Catalyzes the addition of an amino acid to the nucleotide precursor UDP-N-acetylmuramoyl-L-alanyl-D-glutamate (UMAG) in the biosynthesis of bacterial cell-wall peptidoglycan. Cell wall biogenesis; peptidoglycan biosynthesis. Carboxylation is probably crucial for Mg(2+) binding and, consequently, for the gamma-phosphate positioning of ATP. Belongs to the MurCDEF family. MurE subfamily.