The enzyme has been originally characterized from the protist Euglena gracilis [1,2]. The enzyme from the archaeon Sulfolobus solfataricus can transfer the propylamine moiety from S-adenosyl 3-(methylsulfanyl)propylamine to putrescine, sym-norspermidine and spermidine with lower efficiency [3]. cf. EC 2.5.1.16 (spermidine synthase) and EC 2.5.1.22 (spermine synthase).