Mutually exclusive pathways for biosynthesis of polyamine homospermidine dependent on putrescine or agmatine

Homospermidine is a structural analog of spermidine, a polyamine that is essential for growth and cell proliferation in eukaryotes. The eukaryotic enzyme deoxyhypusine synthase (DHS) can synthesize homospermidine directly from spermidine and putrescine. Nonhomologous bacterial homospermidine synthase (HSS) synthesizes homospermidine directly from two molecules of putrescine. SpeY is a homolog of DHS in bacteria that is essential for homospermidine biosynthesis in a cyanobacterium. Recently, it was shown in the bacterial hyperthermophile Thermus thermophilus, that SpeY couples two molecules of agmatine produced by arginine decarboxylase to form N¹,N⁹-bis(guanidino)homospermidine (G44G), which is then converted to homospermidine by a ureohydrolase. We sought to determine whether synthesis of G44G was a common feature of phylogenetically diverse SpeY homologs. Herein, we demonstrate that SpeY homologs from diverse bacterial phyla, and from species encoding arginine decarboxylases from three different protein folds, with either agmatinase or agmatine iminohydrolase partners, all form G44G. Diverse agmatine, N¹-aminopropylagmatine and presumed G44G ureohydrolases convert G44G to homospermidine but SpeY-associated G44G ureohydrolases do not act on agmatine. Similarly, diverse agmatine, N¹-aminopropylagmatine and presumed G44G iminohydrolases convert G44G to N¹,N⁹-bis(carbamoyl)homospermidine but SpeY-associated G44G iminohydrolases do not act on agmatine. We identified ornithine decarboxylases in both the SpeY and HSS pathways that have become neofunctionalized to arginine decarboxylases, and in the HSS pathway are associated with an agmatinase. Bacterial homospermidine biosynthetic pathways are therefore dependent either on agmatine/SpeY or putrescine/HSS. Our study highlights a new role for agmatine in bacterial polyamine biosynthesis, which also includes the N¹-aminopropylagmatine and carboxyaminopropylagmatine pathways for spermidine production.