Catalyzes the condensation of L-2,3-diaminopropionic acid (L-Dap) and citryl-diaminoethane to form L-2,3-diaminopropionyl-citryl-diaminoethane, the third step in staphyloferrin B biosynthesis. (S)-2,3-diaminopropanoate + 2-[(2-aminoethylcarbamoyl)methyl]-2-hydroxybutanedioate + ATP = 2-[(L-alanin-3-ylcarbamoyl)methyl]-3-(2-aminoethylcarbamoyl)-2-hydroxypropanoate + AMP + diphosphate Siderophore biosynthesis. Forms a mixture of monomer and dimer in solution. Up-regulated under iron-deficient growth conditions. Repressed by Fur under iron-rich growth conditions. Belongs to the IucA/IucC family. Truncated N-terminus.