A thermophilic and cyanide ion-tolerant bacterium, Bacillus stearothermophilus CN3 isolated from a hot spring in Japan, was found to produce thermostable beta-cyano-L-alanine synthase. The enzyme catalyzes the synthesis of beta-cyano-L-alanine from O-acetyl-L-serine and cyanide ions. The purified enzyme has a molecular mass of approximately 70 kDa and consists of two identical sub-units. It was stable in the pH range of 6.0 to 10.0 and up to 70degreesC. The enzyme also catalyzes the synthesis of various beta-substituted-L-alanine derivatives from O-acetyl-L-serine and nucleophilic reagents. The gene encoding the beta-cyano-L-alanine synthase was isolated from B. stearothermophilus CN3. Sequence homology analysis revealed that the beta-cyano-L-alanine synthase of the bacterium is O-acetyl-L-serine sulfhydrylase. A recombinant plasmid, constructed by ligation of the cloned gene and an expression vector, pKK223-3, was introduced into E. coli JM109. The transformed E. coli cells overexpressed beta-cyano-L-alanine synthase. Heat stable beta-cyano-L-alanine synthase can be applied to the synthesis of [4-C-11]L-2,4-diaminobutyric acid as a tracer for positron emission tomography.