Involved in lipopolysaccharide (LPS) biosynthesis. Catalyzes the transfer of two 3-deoxy-D-manno-octulosonate (Kdo) residues from CMP-Kdo to lipid IV(A), the tetraacyldisaccharide-1,4'-bisphosphate precursor of lipid A. CMP-3-deoxy-beta-D-manno-octulosonate + lipid IVA (E. coli) = alpha-Kdo-(2->6)-lipid IVA + CMP + H(+) alpha-Kdo-(2->6)-lipid IVA + CMP-3-deoxy-beta-D-manno-octulosonate = alpha-Kdo-(2->4)-alpha-Kdo-(2->6)-lipid IVA + CMP + H(+) Catalytic activity is inhibited by the antibiotic polymixin B and by Re endotoxin. Optimum pH is 7. Glycolipid biosynthesis; KDO(2)-lipid A biosynthesis; KDO(2)-lipid A from CMP-3-deoxy-D-manno-octulosonate and lipid IV(A): step 1/4. Glycolipid biosynthesis; KDO(2)-lipid A biosynthesis; KDO(2)-lipid A from CMP-3-deoxy-D-manno-octulosonate and lipid IV(A): step 2/4. Bacterial outer membrane biogenesis; LPS core biosynthesis. The N-terminal half of KdtA is responsible for determining the number of Kdo residues that are transferred to lipid IVA. Degraded by the protease FtsH; therefore FtsH regulates the addition of the sugar moiety to the LPS and thus the maturation of the LPS precursor. Cells lacking this gene display growth defects, absence of Kdo transferase activity, and accumulate massive amounts of lipid IV(A). Belongs to the glycosyltransferase group 1 family. Glycosyltransferase 30 subfamily.