Involved in the synthesis of the bacterial cell wall. Catalyzes the addition of alanine into the interchain peptide bridge of peptidoglycan precursor using aminoacyl-tRNA(Ala) as amino acid donor. This alanine is added to the epsilon-amino group of the L-lysine of the peptidoglycan UDP-N-acetyl-alpha-D-muramoyl-L-alanyl-D-glutamyl-L-lysyl-D-alanyl-D-alanine, in a ribosome-independent mechanism (PubMed:11083873, PubMed:4248527, PubMed:12679335, PubMed:15901708, PubMed:23744707). Specific for UDP-N-acetyl-muramoyl-pentapeptide. Has no activity toward UDP-N-acetyl-muramoyl-tetrapeptide or UDP-N-acetyl-muramoyl-tripeptide (PubMed:15901708). Also acts on L-seryl-tRNA(Ser) (PubMed:4248527). L-alanyl-tRNA(Ala) + UDP-N-acetyl-alpha-D-muramoyl-L-alanyl-gamma-D-glutamyl-L-lysyl-D-alanyl-D-alanine = H(+) + tRNA(Ala) + UDP-N-acetyl-alpha-D-muramoyl-L-alanyl-gamma-D-glutamyl-N(6)-(L-alanyl)-L-lysyl-D-alanyl-D-alanine Optimum pH is 7.0-8.0. Belongs to the FemABX family.