CLONING OF AN ARABIDOPSIS-THALIANA CDNA-ENCODING CYSTATHIONINE BETA-LYASE BY FUNCTIONAL COMPLEMENTATION IN ESCHERICHIA-COLI

Cystathionine beta-lyase, the second enzyme involved in the methionine biosynthetic pathway in plants, catalyses the synthesis of homocystei ne from cystathionine. A cDNA encoding cystathionine beta-lyase was cl oned from an Arabidopsis thaliana expression library by complementatio n of an Escherichia coli mutant deficient in this enzyme. As deduced f rom the full-length nucleotide sequence (1.7 kb), the polypeptide cont ains 464 amino acids and presents a predicted M(r) of 50372. A. thalia na cystathionine beta-lyase exhibits 22% sequence identity with the E. coli corresponding enzyme and contains a 70 amino acid N-terminal add itional sequence compared with the bacterial protein. Since the genera l features of chloroplast transit peptides could be observed in this a mino-terminal extension, we propose a chloroplast localization for the cDNA-encoded enzyme. Southern blot analysis suggested that cystathion ine beta-lyase is encoded by a single copy gene in A. thaliana.