MOLECULAR-CLONING AND SEQUENCE-ANALYSIS OF THE CDNA-ENCODING RAT-LIVER CYSTEINE SULFINATE DECARBOXYLASE (CSD)

The taurine biosynthesis enzyme, cysteine sulfinate decarboxylase (CSD ), was purified to homogeneity from rat liver. Three CSD peptides gene rated by tryptic cleavage were isolated and partially sequenced. Two o f them showed a marked homology with glutamate decarboxylase and their respective position on the CSD amino acid sequence was postulated acc ordingly. Using appropriate degenerated primers derived from these two peptides, a PCR amplified DNA fragment was generated from liver poly( A)(+) mRNA, cloned and used as a probe to screen a rat liver cDNA libr ary. Three cDNAs, length around 1800 bp, were isolated which all conta ined an open reading frame (ORF) encoding a 493 amino acid protein wit h a calculated molecular mass of 55.2 kDa close to the experimental va lues for CSD. The encoded protein contained the sequence of the three peptides isolated from homogenous liver CSD. Our data confirm and sign ificantly extend those recently published (Kaisaki et al. (1995) Bioch im. Biophys. Acta 1262, 79-82). Indeed, an additional base pair found 1371 bp downstream from the initiation codon led to a shift in the ope n reading frame which extended the carboxy-terminal end by 15 amino ac id residues and altogether modified 36 amino acids. The validity of th is correction is supported by the finding that the corrected reading f rame encoded a peptide issued from CSD tryptic cleavage that was not e ncoded anywhere in the CSD sequence previously reported.