Lipid A from E. coli, F. novicida, and the F. novicida lpxF mutant. A. The predominant lipid A moiety of wild-type E. coli LPS is a hexa-acylated disaccharide of glucosamine, substituted with phosphate groups at the 1- and 4′-positions (4). In polymyxin-resistant mutants of E. coli, an l-Ara4N residue is attached to the 4′-phosphate group (not shown) by ArnT, a glycosyltransferase that uses undecaprenyl phosphate-α-l-Ara4N (Fig. 2) as its donor substrate (4). B. The major lipid A species of F. novicida is a tetra-acylated disaccharide of glucosamine lacking the 3′-hydroxyacyl chain and 4′-phosphate group (14, 15). An alpha-linked d-GalN residue, which appears to be unique to strains of Francisella, is attached to the 1-phosphate group (14, 15). Over 90% of Francisella lipid A is termed “free” in the sense that it is not linked to a core oligosaccharide and O-antigen (15). However, some LPS is also synthesized, and its lipid A is modified with GalN in the same manner (14). C. Free lipid A of a Francisella mutant lacking LpxF, the 4′-phosphatase, retains both the 4′-phosphate group and the 3′-hydroxyacyl chain, consistent with an obligatory order of processing (19). The active site of the inner membrane protein LpxF faces the periplasm (20). The gene encoding the 3′-deacylase has not been identified, but it may be an outer membrane protein.