In the extreme thermophile Thermus thermophilus, a disruption mutant of a gene homologous to speB (coding for agmatinase ؍ agmatine ureohydrolase) accumulated N 1 -aminopropylagmatine (N 8 -amidino-1,8-diamino-4-azaoctane, N 8 -amidinospermidine), a new compound, whereas all other polyamines produced by the wild-type strain were absent from the cells. Double disruption of speB and speE (polyamine aminopropyltransferase) resulted in the disappearance of N 1 -aminopropylagmatine and the accumulation of agmatine. These results suggested the following. 1) N 1 -Aminopropylagmatine is produced from agmatine by the action of an enzyme coded by speE. 2) N 1 -Aminopropylagmatine is a metabolic intermediate in the biosynthesis of unique polyamines found in the thermophile. 3) N 1 -Aminopropylagmatine is a substrate of the SpeB homolog. They further suggest a new biosynthetic pathway in T. thermophilus, by which polyamines are formed from agmatine via N 1 -aminopropylagmatine. To confirm our speculation, we purified the expression product of the speB homolog and confirmed that the enzyme hydrolyzes N 1 -aminopropylagmatine to spermidine but does not act on agmatine.Polyamines play important roles in cell proliferation and cell differentiation. Common polyamines such as putrescine, spermidine, and spermine are distributed ubiquitously in cells and tissues at relatively high concentrations (1, 2).Thermus thermophilus, of which the genome project was completed using two strains, HB8 and HB27 (Structural-Biological Whole Cell Project at www.srg.harima.riken.go.jp/ thermus/j_index.htm and see Ref. 3, respectively), produces a variety of polyamines including unusually long polyamines and branched ones (4) (see Fig. 9C). These long and branched polyamines have a marked effect of protecting and stabilizing nucleic acids (5, 6) and of activating cell-free polypeptide synthesis at high temperature (7-9).In many organisms, such as bacteria, yeast, animals, and plants, the first step of polyamine biosynthesis is production of putrescine by decarboxylation of L-ornithine (see Fig. 9A). An additional or alternative pathway of putrescine biosynthesis that is often seen in plants and sometimes in bacteria is decarboxylation of L-arginine followed by hydrolysis of agmatine. Agmatine ureohydrolase or agmatinase, coded by the speB gene, catalyzes this second reaction. The next step is production of spermidine and spermine by the addition of an aminopropyl group to putrescine and spermidine, respectively. This reaction is catalyzed by spermidine or spermine synthase (putrescine/spermidine aminopropyltransferase) coded by the speE gene (1).To investigate the polyamine biosynthetic pathway in T. thermophilus, we constructed a disruption strain of the speB gene homolog of T. thermophilus. Disruption of the speB gene homolog resulted in drastic reduction of triamines, longer and branched polyamines without accumulation of agmatine, and in accumulation of an unknown compound. Double disruption of speB and speE gene homologs resulted in disappear...