Catalyzes the decarboxylation of carboxynorspermidine and carboxyspermidine in vitro. In vivo, responsible for synthesizing spermidine, but not sym-norspermidine. carboxynorspermidine + H(+) = CO2 + norspermidine carboxyspermidine + H(+) = CO2 + spermidine KM values are given with the protein sequence containing Glu instead of Lys at position 184, the effect of the variation on activity is unclear. Homodimer. Growth is highly compromised in polyamine auxotrophic bacteria, but can be restored by exogenous spermidine, sym-homospermidine and to a lesser extent by sym-norspermidine. Belongs to the Orn/Lys/Arg decarboxylase class-II family. NspC subfamily.