Catalyzes the synthesis of N-((2S)-2-amino-2-carboxyethyl)-L-glutamate (ACEGA) from O-phospho-L-serine and L-glutamate. L-glutamate + O-phospho-L-serine = H(+) + N-[(2S)-2-amino-2-carboxyethyl]-L-glutamate + phosphate Siderophore biosynthesis. Homodimer. Belongs to the cysteine synthase/cystathionine beta-synthase family. SbnA subfamily.