The previously described Streptomyces tendae nikC::aph mutant was used to mutasynthesize nikkomycins B_x and B_z. The mutant is deficient in L-lysine 2-aminotransferase, which transaminates lysine to form piperideine 2-carboxylate, the precursor of the peptidyl side chain of the biologically active nikkomycins I, J, X, and Z, and is therefore unable to produce these nikkomycins. The mutant accumulates the biologically inactive biosynthetic nucleoside precursors nikkomycins C_x and C_z. Resting cell cultures of the mutant fed with benzoic acid produced the biologically active nikkomycins B_x and B_z, which contain 2-amino-4-hydroxy-3-methyl-4-(4'-hydroxyphenyl)butanoic acid as the peptidyl side chain. The structures of nikkomycins B_x and B_z were confirmed by mass spectrometry and NMR. Nikkomycins B_x and B_z exhibit significantly higher pH stability than their analogues nikkomycins X and Z.