The S. meliloti acpXL-lpxXL cluster mutants have alterations in their lipid A biosynthesis. Shown is MALDI-TOF MS of the lipid A species produced by each strain (acpXL (A), Rm1021 (B), fabF2XL without and with pSmfabF2XL (C), fabF1XL without and with pSmfabF1XL (D), adhA2XL without and with pSmadhA2XL (E), and fabZXL without and with pSmfabZXL (F)). The capital letters indicate the major lipid A species (see Fig. 3 for structures) in each sample (pSmacpXL, pSmfabZXL, pSmfabF2XL, pSmfabF1XL, pSmadhA2XL, and pSmlpxXL are wild-type versions of the disrupted genes in the mutant strains cloned into pRF771).