Removes the 1-phosphate group from (tetraacyl) lipid A species, has no requirement for the Kdo(2) moiety of lipid A. Has no 4'-phosphatase activity. Reduces sensitivity of S.meliloti strain 1021 to the cationic antimicrobial peptide (CAMP) polymyxin B. Bacterial outer membrane biogenesis; LPS lipid A biosynthesis. In this strain lipid A lacks phosphate groups, and both tetra- and pentaacylated species exist. Belongs to the lipid A LpxE 1-phosphatase family.