beta-Cyanoalanine synthase and cysteine synthase from potato: molecular cloning, biochemical characterization, and spatial and hormonal regulation

beta -Cyanoalanine synthase (CAS, L-3-cyanoalanine synthase; EC 4.4.1.9) is the most important enzyme in cyanide metabolism. In addition to CAS, cyste ine synthase (CS, EC 4.2.99.8) possesses CAS activity. To explore the physi ological significance of cyanide metabolism, we isolated the cDNA clones co rresponding to purified CAS (designated PCAS-1 and PCAS-2) and CS (designat ed PCS-1 and PCS-2) from potato using the information of these amino acid s equences. The recombinant proteins of PCS-1, PCS-2 and PCAS-1 catalyzed bot h CAS and CS reactions, although the ratios between CAS and CS activity wer e remarkably different. PCAS-1 preferred the substrates for the CAS reactio n to the substrates for the CS reaction. From the kinetic characters and ho mology of amino acid sequences with known CS-like proteins, PCS-1, PCS-2 an d PCAS-1 were identified as cytosolic CS, plastidic CS and mitochondrial CA S, respectively. The highest level of CAS activity, CAS protein and its mRN A were detected in potato buds. Stimulation of CAS activity and protein acc umulation by ethylene without the concomitant increase of its mRNA suggeste d that ethylene induces CAS protein accumulation at the post-transcriptiona l level.