Presentation Information
[3Open-03]A Gdx-mediated Cs+ efflux mechanism confers Cs+ resistance in Escherichia coli
○Daiki Kojima1,2, Masahiro Ito1,2 (1. Toyo University, Graduate school of Life Sciences (Japan), 2. Toyo University, Bio-Resilience Research Project (Japan))
Keywords:
Cesium-resistant mechanism,Escherichia coli,Small multidrug resistance (SMR),Mutant
Cesium (Cs+), released into the environment following nuclear power plant accidents, has widely contaminated soil and water, raising concerns about its environmental and health impacts. Various approaches have been explored for Cs+ removal, and microbial remediation has attracted attention because of its lower environmental burden compared to conventional chemical methods (1).
However, the application of microorganisms is limited by the intrinsic sensitivity of bacterial cells to Cs+. In Escherichia coli, Cs+ enters the cell via the K+/H+ symporter Kup due to its similarity to K+. Because no dedicated Cs+ efflux system has been identified, Cs+ accumulates intracellularly, leading to K+ depletion and growth inhibition.
In our laboratory, a Cs+-resistant E. coli strain, ZX-1, capable of growing under high Cs+ concentrations, was obtained. ZX-1 maintains a low intracellular Cs+ concentration (2), suggesting the presence of an active resistance mechanism. In this study, we aimed to elucidate the molecular basis of Cs+ resistance in ZX-1.
RNA-seq analysis revealed that gdx, encoding a guanidinium exporter, was markedly upregulated in ZX-1 compared to the parental strain Mach1. Overexpression of gdx in Mach1 conferred Cs+ resistance comparable to or greater than that of ZX-1. Intracellular ion measurements showed that the gdx-overexpressing strain maintained lower Cs+ levels while preventing K+ depletion. Antiport assays further demonstrated that Gdx transports not only guanidinium, its native substrate, but also Cs+. In addition, Gdx variants carrying mutations at aromatic residues (W16 and W62) were constructed to investigate substrate recognition.
These results indicate that although Gdx exhibits high specificity for guanidinium, it can also mediate Cs+ efflux with low affinity. One possible explanation is that both guanidinium and Cs+ are relatively weakly hydrated ions, which may facilitate their interaction with the aromatic binding pocket of Gdx.
Taken together, our findings suggest that E. coli possesses a previously unrecognized membrane protein-mediated Cs+ efflux mechanism. The ability of Gdx to transport Cs+ expands its functional scope beyond guanidinium export and provides new insights into microbial Cs+ resistance, which may contribute to the development of biotechnological strategies for Cs+ remediation (3).
(1) Kuppan N et. Al., (2024), Waste Manag. Bull., 2:154-171.
(2) Kojima D et. al., (2024), Front Microbiol., 14:1340033.
(3) Kojima D and Ito M, (2026), Eng. Microbiol., 6:100251.
However, the application of microorganisms is limited by the intrinsic sensitivity of bacterial cells to Cs+. In Escherichia coli, Cs+ enters the cell via the K+/H+ symporter Kup due to its similarity to K+. Because no dedicated Cs+ efflux system has been identified, Cs+ accumulates intracellularly, leading to K+ depletion and growth inhibition.
In our laboratory, a Cs+-resistant E. coli strain, ZX-1, capable of growing under high Cs+ concentrations, was obtained. ZX-1 maintains a low intracellular Cs+ concentration (2), suggesting the presence of an active resistance mechanism. In this study, we aimed to elucidate the molecular basis of Cs+ resistance in ZX-1.
RNA-seq analysis revealed that gdx, encoding a guanidinium exporter, was markedly upregulated in ZX-1 compared to the parental strain Mach1. Overexpression of gdx in Mach1 conferred Cs+ resistance comparable to or greater than that of ZX-1. Intracellular ion measurements showed that the gdx-overexpressing strain maintained lower Cs+ levels while preventing K+ depletion. Antiport assays further demonstrated that Gdx transports not only guanidinium, its native substrate, but also Cs+. In addition, Gdx variants carrying mutations at aromatic residues (W16 and W62) were constructed to investigate substrate recognition.
These results indicate that although Gdx exhibits high specificity for guanidinium, it can also mediate Cs+ efflux with low affinity. One possible explanation is that both guanidinium and Cs+ are relatively weakly hydrated ions, which may facilitate their interaction with the aromatic binding pocket of Gdx.
Taken together, our findings suggest that E. coli possesses a previously unrecognized membrane protein-mediated Cs+ efflux mechanism. The ability of Gdx to transport Cs+ expands its functional scope beyond guanidinium export and provides new insights into microbial Cs+ resistance, which may contribute to the development of biotechnological strategies for Cs+ remediation (3).
(1) Kuppan N et. Al., (2024), Waste Manag. Bull., 2:154-171.
(2) Kojima D et. al., (2024), Front Microbiol., 14:1340033.
(3) Kojima D and Ito M, (2026), Eng. Microbiol., 6:100251.
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