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Francisella tularensis and related intracellular pathogens synthesize lipid A molecules that differ from their Escherichia coli counterparts. Although a functional orthologue of lpxK, the gene encoding the lipid A 4′-kinase, is present in Francisella, no 4′-phosphate moiety is attached to Francisella lipid A. We now demonstrate that a membrane-bound phosphatase present in Francisella novicida U112 selectively removes the 4′-phosphate residue from tetra- and pentaacylated lipid A molecules. A clone that expresses the F. novicida 4′-phosphatase was identified by assaying lysates of E. coli colonies, harboring members of an F. novicida genomic DNA library, for 4′-phosphatase activity. Sequencing of a 2.5-kb F. novicida DNA insert from an active clone located the structural gene for the 4′-phosphatase, designated lpxF. It encodes a protein of 222 amino acid residues with six predicted membrane-spanning …