cDNA for S-Adenosyl-L-Homocysteine Hydrolase from Catharanthus roseus

The majority of the Met synthesized in plants is utilized for methylations performed with S-adenosyl-L-Met. These reactions play a major role in the modification of a large variety of acceptor molecules, such as lipids, polysaccharides, nucleic acids, proteins, and secondary plant products (reviewed by Giovanelli, 1987). The hydrolysis of SAH to adenosine and L-homocysteine by SAH hydrolase is one of the steps in the regeneration of S-adenosyl-L-Met. In cell cultures of Catharanthus roseus, a change from MX growth medium to a solution of 8% Suc induces the expression of many proteins (Vetter et al., 1992; our Ünpublished results). In a differential screen for induced mRNAs we identified a cDNA coding for a protein of more than 90% identity with the two SAH hydrolases known from plants (Table I). The similarity to the enzymes from vertebrates (human, AC P23526; rat, AC M15185), Caenorhabditis elegans (AC P27604), Dictyostelium discoideum (AC P10819), and the bacterium Rhodobacter capsulatus (AC M80630) was less pronounced (60-63%). SAH hydrolases are highly conserved enzymes (Sganga et al., 1992), and two consensus patterns have been defined previously (Bairoch, 1992). The sequences of the plant SAH hydrolases suggest that the consensus patterns should be extended to allow additional conservative amino acid replacements in the two motifs. The C. roseus protein deviates by one exchange from consensus pattern 1 (C-N-I-F-S-T-Q-EXaa-A-A-A-A-I-A instead of C-N-I-F-S-T-Q-@ or NI-XaaA-A-A-A-I-A), and the parsley protein differs in two amino acids from consensus pattern 2 (G-K-V-A-L-1-Xaa-G-Y-G-DV-G-K-G instead of G-K-V-A-y-y-Xaa-G-Y-G-D-V-G-K-G). In view of the high overall conservation of SAH hydrolases, it is interesting that there are distinct size differences. The three plant enzymes contain 485 amino acids, whereas the proteins from vertebrates (man, rat), D. discoideum, or C. elegans are smaller by about 50 amino acids (430-437 residues). Apart from a shorter NH2-terminal region, they lack a specific stretch of 40 amino acids (positions 150-190 in the plant enzymes). It seems possible that the different subunit composition of the enzymes may be one of the reasons (see Ogawa et al., 1987, for overview). The enzyme from the bacterium R. capsulatus appears to be more closely related to Table 1. Characteristics of CRSAHH from C. meus