Catalyzes the decarboxylation of citryl-L-2,3-diaminopropionic acid to citryl-diaminoethane, the second step in staphyloferrin B biosynthesis. 2-[(L-alanin-3-ylcarbamoyl)methyl]-2-hydroxybutanedioate + H(+) = 2-[(2-aminoethylcarbamoyl)methyl]-2-hydroxybutanedioate + CO2 Siderophore biosynthesis. Homodimer. Up-regulated under iron-deficient growth conditions. Repressed by Fur under iron-rich growth conditions. Belongs to the Orn/Lys/Arg decarboxylase class-II family.