano-alpha-aminobutyric acid synthase can be applied to the synthesis of [5-C-11]L-glutamic acid used as a tracer for positron emission tomography.