S-adenosyl 3-(methylsulfanyl)propylamine + spermidine = H(+) + S-methyl-5'-thioadenosine + spermine Amine and polyamine biosynthesis; spermine biosynthesis; spermine from spermidine: step 1/1. Heterodimer. Component of a multiprotein complex. Interacts with SPDSYN1 and SPDSYN2. A number of isoforms are produced. According to EST sequences. Expressed predominantly in stem internodes, flower buds and roots. Up-regulated by abscisic acid and salt stress. Belongs to the spermidine/spermine synthase family. Truncated N-terminus.