(a) Biosynthetic reactions from 3-deoxy-d-manno-oct-2-ulosonic acid (Kdo)2-lipid A to Hep2-Kdo2-lipid A. WaaC adds an l-d-heptose to Kdo2-lipid A, forming Hep-Kdo2-lipid A, and WaaF adds another l-d-heptose to Hep-Kdo2-lipid A, forming Hep2-Kdo2-lipid A. The donor of l-d-heptose is ADP-l-d-heptose, which is converted from ADP-d-d-heptose by ADP-l-glycero-d-manno-heptose-6-epimerase RfaD [19]. The lipid A, Kdo and heptose groups are shown in black, red and blue, respectively; (b) the comparison of the location and inactivation of genes rfaC, rfaF and rfaD in the chromosome of E. coli strains W3110, WBB06 and WJW00.