Abstract:Diverse lipid A structures have been observed in a multitude of Gram-negative bacteria, but the metabolic logic of lipid A biosynthesis is widely conserved. This chapter will start by describing the nine constitutive enzymes of the Raetz pathway, which catalyze conserved lipid A biosynthetic reactions that depend on cytoplasmic cofactors. Concomitant with lipid A export and assembly on the cell surface, a number of regulated covalent modifications of lipid A can occur in the extracytoplasmic compartments. The narrow phylogenetic distribution of the lipid A modification enzymes, combined with the diverse regulatory signals governing their expression, is responsible for most of the lipid A structural diversity that is observed in nature. By focusing on E. coli as a model system, the general principles of lipid A biosynthesis and assembly are revealed to inform related processes that occur in more divergent organisms.