Modification of the phosphate groups of lipid A with 4-amino-4-deoxy-L-arabinose (L-Ara4N Lipopolysaccharide (LPS)1 is an immunogenic glycolipid that constitutes most of the outer leaflet of the outer membrane of Gram-negative bacteria (1-4). LPS consists of three domains, which are the O-antigen, the core oligosaccharide, and the lipid A moiety (1-4). The O-antigen functions as a protective barrier, whereas the core sugars maintain outer membrane integrity and provide an attachment site for the O-antigen (1-4). Lipid A is the hydrophobic membrane anchor of LPS, and it is the active (endotoxin) component of LPS, accounting for many of the pathophysiological effects associated with Gram-negative sepsis (5-7).The Kdo 2 -lipid A portion of LPS is sufficient to support growth in Escherichia coli and Salmonella typhimurium (2). Covalent modifications to Kdo 2 -lipid A can be induced by environmental stimuli, such as low Mg 2ϩ concentrations or low pH (8 -10). As shown in Fig. 1 for S. typhimurium, these modifications include the incorporation of palmitate (11, 12), the addition of phosphoethanolamine (pEtN) (13-15), and/or the addition of 4-amino-4-deoxy-L-arabinose (L-Ara4N) moieties (13,16,17). The modification of at least one phosphate residue with L-Ara4N is required for maintaining resistance to certain cationic antimicrobial peptides of the innate immune system and to the antibiotic polymyxin (18,19). Resistance is due, in part, to the neutralization of the negative charges of lipid A by L-Ara4N, reducing the affinity of lipid A for cationic substances (20) and preventing these anti-microbial compounds from penetrating the outer membrane.The addition of pEtN and L-Ara4N groups to lipid A is controlled by the PmrA/PmrB two-component regulatory system, which is activated by low pH, high Fe 3ϩ levels, or indirectly, by low concentrations of Mg 2ϩ via the PhoP/PhoQ system through the action of PmrD (9). Activated PmrA stimulates transcription at the pmrCAB, pmrE(ugd) , and pmrHFIJKLM loci (19,21). Constitutive pmrA (pmrA c ) mutants of E. coli and S. typhimurium are polymyxin-resistant, and they modify their lipid A with L-Ara4N and pEtN groups under all growth conditions (17,18,22). Inactivation of either pmrE or the genes in the pmrHFIJKLM operon results in complete loss of polymyxin resistance and of L-Ara4N-modified lipid A in pmrA c bacterial cells (19,21,23). Similarly, pmrA c mutants harboring a nonpolar disruption of the pmrC(eptA) gene are unable * This research was supported by National Institutes of Health Grant GM-51310 (to C. R. H. R.). The Duke University NMR Center is partially supported by P30-CA-14236. NMR instrumentation in the Center was funded by the National Science Foundation, the National Institutes of Health, the North Carolina Biotechnology Center, and Duke University. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fac...

Addition of the 4-amino-4-deoxy-L-arabinose (L-Ara4NPmrI (renamed ArnA) was overexpressed using a T7 construct, and shown by itself to catalyze the unprecedented oxidative decarboxylation of UDP-glucuronic acid to form uridine 5-(␤-L-threo-pentapyranosyl-4؆-ulose diphosphate). A 6-mg sample of the latter was purified, and its structure was validated by NMR studies as the hydrate of the 4؆ ketone. ArnA resembles UDP-galactose epimerase, dTDP-glucose-4,6-dehydratase, and UDP-xylose synthase in oxidizing the C-4؆ position of its substrate, but differs in that it releases the NADH product.

ArnB was overexpressed using a T7lac promoter-driven construct and shown to catalyze the reversible transfer of the amino group from glutamate to the acceptor, uridine 5-(␤-L-threo-pentapyranosyl-4؆-ulose diphosphate), the intermediate that is synthesized by ArnA from UDPglucuronic acid. A 1.7-mg sample of the putative UDP-LAra4N product generated in vitro was purified by ion exchange chromatography, and its structure was confirmed by 1 H and 13 C NMR spectroscopy. ArnB, which is a cytoplasmic protein, was purified to homogeneity from an overproducing strain of E. coli and shown to contain a pyridoxal phosphate cofactor, as judged by ultraviolet/visible spectrophotometry. The pyridoxal phosphate was converted to the pyridoxamine form in the presence of excess glutamate. A simple quantitative radiochemical assay was developed for ArnB, which can be used to assay the enzyme either in the forward or the reverse direction. The enzyme is highly selective for glutamate as the amine donor, but the equilibrium constant in the direction of UDP-L-Ara4N formation is unfavorable (ϳ0.1). ArnB is a member of a very large family of aminotransferases, but closely related ArnB orthologs are present only in those bacteria capable of synthesizing lipid A species modified with the L-Ara4N moiety.