The Pseudomonas aeruginosa Type VI secretion system toxin Tse8 evolved from a novel N-carbamoylputrescine amidohydrolase.

The polyamine putrescine is synthesized primarily from L-arginine via agmatine in bacteria. There are currently three known routes from agmatine to putrescine, including direct conversion by agmatinase. The other two routes use agmatine deiminase to produce N-carbamoylputrescine from agmatine, then one of two nonhomologous enzymes, putrescine transcarbamylase or N-carbamoylputrescine amidohydrolase (NCPAH) convert N-carbamoylputrescine to putrescine. Here, we functionally identify enzymes from phylogenetically distant bacteria, the gamma-proteobacterium Shewanella oneidensis, and the actinomycetota species Microterricola gilva, that are novel alternative, nonhomologous, noncanonical NCPAHs that we term AguY, which have emerged by convergent evolution. Kinetic analysis indicates that the AguY enzymes are as efficient as the canonical NCPAH from Pseudomonas aeruginosa, in converting N-carbamoylputrescine to putrescine. Genomic evidence suggests that the AguY enzymes may participate in putrescine biosynthetic or agmatine catabolic pathways, and are occasionally encoded in genomes that also encode agmatinase. We show that the Type VI secretion system toxin Tse8 from Pseudomonas aeruginosa has evolved from AguY. It is formally possible that AguY evolved directly or indirectly from the ancient glutamine amidohydrolase GatA, a component of the transamidosome, an RNA/protein complex required for production of glutamine-charged tRNA. Our study provides a further example of the prevalence of convergent evolution and horizontal gene transfer in polyamine biosynthesis, suggesting pervasive selective pressure to evolve polyamine metabolism in bacteria.