(p)ppGpp and moonlighting RNases influence the first step of lipopolysaccharide biosynthesis in Escherichia coli

The role of the essential GTPase ObgE in regulating lipopolysaccharide synthesis in Escherichia coli

During growth, cells need to synthesize and expand their envelope, a process that requires careful regulation. Here, we show that the GTPase ObgE of E. coli contributes to the regulation of lipopolysaccharide (LPS) synthesis, an essential component of the Gram-negative outer membrane. Using a dominant-negative mutant (named 'ObgE*'), we show a direct interaction between ObgE and LpxA, which catalyzes the first step in LPS synthesis. This interaction is enhanced by the mutation in ObgE* which, when bound to GTP, leads to inhibition of LpxA, decreased LPS synthesis, and cell death. Although wild-type ObgE does not exert the same strong effects as ObgE* on LpxA or LPS synthesis, our data indicate that ObgE participates in the regulation of cell envelope synthesis in E. coli. Because ObgE also influences other cellular functions (i.e., ribosome assembly, DNA replication, etc.), it seems increasingly plausible that this GTPase coordinates several processes to finetune cell growth.

Common and varied molecular responses of Escherichia coli to five different inhibitors of the lipopolysaccharide biosynthetic enzyme LpxC

A promising yet clinically unexploited antibiotic target in difficult-to-treat Gram-negative bacteria is LpxC, the key enzyme in the biosynthesis of lipopolysaccharides, which are the major constituents of the outer membrane. Despite the development of dozens of chemically diverse LpxC inhibitor molecules, it is essentially unknown how bacteria counteract LpxC inhibition. Our study provides comprehensive insights into the response against five different LpxC inhibitors. All compounds bound to purified LpxC from Escherichia coli. Treatment of E. coli with these compounds changed the cell shape and stabilized LpxC suggesting that FtsH-mediated proteolysis of the inactivated enzyme is impaired. LpxC inhibition sensitized E. coli to vancomycin and rifampin, which poorly cross the outer membrane of intact cells. Four of the five compounds led to an accumulation of lyso-phosphatidylethanolamine, a cleavage product of phosphatidylethanolamine, generated by the phospholipase PldA. The combined results suggested an imbalance in lipopolysaccharides and phospholipid biosynthesis, which was corroborated by the global proteome response to treatment with the LpxC inhibitors. Apart from LpxC itself, FabA and FabB responsible for the biosynthesis of unsaturated fatty acids were consistently induced. Upregulated compound-specific proteins are involved in various functional categories, such as stress reactions, nucleotide, or amino acid metabolism and quorum sensing. Our work shows that antibiotics targeting the same enzyme do not necessarily elicit identical cellular responses. Moreover, we find that the response of E. coli to LpxC inhibition is distinct from the previously reported response in Pseudomonas aeruginosa.