Two S-adenosylmethionine synthetase-encoding genes differentially expressed during adventitious root development in Pinus contorta

Two S-adenosylmethionine synthetase (SAMS) cDNAs, PcSAMS1 and PcSAMS2, have been identified in Pinus contorta. We found that the two genes are differe ntially expressed during root development. Thus, PcSAMS1 is preferentially expressed in roots and exhibits a specific expression pattern in the merist em at the onset of adventitious root development, whereas PcSAMS2 is expres sed in roots as well as in shoots and is down-regulated during adventitious root formation. The expression of the two SAMS genes is different from the SAMS activity levels during adventitious root formation. We conclude that other SAMS genes that remain to be characterized may contribute to the obse rved SAMS activity, or that the activities of PcSAMS1 and PcSAMS2 are affec ted by post-transcriptional regulation. The deduced amino acid sequences of PcSAMS1 and PcSAMS2 are highly divergent, suggesting different functional roles. However, both carry the two perfectly conserved motifs that are comm on to all plant SAMS. At the protein level, PcSAMS2 shares about 90% identi ty to other isolated eukaryotic SAMS, while PcSAMS1 shares less than 50% id entity with other plant SAMS. In a phylogenetic comparison, PcSAMS1 seems t o have diverged significantly from all other SAMS genes. Nevertheless, PcSA MS1 was able to complement a Saccharomyces cerevisiae sam1 sam2 double muta nt, indicating that it encodes a functional SAMS enzyme.