Reciprocal Control between a Bacterium's Regulatory System and the Modification Status of Its Lipopolysaccharide

Abstract

SUMMARY Gram-negative bacteria often modify their lipopolysaccharide (LPS) thereby increasing resistance to antimicrobial agents and avoidance of the host immune system. However, it is unclear how bacteria adjust the levels and activities of LPS-modifying enzymes in response to the modification status of their LPS. We now address this question by investigating the major regulator of LPS modifications in Salmonella enterica. We report that the PmrA/PmrB system controls expression of a membrane peptide that inhibits the activity of LpxT, an enzyme responsible for increasing the LPS negative charge. LpxT’s inhibition and the PmrA-dependent incorporation of positively charged L-4-aminoarabinose into the LPS decrease Fe3+ binding to the bacterial cell. Because Fe3+ is an activating ligand for the sensor PmrB, transcription of PmrA-dependent LPS-modifying genes is reduced. This mechanism enables bacteria to sense their cell surface by its effect on the availability of an inducing signal for the system regulating cell surface modifications.

Deciphering the Metabolism of Undecaprenyl-Phosphate: The Bacterial Cell-Wall Unit Carrier at the Membrane Frontier

“…5). 54,106,111 The identification of C 55 -PP as the physiological phosphate donor was clearly established by demonstrating that the antibiotic bacitracin, which specifically binds C 55 -PP, inhibited the formation of lipid A 1-diphosphate in vivo. 106 Thus, YeiU also displayed a C 55 -PP phosphatase activity that was coupled to a phosphotransfer reaction, creating an unexpected link between C 55 -P metabolism and the lipid A remodeling pathway.…”

“…84 Thus, EptA and LpxT accomplish two different modifications, that is, the addition of a pEtN or a phosphate group, at the same (1-phosphate) position of the lipid A. While EptA transcription is increased under PmrA/PmrB-induction conditions, LpxT activity was recently demonstrated to be post-translationally repressed via the binding of a small membrane peptide called PmrR, 54 whose expression is induced by phosphorylated PmrA, thus favoring the addition of the pEtN group instead of a phosphate group in these conditions. It was hypothesized that this mode of regulation of LpxT by PmrR was more efficient than a transcriptional repression to enable a rapid fine-tuning of cell surface properties (charges content).…”

Fe³⁺-dependent epistasis between the CpxR-activated loci and the PmrA-activated LPS modification loci in <i>Salmonella enterica</i>

“…Moreover, we have demonstrated that the formation of 1-PP lipid A is essential to the reciprocal control phenomenon and iron binding to the cell surface because they are completely abolished by mutating the lpxT gene (Kato et al, 2012). Additionally, while attempting to identify the PmrR peptide target, a pull-down with GST-PmrR resulted in the identification of several proteins, including DegP that is under the control of the CpxR/CpxA system in E. coli and S. enterica, as co-purified proteins (Kato, A., unpubl.).…”

“…The PmrA/PmrB two-component regulatory system confers Salmonella resistance to cationic peptide polymyxin B and ferric iron by modifying the phosphoryl groups of lipopolysaccharide (LPS) (Chen and Groisman, 2013), the Our group has reported the reciprocal control of the PmrA/PmrB system and the LPS modification status where, together with the Ugd/PbgPE-mediated positively charged L-Ara4N modification at the 1 and 4¢ positions of lipid A, the PmrC-mediated pEtN modification at the 1 position and the PmrR dependent inhibition of the 1-PP lipid A formation, which is catalyzed by LpxT, decreases the net negative charge of the LPS and, in turn, hinders the accessibility of Fe 3+ to the cell surface as well as to the Fe 3+ sensor PmrB (Kato et al, 2012). Moreover, we have demonstrated that the formation of 1-PP lipid A is essential to the reciprocal control phenomenon and iron binding to the cell surface because they are completely abolished by mutating the lpxT gene (Kato et al, 2012).…”

“…Ugd/PbgPE modifies phosphates of lipid A with L-4-aminoarabinose (L-Ara4N) at the 1 and 4¢ positions (Gunn et al, 1998), whereas PmrC adds phosphoethanolamine (pEtN) primarily to phosphate at the 1 position of lipid A (Kato et al, 2012;Lee et al, 2004). The PmrCmediated pEtN modification at position 1 of lipid A requires the concomitant action of a novel membrane peptide PmrR (Kato et al, 2012), which inhibits the formation of the LpxT-mediated pyrophosphate at position 1 (1-PP) (Herrera et al, 2010;Touze et al, 2008). In addition, in the presence of both high Fe 3+ , and low Mg 2+ , signals, phospho-PmrA binds to the pmrD promoter to exert partial inhibition of PhoP-dependent transcriptional initiation (Kato et al, 2003).…”