Catalyzes the production of spermidine from putrescine and decarboxylated S-adenosylmethionine (dcSAM). Has a strong preference for putrescine as substrate, and has very low activity towards 1,3-diaminopropane. Has extremely low activity towards spermidine.
Amine and polyamine biosynthesis; spermidine biosynthesis; spermidine from putrescine: step 1/1.
Belongs to the spermidine/spermine synthase family. Contains 1 PABS (polyamine biosynthesis) domain.