Fig 7.

Schematic diagram of phosphoethanolamine modification of lipopolysaccharide by PmrAB of A. baumannii in response to metal ions. (A) PmrAB of A. baumannii is a two-component system that governs responses to Fe²⁺, Zn²⁺, and Al³⁺. The 35-ExxE and 131-ExxxE motif sequences located in the periplasmic region of PmrB, a sensor kinase, are critical to these responses. (B) Among the genes activated by PmrAB are those responsible for LOS modification, which include pmrC and eptA genes for pEtN modification and naxD operon for GalN modification. (C) The pEtN modification of LOS is crucial for reducing Al³⁺ toxicity and for resistance to colistin. In A. baumannii, PmrAB is activated in response to metal ions, which leads to an upregulation of pEtN modification genes. This upregulation contributes to colistin and Al³⁺ toxicity avoidance by facilitating the modification of LOS with pEtN.