Accumulation of the lipid A precursor UDP-2,3-diacylglucosamine in an Escherichia coli mutant lacking the lpxH gene

Abstract

The lpxH gene encodes the UDP-2,3-diacylglucosamine-specific pyrophosphatase that catalyzes the fourth step of lipid A biosynthesis in Escherichia coli. To confirm the function of lpxH, we constructed KB21/ pKJB5. This strain contains a kanamycin insertion element in the chromosomal copy of lpxH, complemented by plasmid pKJB5, which is temperature-sensitive for replication and harbors lpxH+. KB21/pKJB5 grows at 30°C but loses viability at 44°C, demonstrating that lpxH is essential. CDP-diglyceride hydrolase (Cdh) catalyzes the same reaction as LpxH in vitro but is nonessential and cannot compensate for the absence of LpxH. The presence of Cdh in cell extracts interferes with the LpxH assay. We therefore constructed KB25/ pKJB5, which contains both an in-frame deletion of cdh and a kanamycin insertion mutation in lpxH, covered by pKJB5. When KB25/pKJB5 cells are grown at 44°C, viability is lost, and all in vitro LpxH activity is eliminated. A lipid migrating with synthetic UDP-2,3-diacylglucosamine accumulates in KB25/pKJB5 following loss of the covering plasmid at 44°C. This material was converted to the expected products, 2,3-diacylglucosamine 1-phosphate and UMP, by LpxH. Pseudomonas aeruginosa contains two proteins with sequence similarity to E. coli LpxH. The more homologous protein catalyzes UDP-2,3-diacylglucosamine hydrolysis in vitro. The corresponding gene complements KB25/pKJB5 at 44°C, but the less homologous gene does not. The accumulation of UDP-2,3-diacylglucosamine in our lpxH mutant is consistent with the observation that the lipid A disaccharide synthase LpxB, the next enzyme in the pathway, cannot condense two UDP-2,3-diacylglucosamine molecules, but instead utilizes UDP-2,3-diacylglucosamine as its donor and 2,3-diacylglucosamine 1-phosphate as its acceptor.