Elucidating the LPS modification repertoire of Pseudomonas aeruginosa
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Gram-negative bacteria enhance their survival in harmful environments by outer membrane remodeling, particularly at the lipid A moiety of LPS. We recently identified a functional ortholog of the lipid A kinase, lpxT, in Pseudomonas aeruginosa. LpxT[subscript Pa] is unique from previously characterized LpxT enzymes in that it is able to phosphorylate both lipid A phosphate groups as well as generate a novel 1-triphosphate species. Low Mg²⁺ results in modulation of LpxT[subscript Pa] activity and is influenced by transcription of lipid A aminoarabinose (L-Ara4N) transferase ArnT, which is induced when Mg²⁺ is limiting (Nowicki et al., Mol Micro, 2014). We have also revealed the identity of a functional phosphoethanolamine (pEtN) transferase, EptA[subscript Pa], in P. aeruginosa, and the first report of pEtN-modified lipid A in this organism. EptA[subscript Pa] adds pEtN strictly to the non-canonical position of lipid A. Transcription of EptA[subscript Pa] is regulated by Zn²⁺ via the ColRS twocomponent system, contrasting from EptA regulation in enteric bacteria such as Salmonella enterica and Escherichia coli. Further, although L-Ara4N modification readily occurs at the same site of pEtN addition under several environmental conditions, Zn²⁺exclusively induces pEtN addition to lipid A and downregulates transcription of the L-Ara4N transferase gene (Nowicki et al., Mol Micro, 2015). The existence and specificity of these modification enzymes suggests that coordinated regulation of P. aeruginosa outer membrane remodeling occurs to permit adaptation to a changing environment.