beta-cyanoalanine synthase is a mitochondrial cysteine synthase-like protein in spinach and Arabidopsis

beta-Cyano-alanine synthase (GAS; EC 4.4.1.9) plays an important role in cy anide metabolism in plants. Although the enzymatic activity of beta-cyano-A la synthase has been detected in a variety of plants, no cDNA or gene has b een identified so far. We hypothesized that the mitochondrial cysteine synt hase (CS; EC 4.2.99.8) isoform, Bsas3, could actually be identical to CAS i n spinach (Spinacia oleracea) and Arabidopsis. An Arabidopsis expressed seq uence tag database was searched for putative Bsas3 homologs and four new CS -like isoforms,ARAth;Bsas1;1, ARAth;Bsas3;1, ARAth;Bsas4;1, and ARAth;Bsas4 ;2, were identified in the process. ARAth;Bsas3;1 protein was homologous to the mitochondrial SPIol;Bsas3;1 isoform from spinach, whereas ARAth;Bsas4; 1 and ARAth;Bsas4;2 proteins defined a new class within the CS-like protein s family. In contrast to spinach SPIol;Bsas1;1 and SPIol;Bsas2;1 recombinan t proteins, spinach SPIol;Bsas3;1 and Arabidopsis ARAth; Bsas3;1 recombinan t proteins exhibited preferred substrate specificities for the CAS reaction rather than for the CS reaction, which identified these Bsas3 isoforms as GAS. Immunoblot studies supported this conclusion. This is the first report of the identification of CAS synthase-encoding cDNAs in a living organism. A new nomenclature for CS-like proteins in plants is also proposed.