Relationships between ACC synthase isozymes calculated using phenetic data. The catalytical and evolutionary aspects

The study was undertaken as an attempt at explaining interrelations between the presently found much diversified isoforms of ACC synthase, a key enzyme of the ethylene synthesis pathway in higher plants. The results of our study analysed on a background of current knowledge implied some not yet discussed, physiological and evolutionary aspects of the plant ACS isozymes. The computer methods applied are based on analysis of phenetic data. The subjects of the study were 159 synthases from higher plants and the only one known from the fungus Penicillium citrinum. The phenograms of 95 full-length sequence ACC synthases from higher plants and the synthase from Penicillium citrinum were made for cDNA and polypeptides by the two different techniques: UPGMA and Kitsch’s method and were almost identical. They indicate the presence of three significantly different types of ACS plant isozymes denoted as type A, type B and A3 group. A number of arguments are given showing that the synthases denoted as A1 and A2 group, hitherto treated by many authors as separate evolutionary lineages, are of the same type A. The presence of a new poorly recognised and probably evolutionary separate group of synthases, denoted as A3, is for the first time evidenced. The type B isozymes are shown to comprise two distinct groups B1 and B2, and B1 group can be divided into subgroups. A comparison of the proportion of genes encoding different type synthases in taxa distinguished by molecular systematicians has shown that similar sets and numbers of genes of type A and A3 group synthases are conserved in the genomes of eudicots and noneudicots. In the genomes of noneudicots the genes of B1 (B1a and B1b) group synthaseswere not found. The genes of B1(B1a and B1b) group in full diversity were established to occur in Rosidae and as B1a subgroup in Asteridae, the subclasses representing eudicots. It is evidenced that the C-terminal region of the enzyme, hitherto treated as highly variable, and the last aa residue are conserved within the B type and A1 synthases. The acidic character of the polypeptides and the lack of conservation of the last aa residue in the synthases of A2 group are explained by the loss of the 3′-terminal fragment by some A type genes. The C-terminal region of B type and A1 group synthases was found to contain a fragment similar to the so-called MAPK-docking domain, which suggests that these isozymes are controlled by the processes of phosphorylation. The aa residues forming the catalytically important three-dimensional structure called the hydrophobic pocket for the adenine ring of SAM can differ slightly in the particular type or groups of ACS. Moreover, A3 group differs from the other synthase groups by the aa residues required for correct orientation of PLP in the active site. Probably, particular ACS groups can differ in the kinetic features and the half-life.