Involved in the modification of the lipid A domain of lipopolysaccharides (LPS). Transfers a phosphate group from undecaprenyl pyrophosphate (C55-PP) to lipid A to form lipid A 1-diphosphate. Contributes to the recycling of undecaprenyl phosphate (C55-P) (PubMed:18047581). In vitro, has low undecaprenyl-diphosphate phosphatase activity (PubMed:17660416). alpha-Kdo-(2->4)-alpha-Kdo-(2->6)-lipid A + di-trans,octa-cis-undecaprenyl diphosphate = (Kdo)2-lipid A 1-diphosphate + di-trans,octa-cis-undecaprenyl phosphate Inhibited by BasR. This regulation does not occur at the level of transcription, but rather following the assembly of LpxT into the inner membrane. Optimum pH is 7.0. Significant activity is observed from pH 5.5 to pH 7.5. Bacterial outer membrane biogenesis; lipopolysaccharide biosynthesis. Transferase activity takes place on the periplamic side of the inner membrane. Deletion of the gene results in the production of LPS containing only the bis-phosphorylated lipid A species. In E.coli, lipid A contains typically a monophosphate unit at positions 1 and 4' (bis-phosphorylated lipid A). However, one-third of the lipid A contains an unsubstituted diphosphate unit at position 1 (lipid A 1-diphosphate). Belongs to the LpxT phosphotransferase family.