Involved in the biosynthesis of pyridine alkaloid natural products, leading mainly to the production of anabasine, anatabine, nicotine and nornicotine, effective deterrents against herbivores with antiparasitic and pesticide properties (neurotoxins); nornicotine serves as the precursor in the synthesis of the carcinogen compound N'-nitrosonornicotine (NNN) (By similarity). Catalyzes the first and rate-limiting step of polyamine biosynthesis that converts ornithine into putrescine, which is the precursor for the polyamines, spermidine and spermine (PubMed:11736657). Can also use, with a lower efficiency, L-lysine as substrate to produce cadaverine (PubMed:11736657). Polyamines are essential for cell proliferation and are implicated in cellular processes, ranging from DNA replication to apoptosis (By similarity). H(+) + L-lysine = cadaverine + CO2 H(+) + L-ornithine = CO2 + putrescine Repressed by alpha-difluoromethylornithine (DFMO), 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) and salicylaldehyde. kcat is 77.78 sec(-1) with L-ornithine as substrate (at pH 8.0) (PubMed:11736657). kcat is 0.236 sec(-1) with L-lysine as substrate (at pH 6.8) (PubMed:11736657). Optimum pH is 8.0 with L-ornithine as substrate (PubMed:11736657). Optimum pH is 6.8 with L-lysine as substrate (PubMed:11736657). Alkaloid biosynthesis; nicotine biosynthesis. Amine and polyamine biosynthesis; putrescine biosynthesis via L-ornithine pathway; putrescine from L-ornithine: step 1/1. Homodimer (PubMed:11736657). Only the dimer is catalytically active, as the active sites are constructed of residues from both monomers (By similarity). Belongs to the Orn/Lys/Arg decarboxylase class-II family.