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. An efficient aminopropyltransferase activity has been also observed with norspermidine which produces thermine, and spermidine which produces spermine and thermospermine. The aminopropyl activity with homospermidine, mitsubishine, thermine, 1,3-diaminopropane, putrescine (1,4-diaminobutane), spermine and caldopentamine are very low. 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 norspermidine + S-adenosyl 3-(methylsulfanyl)propylamine = H(+) + norspermine + S-methyl-5'-thioadenosine S-adenosyl 3-(methylsulfanyl)propylamine + spermidine = H(+) + S-methyl-5'-thioadenosine + thermospermine S-adenosyl 3-(methylsulfanyl)propylamine + spermidine = H(+) + S-methyl-5'-thioadenosine + spermine kcat is 0.73 sec(-1) for aminopropyl transferase activity with S-adenosylmethioninamine as substrate (at pH 9 and 37 degrees Celsius). kcat is 0.65 sec(-1) for aminopropyl transferase activity with spermidine as substrate (at pH 9 and 37 degrees Celsius). kcat is 0.53 sec(-1) for aminopropyl transferase activity with norspermidine as substrate (at pH 9 and 37 degrees Celsius). kcat is 0.37 sec(-1) for aminopropyl transferase activity with agmatine as substrate (at pH 9 and 37 degrees Celsius). kcat is 0.29 sec(-1) for aminopropyl transferase activity with mitsubishine as substrate (at pH 9 and 37 degrees Celsius). Optimum pH is 8.5 at 60 degrees Celsius. Optimum temperature is above 80 degrees Celsius. Homotetramer. Cells lacking both speB and speE genes show defective growth at 70 degrees Celsius and significantly defective growth at 78 degrees Celsius. They accumulate agmatine and N1-aminopropylagmatine. In T.thermophilus, the biosynthetic pathways of spermidine operates via N1-aminopropylagmatine without the production of putrescine. Belongs to the spermidine/spermine synthase family.