Extragenic suppressor mutations map to lpxA and glmU . Four of the five independently derived extragenic suppressor mutations mapped to lpxA. lpxA is centrally located in an essential operon containing five genes. The amino acid substitutions in LpxA were mainly located along the left-handed helix of the short parallel β-sheet. LpxA contains hexapeptide repeats that are a signature of an acetyltransferase enzyme (shown in gray). One extragenic suppressor mutation mapped to glmU, which is also centrally located in an essential operon containing 4 genes. The amino acid substitution mapped within the uridyltransferase domain located close to the N-terminus. LpxA and GlmU are both involved in membrane synthesis, and both proteins contain hexapeptide transferase signature domains.