Three differentially expressed S-adenosylmethionine synthetases from Catharanthus roseus: molecular and functional characterization.

We describe the molecular and functional characterization of three closely related S-adenosyl-L-methionine synthetase (SAMS) isoenzymes from Catharanthus roseus (Madagascar periwinkle). The genes are differentially expressed in cell cultures during growth of the culture and after application of various stresses (elicitor, nutritional down-shift, increased NaCl). Seedlings revealed organ-specific expression and differential gene regulation after salt stress. A relationship analysis indicated that plant SAMS group in two main clusters distinguished by characteristic amino acid exchanges at specific positions, and this suggested differences in the enzyme properties or the regulation. SAMS1 and SAMS2 are of type I and SAMS3 is of type II. The properties of the isoenzymes were compared after heterologous expression of the individual enzymes, but no significant differences were detected in a) optima for temperature (37 to 45 degrees C) or pH (7 to 8.3); b) dependence on cations (divalent: Mg2+, Mn2+, Co2+; monovalent: K+, NH4+, Na+); c) K(m)s for ATP and L-methionine; d) inhibition by reaction products (S-adenosyl-L-methionine, PPi, Pi), by the reaction intermediate tripolyphosphate, and by the substrate analogues ethionine and cycloleucine; e) response to metabolites from the methyl cycle (L-homocysteine) or from related pathways (L-ornithine, putrescine, spermidine, spermine); f) native protein size (gel permeation chromatography). The results represent the first characterization of plant SAMS isoenzyme properties with individually expressed proteins. The possibility is discussed that the isoenzyme differences reflect specificities in the association with enzymes that use S-adenosyl-L-methionine.