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 agmatine to yield N1-aminopropylagmatine. It can also use cadaverine (1,5-diaminopentane) and putrescine (1,4-diaminobutane) as substrate with a lower activity than that of agmatine. The reaction involves a nucleophilic attack on the C-3 methylene of the propylamine moiety adjacent to the positively charged sulfur of decarboxy-AdoMet. agmatine + S-adenosyl 3-(methylsulfanyl)propylamine = H(+) + N(1)-aminopropylagmatine + S-methyl-5'-thioadenosine putrescine + S-adenosyl 3-(methylsulfanyl)propylamine = H(+) + S-methyl-5'-thioadenosine + spermidine cadaverine + S-adenosyl 3-(methylsulfanyl)propylamine = aminopropylcadaverine + H(+) + S-methyl-5'-thioadenosine kcat is 0.3 sec(-1) for aminopropyl transferase activity with putrescine as substrate (at pH 9 and 70 degrees Celsius). kcat is 0.28 sec(-1) for aminopropyl transferase activity with agmatine as substrate (at pH 9 and 70 degrees Celsius. kcat is 0.02 sec(-1) for aminopropyl transferase activity with cadaverine as substrate (at pH 9 and 70 degrees Celsius. Amine and polyamine biosynthesis; spermidine biosynthesis; spermidine from putrescine: step 1/1. Homodimer or homotetramer. Cells lacking this gene show decreased growth at 85 and 93 degrees Celsius. The growth rate is slightly restored at 85 degrees Celsius by the addition of spermidine, however growth at 93 degrees Celsius is not restored even when spermidine is added. This mutant accumulates agmatine at 85 degrees Celsius. In T.kodakarensis, two kinds of synthetic pathways from agmatine to spermidine are predicted. One is the pathway via putrescine (pathway I), and the other is that via N1-aminopropylagmatine (pathway II). Belongs to the spermidine/spermine synthase family.