Involved in the biosynthesis of polyamines which are thought to support the growth of thermophilic microorganisms under high-temperature conditions. It seems that long-chain and branched-chain of polyamines effectively stabilize DNA and RNA, respectively. Catalyzes the irreversible transfer of a propylamine group from the amino donor S-adenosylmethioninamine (decarboxy-AdoMet) to 1,3-diaminopropane to yield sym-norspermidine (bis(3-aminopropyl)amine). It can also synthesize thermospermine from spermidine with a very low activity. propane-1,3-diamine + S-adenosyl 3-(methylsulfanyl)propylamine = H(+) + norspermidine + S-methyl-5'-thioadenosine Homodimer or homotetramer. Belongs to the spermidine/spermine synthase family.