Catalyzes the synthesis of N-((2S)-2-amino-2-carboxyethyl)-L-glutamate (ACEGA) from O-phospho-L-serine and L-glutamate. Involved in the biosynthesis of L-2,3-diaminopropionic acid (L-Dap), a precursor of staphyloferrin B and antibiotics. L-glutamate + O-phospho-L-serine = H(+) + N-[(2S)-2-amino-2-carboxyethyl]-L-glutamate + phosphate Siderophore biosynthesis. Homodimer. Up-regulated under iron-deficient growth conditions. Repressed by Fur under iron-rich growth conditions. Belongs to the cysteine synthase/cystathionine beta-synthase family. SbnA subfamily. Truncated N-terminus.