The lipid A moiety of lipopolysaccharide (LPS) forms the outer monolayer of the outer membrane of most Gram-negative bacteria. Escherichia coli lipid A is synthesized on the cytoplasmic surface of the inner membrane by a conserved pathway of nine constitutive enzymes, which were discovered by the PI. Following attachment of the core oligosaccharide, the core-lipid A complex is flipped to the outer surface of the inner membrane by the ABC transporter MsbA, where the O-antigen polymer is attached. Subsequent trafficking of nascent LPS to the outer membrane involves a second ABC transporter and two outer membrane proteins, encoded by the lpt genes. Many additional covalent modifications of lipid A may occur during its transit from the outer surface of the inner membrane to the outer membrane. Lipid A modification enzymes are therefore excellent reporters for LPS trafficking within the bacterial envelope. However, modification systems are quite variable between different Gram-negative organisms and are often regulated by environmental factors. Although not required for growth, the lipid A modification enzymes can modulate the virulence of some pathogens. Mutation or heterologous expression of the genes encoding the lipid A modification enzymes facilitates the re-engineering of lipid A structure in diverse bacterial strains and can enable the development of new vaccines, as illustrated by the PI's recent genetic studies with Francisella novicida, a mouse-specific model organism for human tularemia. Given the importance of Francisella and the fact that it contains many unprecedented lipid A modification enzymes, the specific aims for the coming grant period will be: 1) the purification and characterization of the lipid A phosphatases and glycosyltransferases of F. novicida;2) the biochemical and genetic analysis of LPS core assembly enzymes of F. novicida;3) the elucidation of the regulation of the free lipid A versus LPS content of F. novicida;and 4) the characterization of modification enzymes that hydroxylate or oxidize Kdo2- lipid A, a defined LPS substructure that supports E. coli growth and potently activates toll-like receptor 4 (TLR4). The availability of diverse modification enzymes and their structural genes will enable the large-scale, combinatorial biosynthesis of novel, Kdo2-lipid A derivatives in E. coli. In addition to creating new opportunities for vaccine development, these studies will provide fundamental insights into lipid A biology, chemistry and outer membrane biogenesis.
Half of all human bacterial pathogens are classified as """"""""Gram-negative"""""""";they appear as pale, pink rods in the light microscope because of their inability to take up a purple dye, called Gram-stain. Bacteria of this kind, which include all strains of Escherichia coli, Salmonella, Pseudomonas, and Francisella, contain an outer membrane that makes them impermeable to certain dyes and antibiotics. The outer surfaces of the outer membranes of Gram-negative bacteria contain large amounts of a unique substance known as lipopolysaccharide (LPS), which is held in place by lipid A. The enzymes that assemble the lipid A anchor of LPS are conserved and are excellent targets for designing new antibiotics with activity against Gram-negative pathogens that have become resistant to commercial antibiotics. In addition, the modification of lipid A structure in live bacteria by manipulation of the genes encoding the enzymes that assemble the lipid A domain of LPS are useful for attenuating bacterial pathogens so that they can be used as vaccines.
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