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 decarboxylation of N1-(3-aminopropyl)agmatine to yield spermidine and urea. It cannot use agmatine as substrate. H2O + N(1)-aminopropylagmatine = spermidine + urea Binds 2 manganese ions per subunit. Amine and polyamine biosynthesis; spermidine biosynthesis. Cells lacking this gene show a significant defect of growth at 78 degrees Celsius and accumulate N1-aminopropylagmatine. The double mutant of speB and speE shows defective growth at 70 degrees Celsius and significant defective growth at 78 degrees Celsius. It accumulates agmatine and N1-aminopropylagmatine. In T.thermophilus, the biosynthetic pathways of spermidine operates via N1-aminopropylagmatine without the production of putrescine. Belongs to the arginase family.