Involved in the biosynthesis of polyamines which play a significant role in the structural and functional organization in the chromoid of E.coli by compacting DNA and neutralizing negative charges. Catalyzes the irreversible transfer (ping-pong mechanism) of a propylamine group from the amino donor S-adenosylmethioninamine (decarboxy-AdoMet) to putrescine (1,4-diaminobutane) to yield spermidine. Cadaverine (1,5-diaminopentane) and spermidine can also be used as the propylamine acceptor. putrescine + S-adenosyl 3-(methylsulfanyl)propylamine = H(+) + S-methyl-5'-thioadenosine + spermidine S-adenosyl 3-(methylsulfanyl)propylamine + spermidine = H(+) + S-methyl-5'-thioadenosine + spermine cadaverine + S-adenosyl 3-(methylsulfanyl)propylamine = aminopropylcadaverine + H(+) + S-methyl-5'-thioadenosine S-adenosyl 3-(methylsulfanyl)propylamine + spermidine = H(+) + S-methyl-5'-thioadenosine + thermospermine The activity is thought to be regulated mainly by the availability of decarboxylated S-adenosylmethionine. Inhibited by dicyclohexylamine, S-adenosyl-1,8-diamino-3-thiooctane (AdoDATO), sulfonium-deaminated analogs of S-adenosyl(5')-3-methylthiopropylamine, p-hydroxymercuribenzoate and N-ethylmaleimide. Optimum pH is 7.5. Optimum temperature is 50 degrees Celsius. Amine and polyamine biosynthesis; spermidine biosynthesis; spermidine from putrescine: step 1/1. Homodimer. Belongs to the spermidine/spermine synthase family.